Work on biological attributes of water quality is an active field of research and most documents more than about five years old are of limited value. The list of protozoan pathogens gives some indication of the scope of the field. This document deals primarily with the pathogenic protozoans; while helminth worms, fungi, bacteria, cyanophytes, toxic algae and viruses are mentioned in passing, they will be dealt with in other reports. This report could not have been written in its present state as little as five years ago since the necessary protozoan research work had not yet been carried out. It may still be somewhat premature and this document may need to be revised before too long, especially the taxonomy. The understanding of protozoans is growing rapidly as a result of new research, especially genome analysis. However, there is a growing need for this information to be consolidated and reviewed, some guidelines are needed now. The following tables give a brief capsular overview of the names of the diseases, the organisms responsible, a broad distribution range, some transmission mechanisms, a general prognosis of patients and some pre-disposing epidemiology factors of the diseases.
|Disease Caused||Amoebiasis, intestinal infection|
|Organism Responsible||Entamoeba histolytica|
|Distribution Range||Cosmopolitan, more common in tropics and sub-tropics|
|Transmission Mechanisms||Water, food, fecal/oral, sex, personal contact|
|Prognosis-Healthy People||Asymptomatic intestinal infection|
|Epidemiology Factors||Slums, communes, institutions, crowding, poor hygiene, promiscuous homosexuals|
|Disease Caused||Encephalitis, Keratitis of the eye, dermal ulcers|
|Organisms Responsible||Acanthamoeba culbertsoni
|Distribution Range||Cosmopolitan in warm waters|
|Transmission Mechanisms||Water, aerosols|
|Prognosis-Healthy People||Primarily keratitis, encephalitis is rare but fatal|
|Prognosis-HIV/AIDS||Fatal encephalitis in most cases|
|Epidemiology Factors||Warm water, inhalation of dust and aerosols, immersion of head or open sores in warm water, contaminated contact lenses|
|Disease Caused||Encephalitis, pneumonitis, dermal ulcers|
|Organism Responsible||Balamuthia mandrillaris|
|Distribution Range||Cosmopolitan in warm waters|
|Transmission Mechanisms||Water, aerosols|
|Prognosis-Healthy People||Primarily pneumonitis, encephalitis is rare but fatal|
|Prognosis-HIV/AIDS||Fatal encephalitis in most cases|
|Epidemiology Factors||Warm water, inhalation of dust and aerosols, immersion of head or open sores in warm water|
|Disease Caused||Balantidiasis, intestinal infection|
|Organism Responsible||Balantidium coli|
|Distribution Range||Cosmopolitan, more common in warm climates|
|Transmission Mechanisms||Food, water, fecal/oral, sex, personal contact, pigs|
|Prognosis-Healthy People||Dysentery, rare with good hygiene|
|Epidemiology Factors||Absence of starch in the gut, poor hygiene, proximity of pigs, slums, crowding, institutions|
|Disease Caused||Blastocystiasis, intestinal infection|
|Organism Responsible||Blastocystis hominis|
|Distribution Range||Cosmopolitan, more prevalent in the tropics|
|Transmission Mechanisms||Food, water, fecal/oral, sex, personal contact|
|Epidemiology Factors||Fecal/oral, sex, poor hygiene, crowding, sewage contamination of water supplies, institutions|
|Organism Responsible||Cryptosporidium parvum|
|Distribution Range||Cosmopolitan, common in cold climates|
|Transmission Mechanisms||Water, food, fecal/oral, sex|
|Epidemiology Factors||Neonate animals, poor hygiene, livestock in watersheds, breakdowns in drinking water filtration and disinfection, institutions, sewage or fecal contamination, re-cycling filter backwash water in filtration plants|
|Disease Caused||Cyclosporiasis, intestinal infection|
|Organism Responsible||Cyclospora cayetanensis|
|Distribution Range||Cosmopolitan, common in cold climates|
|Transmission Mechanisms||Fecal/oral, sex, water, food, crowding, poor hygiene|
|Epidemiology Factors||Contaminated water and soil used to grow berries and leafy crops, drinking water|
|Disease Caused||Dientamoebiasis, intestinal infection|
|Organism Responsible||Dientamoeba fragilis|
|Transmission Mechanisms||Fecal/oral, water, food, sex, pinworm|
|Epidemiology Factors||Helminth eggs and larvae, pinworm, Enterobias vermicularis, crowding, poor hygiene, institutions, slums|
|Disease Caused||Giardiasis, beaver fever|
|Organism Responsible||Giardia lamblia|
|Transmission Mechanisms||Water, food, fecal/oral, sex|
|Prognosis-Healthy People||Asymptomatic or intestinal infection with diarrhea|
|Epidemiology Factors||Daycare, crowding, institutions, slums, poor hygiene, pre-school children, livestock in watersheds, failure of filtration or disinfection in domestic water treatment|
|Disease Caused||Amoebiasis, hartmannelliasis, encephalitis|
|Organism Responsible||Hartmannella veriformis|
|Transmission Mechanisms||Water, aerosols|
|Prognosis-Healthy People||Rare, fatal|
|Epidemiology Factors||Legionella, hot-tubs, showers, cooling towers, sewage or fecally polluted water|
|Disease Caused||Isosporiasis, intestinal infection|
|Organism Responsible||Isospora belli|
|Distribution Range||Tropics and sub-tropics, rarely temperate zone|
|Transmission Mechanisms||Food, water, fecal/oral, sex|
|Prognosis-Healthy People||Asymptomatic to mild diarrhea|
|Prognosis-HIV/AIDS||Dysentery, sometimes death|
|Epidemiology Factors||Male homosexuality, poor hygiene, fecal contamination of food and water, institutions, crowding, slums|
|Disease Caused||Microsporidiosis, various specific diseases|
|Organisms Responsible||Enterocytozoon bieneusi
|Transmission Mechanisms||Water, food, aerosols, dust, fecal/oral, sex?|
|Prognosis-Healthy People||Rare and usually asymptomatic|
|Prognosis-HIV/AIDS||Diarrhea, sinusitis, nephritis, hepatitis, peritonitis, conjunctivitis, eye, respiratory, genitourinary and gastrointestinal infections|
|Epidemiology Factors||Poor hygiene, crowding, institutions, fecal contamination, slums|
|Organism Responsible||Naegleria fowleri|
|Distribution Range||Cosmopolitan but warm water only|
|Transmission Mechanisms||Water via nasal passages after immersion, aerosols|
|Prognosis-Healthy People||Rapidly fatal but rare|
|Prognosis-HIV/AIDS||Rapidly fatal but rare|
|Epidemiology Factors||Contaminated aerosols, immersion of the face in shallow, stagnant, warm water (28-40), hot springs, thermal pools, opportunistic|
Many medically important protozoa require the invasion, resulting in damage, illness or even death, of a suitable host to complete all or part of their life cycle. Such organisms are therefore termed parasites and medical parasitology is the study of protozoan and helminth worm infections of man. Exceptions include some free-living amoebae that normally live and replicate in the environment, or in other species, but under certain circumstances can infect man causing disease, damage or death, but do not complete their life-cycle when they do so. They are not therefore parasites but are included here as examples of opportunistic pathogens.
Parasite infections affect millions of people worldwide afflicting considerable human suffering and economic hardship. Far from declining, many parasite infections are increasing throughout the world. In first world countries protozoan infections are relatively uncommon. However, outbreaks of cryptosporidiosis and giardiasis associated with drinking water supplies and cyclosporidiasis associated with imported fresh produce are of major concern even in North America. Generally, in immunocompetent people protozoan parasitism is rare and not a serious problem, the immune system has evolved to deal with these natural organisms. In the very young and very old, protozoan parasitism, while still uncommon, is of concern in people who are immunocompromised by HIV, AIDS or anti-transplant rejection drugs; infections are epidemic and often fatal or life threatening.
In many of the descriptions of the life cycles of these parasites there is a note concerning the geographic range of the parasite. This is the original, and still primary or historical, distribution range of the parasite, its area of evolutionary origin and current area of greatest density of infection. However, such geographical limitations are becoming meaningless in the modern world. Travel is easy and fast. Visitors and immigrants bring in diseases and parasites that are then introduced into our health system, our sewage system and our environment. New cultural behaviours, diets and practices may encourage the survival and spread of these pathogens in their new aquatic habitats. The organisms dealt with in this report, although they comprise a fairly long list, are only a few of the very many protozoans that can infect man. Those dealt with here are all transmitted, at least in part, by water.
While many life cycle descriptions do not specifically mention water all the organisms dealt with in this report can be distributed and acquired through water even if water is not the optimal mechanism of spread. If the pathogen is waterborne we are at risk, even though the risk may be small, and should be aware of the potential problems and take the necessary steps to prevent disease outbreaks by treating sewage, irrigation and drinking water as required. Pathogens which are spread only through eating infected meat are not dealt with unless there is also an infective stage found in water. Insect vectors, particularly biting mosquitoes and flies, are the main problem with recreational waters. Many insects, and thus the diseases they transfer, are restricted to the close proximity of aquatic habitats used for recreation such as fishing, swimming and boating. Pathogens spread only via insect vectors, and not directly through contact with the water, are not covered in this document.
Protozoa refers to simple eukaryotic organisms composed of a single cell. Reproduction may be through simple asexual cell division as in the amoeboflagellates or sexual involving the fusion of gametes in part of the life cycle as in the apicomplexa. Some protozoa can form a protective cyst stage capable of withstanding harsh environmental conditions.
These use pseudopodia or flagella for locomotion. They are characterized by a feeding and dividing trophozoite stage that can form a temporary resistant cyst stage. Entamoeba histolytica is the cause of amoebic dysentery producing severe infection of the intestines that can spread to the liver. The organism is characterized by a trophozoite and cyst stage. Entamoeba histolytica is an example of a true parasite in that the organism cannot reproduce outside of its host. Other amoebae occur naturally in soil and water environments, which is their preferred habitat for feeding and replication. These amoebae are termed free-living as they have no natural host in which parasitism occurs. They can infect man opportunistically producing severe and often fatal disease. Such free-living amoebae are the Acanthamoeba, Naegleria fowleri and Balamuthia mandrillaris, all of which can infect the central nervous system. In addition, some Acanthamoeba species can also invade the eye.
These organisms have flagella in the trophozoite stage. Trichomonas vaginalis is a common sexually transmitted organism causing trichomoniasis, an infection of the vagina and urethra. Giardia lamblia causes giardiasis, resulting in diarrhea and other intestinal disturbances. Infection arises from the ingestion of cysts, usually through contaminated water. Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense cause trypanosomiasis, more commonly known as African sleeping sickness. The disease is an insect-borne infection and is spread by the bite of the tsetse fly in which part of the trypanosome life cycle is completed. The eventual invasions of the central nervous system by the trypanosome gives rise a comatose state from which the common name for the disease is derived.
Trypanosoma cruzi causes Chagas disease, American trypanosomiasis. The intermediate hosts in this case are triatomid bugs that feed off the blood of man. Infection results from the inoculation of the bug's feces that contains the organism into the bite wound. Individuals who survive the acute stage of the disease are frequently left with chronic and progressive neuronal and smooth muscle lesions in the heart and gastrointestinal tract. Trypanosoma cruzi has an extensive reservoir in wild and domestic mammals and therefore Chagas disease is a zoonosis; human infections can be caught from animals.
Leishmania species cause leishmaniasis. The disease is spread by the bite of sandflies in which part of the organism's life cycle is completed. In man, the promastigotes from the bite of the sandfly become ingested by macrophages and multiply within them as amastigotes. Cutaneous leishmaniasis occurs if the region of infection remains localized to the dermis as an open sore. In the Old World, Southern Europe, the Middle East, India, the former USSR and parts of Africa, Leishmania major, Leishmania tropica, Leishmania aethiopica and certain subtypes of Leishmania infantum are responsible. In the New World, Mexico southwards and through South America, species responsible include Leishmania braziliensis, Leishmania mexicana and Leishmania amazonensis. If the organism spreads, then mutocutaneous leishmaniasis can occur in which the nose, mouth and palate become destroyed. Infection with members of the Leishmania donovani-Leishmania infantum complex produce the systematic disease of visceral leishmaniasis often known as kala-azar that occurs with a global distribution seen in Old and New World leishmaniasis. The parasites multiply within the macrophages of the liver, spleen, bone marrow and other organs. Untreated, the disease is usually fatal. As with trypanosomiasis, leishmaniasis is a zoonosis as many mammals harbour the parasite.
These possess rows of cilia around the outside of the body that aid motility. The only member of this group known to infect man is Balantidium coli. This is a cyst forming parasite that is a commensal and a non-pathogenic parasite of domestic and wild pigs. It does, however, cause severe diarrhea in humans.
This is a unique group sincee all members are parasitic. The group includes parasites causing malaria, cryptosporidiosis and toxoplasmosis. They lack any visible means of locomotion, most are intracellular, and have complex life cycles involving sexual and asexual reproduction. The common feature of all members is the presence of an apical complex in one or more stages of the life cycle. Although the exact components of the apical complex vary among members, it contains enzymes used to penetrate host tissues.
Plasmodium species cause malaria. The four principal species are Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malariae. The name malaria dates from the time when the disease was thought to be spread from stagnant, foul smelling water. The female Anopheles mosquito, which inhabits such environments, is in fact responsible for the transmission of the disease. In the stomach of the female Anopheles mosquito, male, micro, and female, macro, gametocytes fuse to form a zygote. This in turn forms a motile ookinete that penetrates the mid-gut wall and develops into an oocyst within which are many thousands of sporozoites. When mature, the sporozoites rupture the oocyst and penetrate the salivary glands of the mosquito. When the mosquito next feeds on man, the sporozoites are passed via the blood stream to infect parenchymal cells of the liver. Here they form pre-erythrocytic schizonts in which are several thousand daughter cells called merozoites. These merozoites enter red blood cells to start the asexual intra-erythrocytic cycle and form new gametocytes. The asexual red cell stages are responsible for the pathological changes that occur in malaria, fever, chills, anemia, liver enlargement, encephalitis, renal damage and death. The merozoites need high oxygen tensions to thrive and people with sickle cell anemia have low oxygen tensions in their altered red blood cells which gives them some resistance to the growth and survival of the merozoites. This is a good example of an evolutionary trade-off whereby some resistance to malaria is traded for lower oxygen carrying capacity in the blood. It is also an example of parasitism directing the course of evolution.
Cryptosporidiosis is caused by Cryptosporidium parvum and is a diarrheal disease mainly in infants and small children. It is normally self-limiting but in the immunocompromised host the disease can be severe. Cryptosporidium parvum is enzootic in young calves and is usually passed to man in water containing oocysts of the organism.
Toxoplasmosis caused by Toxoplasma gondii is responsible for the multi-organ infection of toxoplasmosis. The domestic cat is the definitive host for Toxoplasma gondii from which man and other mammals can become infected. Infection commonly arises from the consumption of under cooked meat and in the healthy adult is usually asymptomatic. The most devastating form of toxoplasmosis is seen in congenital infections when a pregnant mother passes the organism to the fetus. This can result in severe fetal abnormalities. The life cycle of Toxoplasma gondii is complex, involving both sexual and asexual reproduction. Three main life forms of Toxoplasma gondii occur, the oocyst which is produced from the sexual cycle in the small intestine of the cat and contains sporozoites, the tachyzoite or the asexual invasive form found in secondary hosts which are derived from pseudocysts and the tissue cyst that contains bradyzoites.
The medical importance of microsporidial infections has only recently been highlighted by the frequent recognition of these obligate intracellular parasites in patients with HIV infection and AIDS. Examples are species of Encephalitozoon, Nosema and Septata intestinalis. Multi-organ infections occur and Septata intestinalis is found in about 2% of all AIDS patients with chronic diarrhea.
|granulomatious amoebic encephalitis
List of Pathogenic and Parasitic Protozoans
Life Cycles and other Pathogen Data
There is a large amount of information on the pathology, diagnosis and treatment of the diseases caused by these pathogens. Virtually none of this data is presented here since the main focus of this document is preventing water borne spread of the diseases. Much of this data can be found on the Internet pages listed in the references.
Naegleria fowleri, Hartmannella sp., Balamuthia mandrillaris and Acanthamoeba sp. are among a large number of free-living amoebae that are opportunistic parasites of man and other aquatic animals. Naegleria and Acanthamoeba are commonly found in lakes, swimming pools, tap water, heated industrial process water and heating and air conditioning units. They are only a two of the many genera of pathogenic free-living amoeba that are ubiquitous in the environment, in damp soil, water, animals and air. These amoebas live freely in soil and in fresh and coastal waters. Their resistant cysts can also be transported in dust.
Naegleria fowleri causes a rare, acute and almost invariably fatal, encephalitis. Several species of Acanthamoeba and Balamuthia mandrillaris can cause lung and skin infections, as well as insidious encephalitis in immunocompromised patients. In addition, Acanthamoeba sp. may cause an ulcerative keratitis of the eye which is usually associated with improper sterilization of soft contact lenses.
Humans and other animals are incidental, non-obligatory hosts. The fact that these are opportunistic parasites and not obligate parasites is indicated by the fact they usually kill their hosts in a few days if the infections are not treated. From an evolutionary point of view no obligate parasite would survive very long if it routinely killed its hosts quickly. Only a handful, 3%, of the known, reported cases have survived. Most diagnoses are made postmortem.
When dealing with ponds, lakes, rivers and streams, one can be almost certain that some bacteria, viruses, worms, cyanophytes and protozoa are present in the water. Some of these organisms are indigenous to natural waters; others are carried from wastewater sources including septic systems and runoff from animal and wildfowl areas. Swimmers themselves are also sources of many pathogens. Some conditions favor the growth of these organisms and hence some water bodies have greater potential for disease spread than others.
Long-term screening of systemic infections with tissue-dwelling amphizoic amoebae has shown the prevalence of amoebae in fish from natural habitats in the Czech Republic to be 6.5% of 1240 specimens examined; 3.3% of 61 specimens in thermally polluted waters and 8.4% of 95 specimens in aquaria. Of the fish in natural habitats, infected cyprinids comprised 5.6% of 785 and Esox lucius 29% of 29. The amoeba species isolated included Vahlkampfia lacustris, Vahlkampfia debilis, Naegleria gruberi, other Naegleria species, Hartmannella vermiformis, Vanella tachypodia, Platyamoeba species, Acanthamoeba griffini, Acanthamoeba hatchetti, Acanthamoeba rhysodes and other Acanthamoeb species. A number of these are also opportunistic human pathogens.
Several species of Acanthamoeba are implicated in human infections, including Acanthamoeba culbertsoni, Acanthamoeba divionensis, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Acanthamoeba rhysodes and Acanthamoeba healyi. Acanthamoeba are free-living amoebas of soil and of fresh and salt water. Since Acanthamoeba are ubiquitous, preventive measures to reduce contact are difficult. Trophozoites are 25 to 40 µm in diameter with characteristic spine-like pseudopodia. Cysts are double-walled, usually polygonal and spherical, and 15 to 20 µm in diameter. Reproduction is by binary fission of the trophozoites. Infective cysts can be transmitted in dust and aerosols. There is no known person-to-person transmission.
Transmission is through contact with warm water, water-based fluids or aerosols; these are not food-borne diseases. Warm water or moist air as found in air conditioning units, humidifiers, dialysis units, physiotherapy pools and industrial process water may also be sources of these parasites. There is a risk of exposure to free-living amoeba in warm, fresh water. Some people have advocated not diving and jumping into these waters. The use of nose plugs for unavoidable exposures may help. Make sure there is adequate disinfection of swimming and wading pools. Contact lenses should be removed before swimming.
Except in the case of keratitis of the eye, the defenses of a healthy host seem sufficient to prevent infection. Free-living amoebas have been isolated from human throats, suggesting that they are generally harmless in healthy individuals. Acanthamoeba usually act as opportunistic pathogens, taking advantage of a loss of metabolic, physiologic or immunologic integrity by the host. Among the most common factors predisposing an individual to Acanthamoeba infection are immunosuppressive therapy, treatment with broad-spectrum antibiotics, diabetes mellitus, various cancers, malnutrition, pregnancy, acquired immune deficiency syndrome, AIDS and chronic alcoholism. Surgical trauma, burns, wounds and radiation therapy can also promote infection. No effective treatment is known for opportunistic Acanthamoeba infections in debilitated and immunosuppressed individuals.
The primary focus of infection for opportunistic Acanthamoeba is usually the lower respiratory tract or skin. The amoebas may enter the respiratory tract by the inhalation of aerosols or dust containing cysts. From the skin or lung lesions the amoebas may spread to the brain to cause insidious, slowly progressive and usually fatal encephalitis. In healthy individuals, Acanthamoeba can cause an ulcerating keratitis, often associated with the use of improperly sterilized contact lenses. The incidence of Acanthamoeba keratitis can be greatly reduced by correct sterilization of contact lenses. Lenses should be cleaned properly, using commercial rather than homemade saline, and should be disinfected by a chemical or, preferably, a thermal process.
Acanthamoeba causes a disease of the central nervous system, granulomatious amoebic encephalitis or GAE, granulomatous skin lesions and keratitis, acanthamoebic uveitis and corneal ulcers following corneal trauma or in association with contact lenses. Acanthamoeba do not have an amoeboflagellate form, and cysts can be found in human infections. GAE is rare; as of October 1996 only 103 cases had been reported in the world, 72 of these in the US, over 50 in AIDS patients, in the 25 years since this disease was recognized. The disease is known only from isolated cases and does not normally occur in clusters or outbreaks.
Although amoebic encephalitis has been reported in an immunosuppressed dog and a sheep, it is primarily a disease of man and the Old World primates. Lesions following infection with Acanthamoeba are predominantly within the central nervous system parenchyma and are multifocal, differing from the fulminant meningitis produced by Naegleria fowleri, also a pathogen of human and non-human primates. The organisms in the brain are generally associated with blood vessels, suggesting vascular dissemination. Infections lead to death within several weeks to a year after the first appearance of symptoms.
In the US, GAE is rare. It is more common in the warmer regions of the US and in the warmer months of spring and summer. Internationally, fewer than 300 cases have been reported. Although rare, cases have been reported worldwide, reflecting the ubiquity of the organisms. Most reports come from the US, Australia and Europe. This is likely due to an identification and reporting bias in countries with good health care infrastructure. These infections are almost always fatal. The high mortality is likely due to the difficulty of diagnosis and poor-to-marginal response to therapy. In most cases, the diagnosis is made after death. In warm third world countries, where the disease is more likely to occur, such postmortem diagnoses are rarely made so the incidence of the disease is certainly under-reported.
These infections show no particular ethnic or racial bias but the male to female ratio is 5:1. GAE affects all ages but appears more common in the very young and very old. Patients may have a history of swimming, diving, bathing or playing in warm, fresh water during the previous few days to 2 weeks. The onset occurs over several hours up to 1-2 days. Occasionally, patients will survive for weeks or months.
Balamuthia mandrillaris, is a recently described free-living leptomyxid amoeba that is morphologically similar to Acanthamoeba. It is capable of causing disease in humans and animals, primarily Old World primates. Because the number of human cases is rapidly increasing, the medical community now considers this infection an important emerging disease. Balamuthia mandrillaris usually acts as an opportunistic pathogen in immunocompromised or debilitated individuals in whom it causes pneumonitis or dermal ulcerations. From these lesions the amoebas may spread to the brain to cause an insidious, slowly progressive and usually fatal condition called granulomatous amoebic encephalitis or GAE.
Animals reported with this infection include Hylobates concolor leucogenys, white cheeked gibbon, Papio sphinx, mandrill, Gorilla gorilla gorilla, western lowland gorilla and Colobus guereza kikuyensis, kikuyu colobus monkey. In all these cases indirect immunofluorescent staining of amoebas in brain sections with a Balamuthia mandrillari-specific polyclonal antibody was positive while indirect immunofluorescent staining for several species of Acanthamoeba, Naegleria fowleri and Hartmannella vermiformis was negative.
The trophozoites are free-living worldwide inhabitants of soil and of fresh and salt water. They reproduce by binary fission. The defenses of a healthy host seem sufficient to prevent infection. Free-living amoebas have been isolated from human throats, suggesting that they are generally harmless in healthy individuals. They may contaminate physiotherapy pools, air-conditioning units, etc. No effective treatment is known for opportunistic Balamuthia mandrillaris infections in debilitated and immunosuppressed individuals. The disease progresses over a period of one to several weeks and usually ends in coma and death. The incubation period of the disease is difficult to determine, as pulmonary and skin lesions containing the organisms may be present for months before encephalitis appears. In many cases, a Balamuthia infection is not diagnosed until after, or at best, shortly before, death.
Balantidium coli is a worldwide parasite of many species of animals, including pigs, rats, guinea pigs, humans and dogs. It appears that the parasite can be transmitted readily among these species, when the appropriate condition of fecal contamination is present. Domestic pigs appear to be the primary reservoir for human infections. Humans are infected when they ingest cysts via food or water contaminated with fecal material. In many respects this parasite resembles Entamoeba histolytia, an important difference, that can have a significant impact on epidemiology, is that trophozoites of Balantidium coli will encyst after being passed in stools, trophozoites of Entamoeba histolytica will not. This is the only ciliate protozoan that is known to produce disease in humans. Most transmission is pig-to-man or man-to-man. The organism can be spread through contaminated water although this is not the most common means of epidemic spread.
Balantidium coli has a simple life cycle consisting of a trophozoite that lives in the flexure areas of the large intestine feeding on the intestinal content and tissues. The infection may be asymptomatic or cause severe dysentery similar to amoebiasis. It is invasive and ulcer forming. There is little evidence of tissue invasion in hogs, the reservoir host. In man and monkeys, the organism penetrates the gut mucosa and causes destruction producing a lesion similar in appearance to that caused by Entamoeba histolytica. These parasites thrive chiefly on starchy foods and if starchy foods are available they do not invade the mucosa. The scarcity of starchy foods in the human colon may be the driving force for the invasion of the mucosa.
Balantidium coli reproduces by binary fission with the micronucleus undergoing mitotic division and the macronucleus undergoing an amitotic division before the cytoplasm divides. The cyst form lacks extracellular cilia and is surrounded by a wall that protects the enclosed organism from drying and other deleterious environmental conditions, it is the infective form. Diagnosis is based upon finding either trophozoites or cysts in the stool, there are no serologic tests. Considering the amount of human exposure to the cysts from pigs, man appears to be relatively resistant to the infection. However, if man-to-man transmission occurs as in institutions or in crowded living conditions such as tropical villages, the spread of the infection can reach epidemic proportions. The usual precautions to break the anal/oral chain of spread are important.
Blastocystis hominis is a protozoan occasionally found in the intestinal tract of humans and in other animals. This unicellular protozoan is worldwide in distribution. Knowledge of the life cycle and transmission is incomplete. There are four known modes of division, all of them asexual. Given what is known about the parasite, infection is presumably acquired primarily by the fecal/oral transmission route. Infections have been associated with intra-familial spread, contaminated water and travel to the tropics. Transmission is believed to have occurred from well water contaminated by sewage. Blastocystis hominis has been confirmed in humans, monkeys, apes, pigs, birds and guinea pigs. It is not clear whether the organism found in dogs, cattle, sheep, horses, pigs, chickens, geese, ducks, ostriches, alpacas, llamas, koala and wombat in Australia are the same species.
Whether or not Blastocystis hominis causes symptomatic infection in humans is currently under investigation. It is not at all clear whether disease in healthy humans is caused by this parasite due to the common occurrence of the organism in both asymptomatic and symptomatic individuals. Most people with this organism in their stools are free of symptoms and the parasite may become undetectable in many of these individuals without any treatment at all. Those who believe symptoms could be related to infection with this parasite have described a spectrum of illness including watery diarrhea, abdominal pain, perianal pruritis and excessive flatulence. It is likely that in normal people with a functional immune system this organism is commensal but it may become pathogenic in immunocompromised people such as AIDS patients. This situation is complicated by the fact that such people will generally have a myriad of other parasites, many of them much more pathogenic.
Cryptosporidium are microscopic water-borne parasites. Cryptosporidium parvum infects the small intestine of an unusually wide range of mammals, including humans, and is the species responsible for human cryptosporidiosis. Although numerous other species of the genus have been described over the years the majority of those reported from mammals are synonyms of Cryptosporidium parvum. Recent studies have shown that Cryptosporidium parvum exists in no less than two distinct genotypes. Genetic markers on different chromosomes reveal there is little or no mixing between the genotypes suggesting that two distinct but morphologically identical species may exist. Either genotype may cause a human disease outbreak.
Cryptosporidium produces oocysts that are very resistant to harsh environmental conditions and can survive for a long time. Oocysts can remain viable for about 18 months in cool, damp or wet environments. They are very small and pass through all but sub-micron filters. In the oocyst stage they are resistant to normal levels of drinking water chlorination. They are quite common in rivers and lakes, especially where there has been sewage or fecal contamination. Desiccation for at least two hours is lethal to the oocysts. Snap-freezing destroys oocysts reliably, but with slow freezing, such as that found in the natural environment, oocysts have been reported to survive at temperatures as low as -22 degrees C. They are also heat sensitive, a temperature of 65 degrees Celcius inactivates oocysts in 5-10 minutes. Cryptosporidium oocysts are remarkably resistant to many common disinfectants, including chlorine-based compounds.
The first human cases of cryptosporidial diarrhea were not reported until the mid-1970s. At the time, people thought of cryptosporidial infections as exotic. Between 1976 and 1982, fewer than a dozen cases of cryptosporidial diarrhea were reported in the literature. They occurred mostly in immunocompromised patients, such as in children with leukemia. During the 1980s, cryptosporidial diarrhea emerged as a major problem in people with AIDS. Water-borne and food-borne outbreaks of diarrhea due to Cryptosporidium in recent years have shown that healthy people can contract cryptosporidial diarrhea and that Cryptosporidium is an important parasitic cause of diarrhea worldwide. The largest documented outbreak occurred in the Milwaukee area in 1993. In that outbreak, which investigators traced to the municipal water system, more than 400,000 people contracted cryptosporidial diarrhea.
The characteristic stages of Cryptosporidium are excystment, asexual budding, schizogony/merogony, sexual multiplication, gamogony, zygote and oocyst formation, and sporogony. Mature oocysts contain 4 sporozoites. The oocysts excyst to liberate sporozoites in the gastrointestinal tract. Excystation is possibly triggered by a combination of environmental conditions such as pH, bile salts, carbon dioxide and temperature. The free sporozoites attach to epithelial cells and at this point are sometimes referred to as trophozoites. The trophozoites then undergo asexual reproduction to form merozoites. Sexual reproduction occurs by the subsequent development of male and female gamonts, which develop into gametocytes.
Following fertilization these give rise to zygotes which undergo further asexual development, producing oocysts containing four sporozoites. These oocysts may now be excreted or begin a new cycle within the host. They do not need any further maturation, unlike many coccidian protozoa. Excretion may stop fairly promptly after the cessation of diarrhea or it may continue at low levels for some weeks, even after all symptoms of illness have abated. An incubation period of 2-14 days follows ingestion of oocysts. Very low doses are able to initiate an infection, probably less than 100 oocysts.
The life cycle ofCryptosporidium parvum begins with ingestion of the sporulated oocyst, the resistant stage found in the environment. Each oocyst contains 4 infective stages termed sporozoites, which exit from a suture located along one side of the oocyst. The preferred site of infection is the ileum, and sporozoites penetrate individual epithelial cells in this region. Parasites reside on the lumenal surface of the cells, and they were once thought to occur extra-cellularly. However, observations have clearly shown these parasites to be intracellular, enclosed by a thin layer of host cell cytoplasm. Multiple fission (merogony or schizogony) occurs, resulting in the formation of 8 merozoites within the meront. These meronts are termed Type I meronts and rupture, releasing free merozoites. Once these merozoites penetrate new cells, they undergo merogony to form additional meronts. Type I merozoites are thought to be capable of recycling indefinitely and thus the potential exists for new Type I meronts to arise continuously.
It is thought that some Type I merozoites are somehow triggered into forming a second type of meront, the Type II meront, which contains only 4 merozoites. Once liberated, the Type II merozoites appear to form the sexual stages. Some Type II merozoites enter cells, enlarge, and form macrogametes, macrogametocyte. Others undergo multiple fission once inside cells, forming microgametocytes containing 16 non-flagellated microgametes. Microgametes rupture from the microgametocyte and penetrate macrogametes, thus forming a zygote. A resistant oocyst wall is then formed around the zygote, the only diploid stage in the life cycle, meiosis occurs, and 4 sporozoites are formed in the process. Formation of sporozoites is termed sporogony. These oocysts are passed in the feces to the environment.
Approximately 20% of the oocysts produced in the gut fail to form an oocyst wall with only a series of membranes surrounding the developing sporozoites. These oocysts devoid of a wall are sometimes termed thin-walled oocysts. It is believed that the resulting sporozoites produced from thin-walled oocysts can excyst while still within the gut and infect new cells. Thus, Cryptosporidium parvum appears to have two autoinfective cycles: the first by continuous recycling of Type I meronts and the second through sporozoites rupturing from thin-walled oocysts.
Development of Cryptosporidium occurs more rapidly than many textbooks imply and each generation can develop and mature in as little as 12-14 hours. Due to the rapidity of the life cycle, plus the autoinfective cycles, huge numbers of organisms can colonize the intestinal tract in several days. The ileum soon becomes crowded and secondary sites are often infected, such as the duodenum and large intestine. In immunosuppressed individuals, parasites can sometimes be found in the stomach, biliary and pancreatic ducts and respiratory tract. Diarrhea, weight loss, and abdominal cramping are clinical signs of the disease and in immunosuppressed individuals electrolyte imbalance may occur. The prepatent period, which is the interval between infection and the first appearance of oocysts in the feces, is generally 4 days, 3 days in heavy infections. Patency, which is the length of time oocysts are shed in the feces, generally lasts 6-12 days in immunocompetent individuals but may be prolonged in immunosuppressed patients.
Because Cryptosporidium has been found in many animals, it is widespread in the environment and can be found in lakes and streams. The organism is known to live, both in rural and urban areas, in cattle, sheep, pigs, goats, dogs, cats, deer, raccoon, foxes, coyotes, beavers, muskrats, rabbits and squirrels. There is a list of 129 known host species at: http://www.ksu.edu/parasitology/hosts. Consequently, when these animals live near water bodies, they may serve as carriers of the parasite and may contaminate water. The time of year when Cryptosporidium becomes a problem in surface waters in most areas of North America is generally March to June, when spring rains increase run-off and many neonate animals are present in the environment to amplify oocyst numbers. It should be noted, however, that studies are now showing that many adult animals continue to produce low levels of oocysts on a regular basis, which enhances the environmental load.
Oocysts from human and animal wastes have been found in rivers and streams, lakes and reservoirs, raw and treated sewage and treated surface waters. Although Cryptosporidium can be found in approximately 95 per cent of the sources of North American surface water, it has also been found in some ground water sources. Concentrations of the organism vary greatly. At low concentrations, the organism most likely will not cause disease in healthy individuals.
Cryptosporidium exists in British Columbia waters, normally in low numbers. Only five out of 137 samples taken in 1994 from the GVRD watersheds showed evidence of Cryptosporidiu oocysts. The number of oocysts found per 100 litres of water was very low, less than 3 oocysts, compared to other North American water supplies. The epidemiology of the known recent cases in the Lower Mainland of British Columbia does not indicate waterborne transmission. The separation of drinking water sources from the water bodies that receive sewage is a significant factor in preventing disease outbreaks. There have been water-borne cryptosporidiosis outbreaks in other parts of British Columbia.
In some US filtration plants, such as the one implicated in the Milwaukee outbreak, backwash water is used to clean the sand filters. This process results in a high concentration of oocysts in this wash water. Since this contaminated water may be subsequently recycled to the beginning of the filtration cycle without intermediate oocyst removal, there is greater chance of Cryptosporidium parvum breakthrough when the water comes back to be filtered. Therefore, current practices do not uniformly guarantee the complete removal of these protozoa from water supplies. Filter backwash water should be sent to waste and not re-cycled within the treatment plant.
Enough surveys have been conducted to gain some idea about prevalence of the parasite in the environment. In industrialized nations, somewhere around 0.4% of the population appears to be passing oocysts in the feces at any one time. Of those patients admitted to hospitals for diarrhea, 2-2.5% is passing oocysts. However, the sero-prevalence is much higher and 30-35%, in one study over 50%, of the US population have antibodies to Cryptosporidium parvum. In third world countries, the sero-prevalence is higher still, up to 60-70%, in some studies up to 85%, of people in these countries may have circulating antibodies to this pathogen.
Breakdowns or overloading of public water utilities have occasionally resulted in community outbreaks of cryptosporidiosis. In most cases, various degrees of diarrhea, some weight loss and abdominal cramping were the extent of illness. In some individuals, however, specifically young children, the elderly, and immunosuppressed patients, cryptosporidiosis became chronic and life threatening. It should be noted that it is nearly impossible to determine the origin of many individual cases of cryptosporidiosis. There are endless numbers of anecdotal reports of the parasite being acquired from public water supplies. Many of these may well represent cases of cryptosporidiosis transmitted to humans by pets such as kittens and puppies or by contact with other humans.
Cryptosporidium parvum is predominately a parasite of neonate animals. Although exceptions occur, older animals generally develop poor infections, even when unexposed previously to this parasite. Experimental laboratory infections in immunosuppressed adult animals have shown that infections build up slowly and only occasionally progress to the level found in neonates. It is likely that a developmentally regulated antigen along the intestinal tract is responsible for neonates, rather than adults, developing severe cryptosporidiosis. Humans, on the other hand, are the one host that can be infected at any time in their lives and only previous exposure to the parasite results in either full or partial immunity to new infections.
The nosocomial transmission of Cryptosporidium infection to hospital personnel and to hospitalized patients is a public health concern. Universal body substance precautions are in use in most hospitals. However, it is not known whether these precautions as currently practiced are sufficient to prevent spread of cryptosporidiosis. With the low inoculum required to initiate disease and the hardy nature of oocysts, small numbers of oocysts on the skin of patients or in the environment may be sufficient to transmit disease.
There are many pathways to infection. People can contract cryptosporidiosis from drinking contaminated water or eating raw or undercooked food contaminated with Cryptosporidium. In addition, exposure to the feces of infected individuals or animals or to fecal contaminated surfaces can cause infection. Outbreaks of cryptosporidiosis from person-to-person transmission have occurred in day-care centers. Not everyone exposed to the disease becomes ill, however.
The intestine is a large place for a protozoan and no one really knows how many oocysts it takes to establish an infection in humans. One study suggested that the infectious dose in humans was around 132 oocysts, although one volunteer was infected with as few as 30 oocysts. Another study using a more aggressive isolate suggests that even lower numbers of oocysts, nine, can sometimes initiate infections and cause disease. Humans, like other animals, appear to have various degrees of susceptibility to this parasite and the effective doses will probably be shown to vary between individuals and among isolates.
The numbers of Cryptosporidium oocysts reported by various groups from public water samples are often highly inaccurate. Concentration techniques for oocysts in environmental samples are poor and detection methods often cross-react with algae or other debris. Numerous other species of Cryptosporidium incapable of infecting humans occur in the environment and may cross-react in diagnostic tests. In addition, many oocysts detected are probably not viable due to age, freezing or UV radiation. Exposure to temperatures high enough to denature proteins, as well as freezing to -10 C, always kills the parasites. One recent report has shown quite clearly that the various laboratories performing diagnostic testing on Cryptosporidium oocysts in water have widely varying degrees of accuracy.
Cryptosporidium parvum appears to make little effort to evade the immune system of the host. Many of the surface proteins, glycoproteins and phospholipids are strongly immunogenic and many molecules on the surface of both sporozoites and merozoites are antigenically cross-reactive. The success of the parasite appears to be in its ability to develop rapidly and flood the environment with oocysts. In fact, if this parasite were not efficiently eliminated from the body, it would quickly kill an animal through dehydration and electrolyte imbalance and rapidly eliminate host species from the environment. Indeed, the only effective therapy for the parasite at this time appears to be a healthy, intact immune system.
In immunocompetent hosts, Cryptosporidium infection results in a self-limited diarrheal illness. Oocysts may be excreted for 1 to 2 weeks after clinical improvement. The most common symptoms of cryptosporidiosis are watery diarrhea, abdominal cramps, nausea, vomiting, anorexia, weight loss, wasting and headaches. Other symptoms include gas, weight loss, vomiting, dehydration and fever. These symptoms occur within 2 to 25 days of infection, generally 7 to 12 days, and for people in good health, can last from one to two weeks or as long as a month. Low-grade fever may be present, but high-grade fever does not occur and its presence necessitates a more thorough evaluation for other infectious diseases. Malabsorption, documented by abnormal d-xylose and abnormalities of 72-hour fecal fat, frequently occurs.
Cattle appear to be the primary source of Cryptosporidium, although they have also been found in humans and other animals. Drinking water sources become contaminated when feces containing the parasites are deposited or flushed into water. If water treatment is inadequate, drinking water may contain sufficient numbers of parasites to cause illness. Other sources include direct exposure to feces of infected humans and animals, eating contaminated food and accidental ingestion of contaminated recreational water. The comparative importance of these various routes of exposure is unknown.
Low levels of Cryptosporidium were detected in a national survey of drinking water conducted by Health Canada. Only a small fraction of the parasites appeared to be viable. Nevertheless, outbreaks linked to drinking water have been reported in several provinces. Their spread in swimming pools has also been reported. Swimming pools and recreational water use as well as tap water in the United States have been associated with sporadic outbreaks of cryptosporidiosis. Use of a sub-micron filter will prevent ingestion of cryptosporidia from tap water. At-risk persons should be cautioned concerning the potential for transmission in recreational waters.
Municipal drinking-water treatment providing filtration and disinfection can reduce the risk of cryptosporidiosis. Public water providers, especially those that use surface water, need multiple barriers to reduce the number of oocysts and kill cryptosporidia in water. Since Cryptosporidiu is resistant to traditional disinfection with chlorine the best protective practices include protecting the watersheds from contamination, optimizing filtration and other water treatment processes. Proper water treatment can protect people from Cryptosporidium. More than 97 per cent of oocysts are removed with proper coagulation followed by filtration. Additional protection is offered when a water system also uses ozone or UV as a subsequent primary disinfectant.
Outdoors, water should be boiled for at least one minute before it is used for drinking, food preparation or dental hygiene. This will destroy Cryptosporidium plus any other disease-causing microorganisms that might be present. Certain types of water filters can remove the parasites. Travelers to countries where the safety of drinking water is suspect should boil or filter and disinfect water that is to be used for drinking, food preparation or dental hygiene. Oocysts are very resistant to the usual antiseptic techniques. Their infectivity is destroyed, however, by 5 to 10% ammonium, 10% formaldehyde, freeze drying and exposure to temperatures below freezing and above 65°F for 30 minutes. Some commercial products also inactivate oocysts, such as Oocide, which produces ammonia and Expor, a chlorine-dioxide sterilant.
Prevention depends on avoiding contaminated water and preventing direct contact with persons or animals with cryptosporidiosis. As these sources may not be apparent, prudence suggests that immunocompromised persons drink distilled or boiled water where the risk of Cryptosporidium in the water is significant, for example, in developing countries, and avoid contact with persons withCryptosporidium diarrhea. Guidelines for pet ownership are difficult to formulate at this time, as there is insufficient evidence that this is a significant route of transmission.
Cryptosporidiu will usually disappear from healthy people within a month. Anti-diarrheal drugs and rehydration therapy may be used to treat the symptoms if diarrhea becomes severe. Cryptosporidiu can pose a more serious threat to immunocompromised people such as those with AIDS, cancer or transplant patients receiving immunosuppressive drugs or malnourished children. For these people the symptoms are more severe and can be life threatening. In AIDS patients, the numbers of individuals suffering from chronic cryptosporidiosis has been about 10% in industrialized nations and up to 40% in some third world countries. In people who are immunosuppressed, such as those with HIV infections, Cryptosporidium can establish persistent infection and cause chronic diarrhea that leads to dehydration, wasting and death.
This organism has previously been designated as big Cryptosporidium or thought to be blue-green algae or cyanobacterium or cyanophyte-like bodies Some other names that have been used include coccidia-like and Cyclospora-like bodies, CLBs. Cyclospora organisms were discovered at the turn of the century and Cyclospora cayetanensis was first described at least as early as 1979 but has been reported with increased frequency since the mid- 1980's, in part because of the availability of better techniques for detecting it in feces. Cyclospora cayetanensis was only fully identified as a human parasite in 1993. Cyclospora cayetanensis is a small, 8-10 microns in diameter, protozoan intestinal parasite but two to three times larger than Cryptosporidium. Although the morphology of the oocyst resembles Cryptosporidium parvum, morphology of the intracellular intestinal forms is more similar to Isospora. Phylogenetically, Cyclospora belongs within the Eimeria and may be a mammalian Eimeria species. The organisms of this genus have been isolated from reptiles, moles, rodents and chimpanzees. Because Cyclospora is a newly recognized infectious organism, many questions remain about its biology, the way it is spread and the resulting disease(s).
The life cycle of Cyclospora cayetanensis begins, like all enteric coccidia, with ingestion of a sporulated oocyst, the resistant stage. This sporulated oocyst contains 2 sporocysts, smaller cysts within the oocyst, each enclosing 2 sporozoites, the infective stages. Each oocyst contains a total of 4 sporozoites. Once inside the gut, these sporozoites exit from the sporocysts and oocyst, eventually penetrating epithelial cells along the small intestine. The preferred site is the jejunum. Sporozoites undergo multiple fission inside cells to form meronts, which contain numerous merozoites. There are two asexual generations: the first having 8-12 merozoites and the second having 4 merozoites. The final generation of merozoites penetrates new cells to form gametes, which can also be found in the jejunum. Most gametes simply enlarge to form the female gamete or macrogamete. Some become microgametocytes, which undergo multiple fission to form numerous flagellated sperm-like microgametes. Mature microgametes exit the microgametocyte, fertilize the macrogametes and a resistant oocyst wall is laid down around the zygote.
In time, the unsporulated oocyst is sloughed from the intestinal wall along with the host cell and passes into the external environment with the feces. Further development of sporocysts and sporozoites is termed sporogony or sporulation and occurs only in the presence of higher atmospheric oxygen concentrations than occur in the human gut. Sporulation is complete in 7-12 days at about 30 degrees C.
Cyclospora cayetanensis is found worldwide and most cases reported in Ontario have occurred after travel outside the province. MDS laboratories have been screening all ova and parasite specimens for this organism since its discovery in 1993. Their records show that although the total number of cases each year is low, there seems to be a seasonal variation in its incidence, with an increase in numbers each year in the early summer months. The first human cases of Cyclospora infection were reported in the 1970s. In the early 1980s, Cyclospora was recognized as a pathogen in patients with AIDS. It is known now that Cyclospora is endemic in many parts of the world, and it appears to be an important cause of traveler's diarrhea. It has a worldwide distribution with a higher prevalence in tropical and subtropical countries.
Cyclospora was implicated in a variety of sporadic and epidemic diarrheal illnesses in Nepal, Peru, Papua New Guinea, Solomon Islands, Morocco and other countries of North, Central and South America, Eastern Europe and Asia, including Indonesia, India, Pakistan and Cambodia. Most of these infections were linked to travel or residence in developing countries, others were acquired locally in temperate climates and developed countries. There was a Chicago outbreak and a sporadic case from the United Kingdom.
Overall prevalence varies from region to region. In Nepal it ranged from 10-20% in individuals with diarrhea to 1% in asymptomatic populations. Prevalence in Lima, Peru was 6-18% while a US lab-based study estimated it as approximately 0.2%. The disease displays a seasonal pattern with peaks during hot and humid months. Several other outbreaks have recently been reported from Florida and New York. In terms of overall prevalence, a study from Lima, Peru demonstrated that only 11-28% of children excreting Cyclospora sp. in their stools had diarrhea. This can be considered by some as evidence for the lack of pathogenicity. However, a similar situation was noted for Cryptosporidium in a study from India and asymptomatic carriage of recognized pathogens such as Giardia and Entamoeba histolytica is well recognized.
In addition to humans, Cyclospora cayetanensis, or a morphologically similar species, has been reported from chimpanzees, Pan troglodytes, from Uganda, and baboons and chimpanzees from Tanzania. It may be that this parasite will be found to infect a wide range of primates. Experimental transmission data, plus further sequencing of other genetic loci, may eventually support the hypothesis of multiple species and such studies are currently underway. There are also reports of the parasite in dogs and poultry but it is likely that the former is either Hammondia heydorni or Neospora caninum and the latter either Eimeria mitis or a pseudoparasite.
The fecal-oral route from contaminated feces via water or food is the most common means of spreading Cyclospora. A number of reports suggest swallowing contaminated water or food may spread the parasite. Two recent clusters of cases in Toronto, Ontario and Houston, Texas have both been linked to contaminated strawberries. It is not known how common the various modes of spread are, or whether infected animals can be sources of infection. Cyclospora infections have been identified in otherwise healthy travelers to developing countries, consumers of perishable food products and water contaminated with Cyclospor, infants and children in developing countries, children in child care centers in the United States and AIDS patients. The magnitude of cyclosporiasis as a form of community-acquired diarrhea in the United States is unknown. Epidemics in the United States have been associated with fecal-oral transmission through contaminated water and food. A large outbreak of cyclosporiasis in 1996, totaling 1465 cases in 20 states, the District of Columbia, and 2 provinces of Canada, was associated with contaminated raspberries from Guatemala, revealing the importance of point source contamination of perishable foodstuffs in widespread epidemics of cyclosporiasis. In 1997 another large outbreak, occurred from the same source. It is not clear how the fruit became contaminated, but it might have happened when fruit touched the ground or when contaminated water was sprayed over fruit fields by sprinkling systems.
In 1996 US Federal health officials investigated Cyclospora outbreaks that infected hundreds of people who ate berries in 11 states over two months. Hundreds of laboratory cases of Cyclospora infection were reported to the CDC in May and June in the East, South and Texas. Prior to 1996 only three outbreaks of the infection had been reported. In 1997 the Bureau of Infectious Diseases reported that seven clusters of cases of Cyclospora infection were currently being investigated in the United States, in addition to an outbreak on a cruise ship.
A cluster of cases reported in foreign residents and tourists in Katmandu, Nepal was epidemiologically linked to the drinking of contaminated water or milk diluted with such water. Cyclospora was isolated from tap water and a head of lettuce found in households of patients diagnosed with Cyclospora diarrhea. In the Katmandu cluster only 28% of infected patients reported drinking untreated water or milk in one study and so, additional, yet unidentified modes of transmission may have to be considered.
Cyclospora infects the small intestine and typically causes an illness characterized by watery diarrhea, with an average of 4 to 8 stools per day. The length of time between becoming infected and developing symptoms has not been well established, it probably averages at least several days, but has been reported to be as short as 1 day and as long as 7 days. Mild infections may produce few or no clinical signs, some persons infected with Cyclospora do not develop any symptoms. Symptoms typically wax and wane for several weeks and may persist for several months. If not treated, the illness may last for a few days to a month or longer and may recur one or more times.
Based on the currently available information about how Cyclospora infection is spread, avoiding water or food that may be contaminated with feces is probably the best way to prevent infection. Infected persons should wash their hands often to prevent the spread of infection to other persons. Persons who have previously been infected with Cyclospora can become infected again.
With the exception of some outbreaks, the overall prevalence of Cyclospora in most populations appear to be far less than 1%. Outbreaks seem to occur most frequently in late spring and summer and these warmer temperatures are clearly needed to get oocysts to sporulate with any rapidity. In addition, this time of year correlates with increased import of fruits and vegetables into North America from tropical regions. Individuals become infected when they ingest contaminated food or water containing viable, sporulated oocysts. Because so many of the foods we consume are shipped over long distances and involve contact by many individuals, transportation of pathogens such as Cyclospora between states and countries has become unavoidable. However, the odds of becoming infected with Cyclospora, and many other food-borne pathogens, can be greatly diminished by simply washing fruits and vegetables well prior to consumption. However, simply washing foods does not removed 100% of the oocysts.
Other outbreaks associated with Cyclospora in the literature have been seasonal outbreaks in Nepal, one was associated with chlorinated drinking water. A US outbreak, which occurred in 1990, consisted of 21 cases of illness among physicians and others working at a Chicago hospital. Contaminated tap water from a physicians' dormitory at the hospital was the probable source of the organism. The tap water probably picked up the organism while in a storage tank at the top of the dormitory after the failure of a water pump. The route of transmission is generally via contaminated water. One reported case linked the disease to an accidental ingestion of water from an aquarium.
Little is known about prevention of Cyclospora at this point. Because Cyclospora is much larger than Cryptosporidium, it may be less of a risk in filtered water supplies than is Cryptosporidium. Precautions to avoid Cryptosporidium parvum while travelling in undeveloped areas include sub-micron-filtered water, eating cooked foods, boiling water and avoiding recreational water use.
Cyclospora cayetanensis is an emerging worldwide cause of diarrhea in immunocompetent persons and AIDS patients. The incidence of Cyclospora as a cause of diarrhea in AIDS patients in the United States is unknown. In Haiti, the prevalence of Cyclospora cayetanensis, 11%, in stools of AIDS patients with diarrhea approximates the incidence of Isospora belli, 12%. Persons of all ages are at risk for infection. It is not known how common Cyclospora infection is in North America. Travelers to tropical countries may be at increased risk for infection. The risk may vary with season, some evidence suggests that infection is most common in spring and summer. It is not known yet whether persons with compromised immune systems, such as persons who have AIDS, more commonly develop severe illness if infected with Cyclospora. Cyclospora diarrheal illness in patients with healthy immune systems can be cured with a week of therapy, people who are immunosuppressed may require longer. Duration and outcome of the disease are highly variable in immunocompromised patients, particularly AIDS patients, ranging from two weeks to several months with full recovery. In one reported case, death occurred within two weeks of diagnosis.
Cyclospora may be associated with prolonged diarrhea in immunocompetent as well as immunocompromised populations. Symptoms in the Haitian study of AIDS patients were indistinguishable from those reported in AIDS patients with isosporiasis or cryptosporidiosis. Cyclospora, like Cryptosporidium, Isospora, and microsporidial species, may cause biliary tract disease in AIDS patients.
Despite its name Dientamoeba fragilis is not an amoeba but a flagellate. This protozoan parasite does not produce cysts, only trophozoites. In some patients infection may be either symptomatic or asymptomatic. As for Blastocystis hominis, there is ongoing debate whether Dientamoeba fragilis is a harmless commensal or an opportunistic pathogen. For many years pathogenicity was unclear however, many reports of symptomatic infection have been recorded. A recent Canadian seroprevalence study showed that 91% of healthy children had specific antibodies to Dientamoeba fragilis suggesting that exposure is nearly universal. Infection with Dientamoeba fragilis is frequently associated with Enterobius vermicularis, pinworm infection. It is thought that Dientamoeba fragilis is transmitted between hosts inside helminth eggs or larvae. Like Blastocystis, Dientamoeba fragilis does not invade the host gut and infection can be asymptomatic. In case reports Dientamoeba has been associated with diarrhea, vague abdominal pain, bloating, fatigue, anorexia, vomiting, weight loss and peripheral eosinophilia. The complete life cycle of this worldwide parasite has not yet been determined.
Amoebiasis is the name of the infection caused by Entamoeba histolytica, a parasitic protozoan that infects predominantly humans and other primates. An estimated 40 million people worldwide develop the disease annually and 40,000 die due to dysentery, intestinal diseases and liver abcess. About 5% of the US population is affected. Humans are the definitive host. There is another morphologically indistinguishable species, Entamoeba dispar, which is not usually, if ever, distiguished clinically. Therefore the majority of the of the quoted prevalence, especially in the US, may actually be attributable to this benign species rather than Entamoeba histolytica. Other mammals such as dogs and cats can become infected but usually do not shed cysts with their feces, and thus do not contribute significantly to transmission. Amoebiasis is transmitted by fecal contamination of drinking water and foods and by direct contact with dirty hands or objects as well as by fecal exposure during sexual contact in which case not only cysts, but also trophozoites are infective. Theoretically, the ingestion of one viable cyst is an infectious dose. House flies spread the cysts and the use of human feces as fertilizer on fruit and vegetable crops is an important mechanism of spread.
The infection is not uncommon in the tropics and polar regions, but also occurs in other crowded situations of poor hygiene in temperate urban environments. The most dramatic incident in the USA was the Chicago World's Fair outbreak in 1933 caused by contaminated drinking water. Defective plumbing permitted sewage to contaminate the drinking water. There were 1,000 cases with 58 deaths. Recently, food handlers are suspected of causing many scattered infections, but there has been no single large outbreak. All people are believed to be susceptible to infection, but individuals with a damaged or undeveloped immunity may suffer more severe forms of the disease. In industrialized countries, travelers, recent immigrants and institutionalized populations are the main populations at risk. AIDS patients are very vulnerable and the disease is common among homosexual men.
The active trophozoite stage exists only in the host and in fresh feces; cysts survive outside the host in water and soils and on foods, especially under moist conditions on the latter. When swallowed they cause infections by excysting to the trophozoite stage in the digestive tract. The trophozoites may be invasive in the gut and cause colitis or be asymptomatic in the colon. In the colon they encyst and pass out with the feces to begin the cycle again. Amoebic trophozoites may reach the liver via the portal vein and cause abscesses. Dissemination to lung, brain and skin may also take place.
The life cycle consists of two stages, cysts and trophozoites. Cysts measure 10 to 15 microns in diameter and typically contain four nuclei. They are spread via the ingestion of fecal-contaminated food or water. During excystation within the lumen of the small intestine, cytoplasmic division, giving rise to eight trophozoites follows nuclear division. Trophozoites, which measure 10 to 50 microns in diameter, reside in the lumen of the caecum and large intestine. Approximately 90% of individuals infected with Entamoeba histolytica are asymptomatically colonized; Re-encystation of the trophozoites occurs within the lumen of the colon, resulting in the excretion of cysts in the feces and continuation of the life cycle. Alternatively, the trophozoites can invade the colonic epithelium, causing amoebic colitis in about 10% of infected people.
Giardia lamblia is a single celled protozoan with five flagella. It has also been known as Lamblia intestinalis. Giardia is generally found worldwide in water and causes an intestinal illness called giardiasis or beaver fever. Giardiasis is the most frequent cause of non-bacterial diarrhea in North America, implicated in 25% of the cases of gastrointestinal disease and may be present asymptomatically. The overall incidence of infection in the United States is estimated at 2% of the population. Giardiasis occurs throughout the population, although the prevalence is higher in children than adults. While food and water can transmit this protozoan, children most commonly acquire the organism by the fecal/oral route. As many as 50% of children in daycare centers carry Giardia. Animals are infected with Giardia but their importance as a reservoir is unclear. Chronic symptomatic giardiasis is more common in adults than children.
Giardi produces cysts that are very resistant to harsh environmental conditions. When ingested in contaminated food or water they germinate, reproduce and cause illness. In the small intestine, excystation releases trophozoites that multiply by longitudinal binary fission. The trophozoites remain in the lumen of the proximal small bowel where they can be free or attached to the mucosa by a ventral-sucking disk. Encystation occurs when the parasites travel toward the colon and cysts are the stage found in normal, non-diarrheal, feces. The cysts are hardy, can survive several months in cold water and are responsible for transmission. Because the cysts are infectious when passed in the stool, or very shortly afterward, person-to-person transmission is possible. Cysts are the environmental survival form and infective stage of the organism. Studies with human volunteers have shown that ingestion of as few as one cyst may cause illness in contrast to most bacterial illnesses where hundreds or thousands of organisms may be necessary for an infective dose. Giardia can be excysted, cultured and encysted in vitro, new isolates have bacterial, fungal and viral symbionts.
Diarrhea, abdominal cramps, gas, malaise and weight loss are the most common symptoms caused by Giardia. Vomiting, chills, headache and fever may also occur. These symptoms usually happen within 6 to 16 days of ingestion of the cyst and can continue as long as a month. Normally illness lasts for 1 to 2 weeks but there are cases of chronic infections lasting months to years. Different individuals show various degrees of symptoms when infected with the same strain and the symptoms of an individual may vary during the course of the disease.
The disease mechanism is unknown, some investigators report that the organism produces a toxin while others are unable to confirm its existence. The organism has been demonstrated inside host cells in the duodenum, but most investigators think this is such an infrequent occurrence that it is not responsible for disease symptoms. Mechanical obstruction of the absorptive surface of the intestine has been proposed as a possible pathogenic mechanism, as has a synergistic relationship with some of the intestinal flora.
Organisms that appear identical to those that cause human illness have been isolated from domestic animals, dogs and cats, and wild animals, such as beavers and bears. A related but morphologically distinct organism infects rodents, although rodents may also be infected with human isolates in the laboratory. Giardia is often found in feces from humans, beaver, muskrat and dogs. Drinking water sources become contaminated when feces containing the parasites are deposited or flushed into water. Giardiasis is most frequently associated with the consumption of contaminated water. If water treatment is inadequate, drinking water may contain sufficient numbers of parasites to cause illness.
Other sources include direct exposure to the feces of infected humans and animals, eating contaminated food and accidental ingestion of contaminated recreational water. Outbreaks have been traced to food contamination from infected food handlers and the possibility of infections from contaminated vegetables that are eaten raw cannot be excluded. The largest reported food-borne outbreak involved 24 of 36 persons who consumed macaroni salad at a picnic. Cool moist conditions favor the survival of the organism. The comparative importance of these various routes of exposure is unknown.
Low levels of Giardia were detected in a national survey of drinking water conducted by Health Canada. Only a small fraction of the parasites appeared to be viable. Nevertheless, outbreaks linked to drinking water have been reported in several provinces. Giardia cysts are extremely resistant to chlorine and may be found in treated municipal drinking water. Their spread in swimming pools has also been reported.
Municipal drinking water treatment providing filtration and disinfection can reduce the risk of giardiasis. Protection of the raw water supply is also beneficial. In the outdoors, water should be boiled for at least one minute before it is used for drinking, food preparation or dental hygiene. This will destroy Giardia plus most other disease-causing microorganisms that might be present. Certain types of water filters can remove the parasites. Major outbreaks are associated with contaminated water systems that do not use sub-micron filtration or which have a defect in the filtration system. Travelers to countries where the safety of drinking water is suspect should boil or filter and disinfect water that is to be used for drinking, food preparation or dental hygiene.
Giardi is usually cleared without treatment from healthy people within a month. Chronic cases, both those with defined immune deficiencies and those without, are difficult to treat. Giardia can pose a serious threat to immunocompromised people such as those with AIDS or cancer or to transplant patients receiving immunosuppressive drugs. For these people the symptoms are more severe and can be life threatening. Giardiasis is more prevalent in children than in adults, possibly because many individuals seem to have a lasting immunity after infection. Infection is common in child daycare centers, especially those in which diapering is done. Giardiasis afflicts many homosexual men, both HIV-positive and HIV-negative individuals. This is presumed to be due to sexual transmission.
The amoeba Hartmannella veriformis is a free-living, freshwater amoeba that has been isolated, cloned, cultured and used as a host for Legionella pneumophila in order to understand how Legionella attaches and enters into amoeba and multiplies. The Hartmannella vermiformi was isolated from a water sample taken during an investigation of legionellosis. This sample was cloned and grown. Individual isolates of the widely distributed amoeba species Hartmannella vermiformis may differ in the details of cyst structure. This is unusual for gymnamoebae and may indicate different species or races. The Legionella pneumophila was isolated from a cooling tower implicated as a source of an outbreak.
Protozoa in water sources have been shown to increase Legionella proliferation. The bacteria multiply intracellularly and therefore require protozoa such as amoeba to aid in reproduction. One amoeba can hold enough Legionella bacteria to cause infection in a human being and can be contained in one droplet of water. Evidence seems to point towards the use of aerosol devices such as hot tubs and showers as the mode of transmission. No reports of human-to-human transmission have been proven. Even though the amoeba may not be pathogenic it may harbour pathogenic bacteria.
Hartmannella vermiformis was isolated from the cerebrospinal fluid of a young male patient from Mexico with meningoencephalitis and bronchopneumonia. The patient began with symptoms and signs of the common flu and 12 days after the onset of his disease he was admitted to the hospital with symptoms and signs of meningoencephalitis. The disease ended in coma and death. Wide-spectrum antibiotics, immunosuppressive and anti-tuberculosis therapy were unsuccessful. His prior activities included repeated contact with polluted water. Active amoebas were isolated from samples of the spinal fluid. All amoebas were Hartmannella vermiformis. Amoebas were apparently not the cause of the disease and were likely opportunistic colonizers that may have caused the disease to become worse.
Isospora is a coccidian parasite related to Cryptosporidium and Cyclospora. Human isosporiasis is caused by Isospora belli. Although there are many species of Isospora onlyIsospora belli is known to infect humans. During World War I, the feces of many soldiers were examined for various reasons and this is when the oocysts of Isospora belli were first seen. However, the parasite had not been named at that time. Isospora belli may also infect the gibbon. In cross transmission studies 2 of 3 gibbons were successfully infected with 1-2,000 oocysts of Isospora belli of human origin. There is a report of finding coccidia in the feces of a gibbon in a zoo in Berlin but no other information was given. Perhaps this was also Isospora belli, but there is no way to know for certain. Other animals such as dog, cat, pig, domestic rabbit, guinea pig, mouse, rat and rhesus monkey are resistant to infection. Other species of Isospora infect a wide range of carnivores, passerine birds, amphibians, reptiles, rodents, swine and primates.
Isospora belli is a single celled protozoan parasite that has a life cycle consisting of rapidly multiplying asexual and sexual stages growing within host cells. Following ingestion of mature oocysts, motile sporozoites are released which infect the mucosal cells of the small bowel. The asexual developmental stages produce motile merozoites that spread the infection within the gut tissues. They complete their life cycle within a single host. The oocysts are excreted, mainly undeveloped, but sometimes partially or fully developed. The sexual cycle also occurs in the human host and culminates when the unsporulated oocyst sheds into the gut lumen and is released to the environment. The immature oocysts contain a single spherical sporoblast. This subsequently divides into two sporocysts that mature to produce four motile banana-shaped sporozoites in each. Isospora, like other coccidia, is an obligate parasite, and cannot be cultured in vitro.
The infection is found primarily in warmer regions of the world such as Africa and Central and South America, but is occasionally reported in temperate regions in people returning from abroad and in AIDS patients. Endemic areas for Isospora belli include some areas of Indochina, South America and the islands of the southwestern Pacific. In developing nations where the organism is much more prevalent in the general population, it is also seen more frequently in patients with advanced HIV disease.
Infection is acquired by the ingestion of oocysts in food or water contaminated with the feces of infected humans. Isospora, like the other coccidia, are unable to grow or multiply in the environment although the oocysts allow the parasite to be transmitted through the environment. Infection follows ingestion of mature oocysts by the fecal/oral route. It is also likely to be transmitted directly through poor hygiene and by contaminated food and water. Significant secondary spread is thought unlikely, except perhaps in homosexual men. Reports describe chronic Isospora diarrhea in immunocompetent travelers but the importance of the organism as a cause of chronic diarrhea is not known. Nonetheless, Isospora should be considered a hazard to the immunocompromised traveler. In those with normal immunity, the symptoms are usually mild and self-limiting although in some patients symptoms may be recurrent or even chronic. Clinical features of isosporiasis are nearly indistinguishable from those of Cryptosporidium infection.
Isospora belli was rarely reported as a cause of human disease prior to the HIV epidemic and those cases were generally from South America or Africa. It has been reported in less than 0.2% of patients with advanced HIV disease in the United States and in 15% of patients with advanced HIV disease in Haiti. Reports describe Isospora occurring in 10%, 12% and 16%, respectively, of patients with HIV disease and diarrhea in Brazil, Zaire and Zambia. Rates of infection in the United States may be high in groups immigrating from areas of greater endemicity. In Los Angeles, Hispanic patients, who accounted for 17% of patients with advanced HIV disease, represented 81% of HIV-related isosporiasis. Eight percent of patients with advanced HIV disease in Los Angeles County who came from Latin America had isosporiasis. The high rate of isosporiasis in HIV infected Latin Americans raised the rate of isosporiasis to 1% of all AIDS patients in Los Angeles. Isosporiasis was more likely to occur in foreign-born patients than in those born in the United States.
Isospora is believed to be relatively uncommon in humans, up to 1%, however, when it is actively looked for many cases are found in humans in Natal. Infections are often missed because those examining human stools do not look for it. There are about 800 reports from the Western Hemisphere, the great majority, 332, from Chile. In other selected reports, Isospora belli infections were found in 5 and 32 persons in two schools for mental defectives in South Carolina, but in none of 1134 persons in a mental hospital in Georgia. Seven percent of 165 children were infected in an orphanage in São Paulo, Brazil, but it was found it in only three of 46,404 fecal examinations in Venezuela. Isospora was found in the feces of 2% of 50 persons from Istanbul, Turkey, but from none of 299 persons from Papance, Ankara or Bolu, Turkey.
Person-to-person transmission via direct oral/anal contact has been suggested. Close contact in barracks or institutions, the presence of other intestinal disease, and poor hygiene appear to favour transmission. However, several lines of evidence suggest that sexual contact and direct fecal/oral contact are not the usual routes of transmission. Firstly, Isospora bell oocysts, unlike Cryptosporidiu oocysts, are usually released unsporulated and usually require 48 hours in the environment for the oocyst to sporulate and become infectious. Secondly, heterosexual partners of Haitian bisexual men with advanced HIV disease and isosporiasis did not show evidence of isosporiasis. It appears likely that fecal contamination of food, water or the environment is the usual source of transmission but definitive epidemiological studies have not been undertaken.
No specific information is available on the effectiveness of water treatment processes for removal of Isospora. However, since Isospora oocysts are considerably larger than are those of Cryptosporidium, for example, they should be efficiently removed by coagulation/sedimentation and especially by sub-micron filtration. No direct information is available on susceptibility to disinfectants, but the oocysts are likely to be more resistant than indicator bacteria such as coliforms.
Although no reports describe nosocomial transmission, knowledge of the transmission mechanisms suggests that it should occur. In addition, there are no reports of animal transmission to humans.
Isospora belli infection is associated with AIDS patients, particularly in Africa, Central and South America and the US. In these patients the infection may be more severe and likely to be recurrent if not treated. In immunocompromised individuals and patients with advanced HIV disease, the diarrhea is chronic and highly fluid and can lead to malabsorption, malnutrition, cachexia and dehydration that require hospitalization. It can sometimes be fatal. However, Isospora belli rarely causes diarrhea in AIDS patients in the United States, 20/14,519, <0.2%, but has a significantly higher prevalence in AIDS patients from other countries such as Haiti, 20/131, 15%.
The term microsporidia is commonly used for the single-celled, spore-forming, obligate, intracellular, parasites belonging to the phylum Microsporidia. They are prokaryotes and lack mitochondria. They are all dealt with here as a group under their accepted scientific name by genus. More than 1,200 species belonging to 143 genera have been described. Microsporidia are ubiquitous parasites of arthropods and all classes of vertebrates. Infections of silkworms, honey bees and salmonids account for significant economic losses while sub-clinical infections in laboratory rabbits, rats, mice and guinea pigs can interfere with scientific experiments. Microsporidia also infect fish, pigs, insects, carnivores, ruminants, birds of the parrot family, rhesus monkeys and man. Thelohania can become parasites of freshwater crayfish in Australian yabby farms and is being used to try and control red and black fire ants from South America that are serious pests in the southern US. A recent study has developed methods for the detection and species determination of microsporidia in the environment. It also confirmed the presence of waterborne human pathogenic microsporidia.
A much smaller number, under 20 are known at present, of microsporidian species have been identified as human pathogens: Enterocytozoon bieneusi, Encephalitozoon intestinalis, Encephalitozoon hellem, Encephalitozoon cuniculi, Pleistophora, Trachipleistophora hominis, Trachipleistophora anthropophthera, Nosema ocularum, Nosema algerae, Nosema corneum, Vittaforma corneae, Microsporidium ceylonensis, Microsporidium africanum, Brachiola vesicularum and Brachiola connori. Microsporidian synonym pairs include Encephalitozoon intestinalis=Septata intestinalis, Brachiola connori=Nosema connori and Vittaforma cornea=Nosema corneum. The microsporidia of the species associated with human infection measure from 1 to 5 microns, which is a useful diagnostic feature.
Microsporidia are being increasingly recognized as opportunistic human infectious agents worldwide, occurring mainly, but not exclusively, in severely immunocompromised patients with AIDS. Cases of microsporidiosis in immunocompromised persons not infected with HIV as well as in immunocompetent individuals are rare but have also been reported. Microsporidiosis has been reported in developed as well as in developing countries, including: Argentina, Australia, Botswana, Brazil, Canada, Czech Republic, France, Germany, India, Italy, Japan, Netherlands, New Zealand, Spain, Sri Lanka, Sweden, Switzerland, Thailand, Uganda, United Kingdom, United States of America and Zambia.
Only 2 genera, Enterocytozoon and Encephalitozoon, are common in man. The remaining genera are rarely reported. Most reports on microsporidia-infected mammalian hosts concern man and only a few other hosts, laboratory rabbits and wild rodents. It is possible that the low prevalence of microsporidia in mammals is an artifact, as some factors such as large body size, effective immune system or low pathogenicity of microsporidia, are making the detection of microsporidia in a mammalian body more difficult as compared with the situation in invertebrates. It is likely that microsporidian infections in mammals are more frequent than the record shows and the paucity is simply our present inability to find them or a lack of effort in looking for them.
The species of mammalian microsporidia fall into two groups, the distinction being the ability to withstand the mammalian body temperature of 37 degrees C. The deep organ parasites, Enterocytozoon bieneusi, Encephalitozoon hellem, Encephalitozoon cuniculi, Vittaforma corneae, Trachipleistophora hominis, Brachiola and Thelohania apodemi probably represent the mammalian microsporidia. The species causing eye infections only, Nosema algerae, Nosema ocularum, Microsporidium ceylonensis, Microsporidium africanum and Microsporidium buyukmihcii represent infections by non-mammalian microsporidia able to grow at temperatures below 37 degrees Celcius in the immunoprivileged site of the mammalian cornea.
|Enterocytozoon bieneusi||acalculous cholecystitis, restricted to the enterocytes of the small intestine, resulting in villous atrophy and malabsorption, chronic watery, non-bloody diarrhea, malaise and weight loss|
|Encephalitozoon intestinalis||Infection of the enterocytes of the gastrointestinal tract causing diarrhea, and dissemination to eye, genitourinary and respiratory tracts, colon, liver and kidney|
|Keratoconjunctivitis, infection of respiratory and genitourinary tract, disseminated infection, sinusitis, nephritis, hepatitis and peritonitis|
|Eye and cornea infections, keratoconjunctivitis in AIDS|
|Thelohania apodemi||Internal organ infections|
Most microsporidia have been described on the basis of only one of their spores, usually an environmental spore. However, it has recently been discovered that a Nosema and a Thelohania from a mosquito host were two distinct entities in one species complex life cycle. A single microsporidium can thus have more than one type of spore. The Nosema was responsible for vertical transmission and the Thelohania spores did not infect mosquito larvae. In 1985 it was discovered that this Thelohania spore infected a copepod intermediate host from which came yet a third type of spore that completed the cycle by infecting the mosquito larva. Since then, copepod intermediate hosts have been identified for several more microsporidia of mosquitoes. Thus, some microsporidia have been shown to have a life cycle that included two generations of a mosquito host and a copepod intermediate host, involving three different developmental pathways to three different spores. It is important to note that these spores were once considered three distinct species in three different genera. Thus, a number of distinct microsporidian species as presently named will likely be reduced to synonyms as these complex life cycles are sorted out.
Although the only microsporidia for which intermediate hosts have been identified are from mosquitoes, Amblyospora and Parathelohania, this situation might be common. Nosema necatrix and Thelohania diazom were long considered to be dual infections in caterpillars. They were finally proven to be two spore types of one species, Vairimorpha necatrix, developing in their own way and time, in the same host. The Nosema type spore seems to be the one usually responsible for transmission to the original host, leaving some doubt as to the role of the meiospores. So few complete life cycles have been thoroughly described that few generalities can be made. But, from what we already know, it seems as though many microsporidia are capable of producing more than one kind of spore and that each of these spores plays a specific role in the life cycle. Amblyospora californica is an example of a complex developmental cycle in which two generations of mosquitoes and a copepod, that act as an intermediate host, are needed to compete it. A different type of spore is formed in each host. The relatively more simple life cycle of Nosema bombycis in which two types of spores are formed in the same host is more common. In the genus Vairimorpha, an internal spore and two external spores, one a meiospore and the other a diplokaryotic external spore, are all produced in the same host. There is much diversity in microsporidian developmental cycles and our present understanding of the taxonomy may only be interim. Identification of microsporidian parasites may well be provisional for some time yet.
Only one species of Naegleria, Naegleria fowleri, is known to infect humans. The trophozoites are worldwide free-living inhabitants of soil and warm fresh water; they are also found in sewage and sludge. No human or animal reservoir is known but birds and aquatic mammals may serve this function. Naegleria fowleri is thermophilic, preferring warm water and reproducing successfully at temperatures up to 46 degrees Celcius. They are found in hot springs, power plant and cooling tower thermal discharges, swimming pools, aquaria, hydrotherapy tanks and other sources of hot water. In temperate climates, the amoebae over-winter as cysts in bottom sediments of lakes, swimming pools and rivers. Cysts remain viable for 8 months at 4 degrees Celsius but do not survive if dried, frozen or heated over 50 degrees Celsius. Infectious cysts may also be carried in dust. They reproduce by binary fission. Naegleria is an opportunistic pathogen that enters the body via nasal passages from water. Most patients with primary amoebic meningoencephalitis have been in a swimming pool, freshwater lake, or pond a few days before the onset of symptoms. Chlorinating of water does not entirely eliminate pathogenic strains. Naegleria fowleri has also been isolated from air conditioning units. Amoebas splashed or inhaled onto the olfactory epithelium migrate up the olfactory nerve to the brain and spread via the sub-arachnoid space. They invade the central nervous system causing fatal primary meningoencephalitis, PAM.
There are 3 stages or forms of Naegleria, amoebic, the parasitic stage in animals, cyst, found in colder than ideal climates for this parasite and flagellated, two flagella at one end to aid in movement. In the environment, Naegleria is a free-living organism and is commonly found in the cyst or flagellated form; the amoebic stage only occurs inside animals during an infection. Naegleria fowleri trophozoite cells, cultured from cerebrospinal fluid of a patient who died from primary amebic meningoencephalitis, have characteristically large nuclei with a large, dark staining, karyosome. PAM is rare; fewer than 100 cases have been reported in the United States in the 25 years since this disease was recognized. More than 175 cases of this disease have been recorded worldwide. The disease usually affects children and young adults and is generally known only from isolated cases. While it does not normally occur in clusters or outbreaks, clusters of cases associated with a given source do occur. A series of infections in children occurred in Czechoslovakia between August 1962 and September 1965 when a swimming pool received heated river water treated only with chlorine. Several cases of PAM occurred in a local area in the US and were all traced to one of three small lakes in the area. Naegleria fowleri has been isolated from the drinking water in Australia. It is clear that the number of infections represents only a small fraction of the number of exposures. It is not clear why primary amebic meningoencephalitis is not found predominantly in the tropics, where the amoeba flourishes. While infrequent, infections appear to occur worldwide.
Naegleria is usually found in warm to hot water, 28 to 40 degrees Celcius. Naegleria is also found in water that is stagnant, shallow and contains algae growth. Small farm ponds are good examples. If weather conditions are very hot and occur for a long period of time, then the temperature in small bodies of water can get warm enough to support abundant Naegleria. If the water feels warm to the touch and seems to be very still, one should consider the possibility of Naegleria being present and limit contact with the water especially making sure to avoid getting water up the nose. Flowing streams and rivers and large lakes, where the water is cool, have not been associated with this disease. A child's wading pool can also be a place for bacteria and protozoans to grow. One should make sure to empty the pools daily, scrub them, and let them dry in the sun. Fresh, cool water in the wading pool everyday will decrease risk to a child's health.
The human or animal host gets infected with the parasite while swimming or diving in contaminated water or by inhalation of aerosols. The amoebae, which have a flagellate stage, enter the host via the mouth or nose. They invade the mucosal lining of the nasal passages, the olfactory neuoepithelium, and migrate up the olfactory nerves, or through the cribriform plate into the brain. The parasite now transforms into the amoebic form that can multiply in the tissues of the central nervous system and may be isolated from spinal fluid. This form migrates around the brain quickly causing severe brain damage, PAM. The resultant pathology causes primary amoebic encephalitis. If untreated it rapidly leads to death, within 1 week in most cases. PAM occurs in persons who are generally healthy prior to infection. Not all opportunistic infections are pathogenic but Naegleri in particular have been linked to several hundred cases of fatal infections in humans, usually as a result of brain lesions.
Recently, there has been a case of amoebic meningoencephalitis caused by Naegleria fowleri in Texas that was contracted by swimming in a small pond. Since the only known way that allows one to get amoebic meningitis is if water containing the protozoa goes up the nose one should prevent getting one's head under water. Transmission is through water-based fluids or aerosols, this is not a food borne disease. Naegleria fowleri is found in nature in warm water bodies as amoeboid and amoeboflagellate trophozoites. Cysts also occur in nature, but not in human infections. Warm moist water or air as found in air conditioning units, humidifiers, dialysis units and industrial process water are also sources of these parasites. These are natural ubiquitous organisms that are not the result of any specific preventable contamination event. They normally occur in temperate to tropic areas in hot springs or thermal pools and the trophozoite prefers at least 35 degrees Celsius; under cooler conditions they will encyst.
Drinking and bathing water should meet or exceed normal standards, as should swimming pool water. There is little if anything that can be done about natural lakes and rivers used for swimming and other facial immersion recreation apart from eliminating contamination by infected human and animal sources. Immunodeficient hosts, especially those infected with HIV, may be at risk for atypical infections. Survival is rare.
Cats are the only definitive hosts for Toxoplasma gondii which will infect virtually all warm-blooded hosts and is a very widespread infection of humans. Infection occurs by ingestion of contaminated food and water or by poor hygiene. Garden soil will become contaminated and is a reservoir for oocycsts to get under finger nails or or crops. Cats which hunt birds and rodents are more at risk of carrying this pathogen. During pregnancy Toxoplasma gondii can multiply in the placenta and be passed to the fetus, effects are more severe during the first trimester.
Oocysts are 10 to 12 microns and highly resistant in the environment to freezing and drying. They resist ammonia, chlorine, formalin and other disinfectants. They are killed by heat over 70 degrees Celsius and by gamma radiation. The disease may include mental retardation, loss of vision and hearing or death in congenitally infected infants. Adults are mostly asymptomatic except those with AIDS or other immunocompromised conditions where it is a serious infection causing encephalitis. It is a major cause of abortion in sheep and goats and is very pathogenicto marsupials and monkeys.
Toxoplasma gondii is an obligate intracellular parasite. Cats excrete non-infective oocysts which require at least a day for the internal development of infective sporozoites. These oocysts can survive for over a year. When a homeotherm ingests the infective oocyst it ruptures in the intestine and releases 8 sporozoites which multiply in the intestinal walls and lymph nodes and form tachyzoites. The 2 x 6 micron tachyzoites disperse throughout the body via blood and lymph eventuallu encysting in the brain, liver and skeletal and cardiac muscles. These encysted forms are called bradyzoites, 7 x 2 microns, and survive for the lifespan of the host. When the host tissue is eaten enzymes digest the cyst walls releasing the bradyzoites which transforn to tachyzoites and repeat the cycle. The life cycle is only complete when a cat eats the bradyzoites cysts and then oocysts are formed and excreted.Return To The Top of the Page
Up to 40 percent of US adults are infected with Toxoplasma gondii and up to three million women have acquired sexually transmitted Trichomonas vaginalis. Many parasitic diseases such as giardiasis and cryptosporidiosis are not always reported to health authorities, so that the extent and impact of parasitic diseases in the United States is underestimated. Accurately defining US populations affected by these diseases would greatly help in the development of strategies to prevent, control and treat parasitic infections not only in the United States, but in developing countries as well.
The Center for Disease Control and Prevention, National Centers for Infectious Diseases, keeps track of the parasites that lead to diarrhea and other diseases in the United States. The most recent parasitic infection to attract public attention was the Milwaukee, Wisconsin epidemic of diarrhea and related gastrointestinal disorders that involved more than 400,000 people, stemming from water contaminated with the Cryptosporidium parvum parasite. More frequent, but less in the public eye, are Giardia lambl infections. This parasite is frequently associated with poor hygiene practices or contaminated water. The CDC estimates between 100,000 and one million US cases of Giardia occur annually. Both Giardia and Cryptosporidium infections are common problems in daycare centers. Hikers and campers drinking from contaminated streams also frequently become infected with Giardia.
Not all parasitic infections routinely cause debilitating illnesses. For example, healthy people who become infected with Toxoplasma gondii usually do not develop toxoplasmosis, which damages the brain and nervous system and sometimes kills. The disease frequently develops in those with weakened immune systems and affects 3 to 10 percent of AIDS patients. The disease also can occur among newborns if their mothers became infected during pregnancy. Toxoplasma gondii naturally infects cats and can spread to humans who handle dirt and litter boxes where cats have defecated, drink contaminated water or eat raw or undercooked meat.
People with AIDS or other immunocompromising conditions are particularly susceptible to Cryptosporidium infections. Cryptosporidiosis affects 3 to 15 percent of AIDS patients, who develop incurable diarrhea that can lead to death. In addition, the parasite Microsporidium causes severe diarrhea in 20 to 25 percent of people with AIDS suffering from chronic diarrhea. Recently, scientists have identified another parasite, Cyclospor that can bring on diarrhea in AIDS patients and healthy individuals.
|Geographic Region||Occurrence [Deaths]|
|United States||2983 (1994 data)|
|World Total||10,004,761 |
|amoebic encephalitis-Balamuthia mandrillaris|
|United States||rare [generally fatal]|
|World Total||rare [generally fatal]|
|United States Parasitic Disease Frequencies|
|Balamuthia mandrillaris||amoebic encephalitis||* rare, few/yr., [usually]|
|Cryptosporidium parvum||cryptosporidiosis||* 33/yr|
|Dirofilaria immitis||pulmonary dirofilariasis||118 (since1961)|
|Entamoeba histolytica||amoebiasis||* 2983 (in 1994)|
|Giardia lamblia||giardiasis||* 141/yr|
|Strongyloides stercoralis||strongyloidiasis||up to 1,000,000/yr|
|Trichinella spiralis||trichinellosis||sporadic, about 40/yr|
|Canada Parasitic Disease Frequencies|
|Diphyllobothrium||diphyllobothriasis||a few isolated cases/yr|
|Entamoeba histolytica||amoebiasis||* 1778/yr|
|Giardia lamblia||giardiasis||* 7042/yr|
|Trichinella spiralis||trichinellosis||sporadic, about 5 to 10/yr|
|Central America Parasitic Disease Frequencies|
|Entamoeba histolytica||amoebiasis||* 5,000,000/yr|
|Strongyloides stercoralis||strongyloidiasis||about 3,300,000/yr|
|Trypanosoma cruzi||chagas, trypanosomiasis||8,000,000/yr|
|Caribbean Parasitic Disease Frequencies|
|Trichuris trichiura||trichuriasis, whipworm disease||1,000,000/yr|
|South America Parasitic Disease Frequencies|
|Leishmania chagasi||visceral leishmaniasis||3000/yr|
|Leishmania braziliensis||cutaneous leishmaniasis||19000/yr|
|Onchocerca volvulus||river blindness||500,000/yr|
|Strongyloides stercoralis||strongyloidiasis||about 7,400,000/yr|
|Trypanosoma cruzi||chagas, trypanosomiasis||16 to 18,000,000/yr |
|Australia Parasitic Disease Frequencies|
|New Zealand Parasitic Disease Frequencies|
|China Parasitic Disease Frequencies|
|Ascaris lumbricoides||ascariasis||about 100,000,000/yr|
|Opisthorchis chinensis||Chinese liver fluke||5,000,000/yr|
|Schistosoma japonicum||schistosomiasis||greater than 870,000/yr|
|Africa Parasitic Disease Frequencies|
|hookworm, ancylostomiasis||greater than 1,800,000/yr|
|Ascaris lumbricoides||ascariasis||greater than 200,000,000/yr|
|Entamoeba histolytica||amoebiasis||* 5,000,000/yr, |
|Leishmania||leishmaniasis||2 to 3,000,000/yr|
|river blindness, filariasis||18,000,000/yr|
|Strongyloides stercoralis||strongyloidiasis||about 16,300,000/yr|
|Trypanosoma||trypanosomiasis||more than 300,000/yr|
|Japan Parasitic Disease Frequencies|
|Clonorchis sinensis||clonorchiasis||about 20,000,000/yr|
|Paragonimus||paragonimiasis||greater than 1,000,000/yr|
|Schistosoma japonicum||schistosomiasis||greater than 1,000,000/yr|
|Southeast Asia Parasitic Disease Frequencies|
|Opisthorchis chinensis||Chinese liver fluke||19,000,000/yr|
|Strongyloides stercoralis||strongyloidiasis||greater than 5,300,000/yr|
|Eurasia Parasitic Disease Frequencies|
|Giardia lamblia||giardiasis||* 10,000,000/yr|
|Middle-East Parasitic Disease Frequencies|
|Ancylostoma duodenale||hookworm, ancylostomiasis||greater than 60,000,000/yr |
|Europe Parasitic Disease Frequencies|
|Strongyloides stercoralis||strongyloidiasis||up to 2,000,000/yr|
|Taenia solium||cystercercosis||sporadic outbreaks|
|Trichinella spiralis||trichinellosis||sporadic outbreaks|
|India, Pakistan and Sri Lanka Parasitic Disease Frequencies|
|Ancylostoma duodenale||hookworm, ancylostomiasis||greater than 300,000,000/yr|
|Dracunculus medinensis||dracunculiasis||39,792 (in 1994) 60 (in 1995)|
|Strongyloides stercoralis||strongyloidiasis||about 7,000,000/yr|
|World Totals Parasitic Disease Frequencies|
|Intestinal roundworms, ascariasis||1,400,000,000|
|Amoebiasis (protozoan)||* 40,000,000|
|Food borne trematode infections||40,000,000|
Disease severity ranges from rapidly fatal encephalitis to low grade chronic infections of the gut that may be non-symptomatic for many years. Most commonly there are respiratory problems or diarrhea which are not life-threatening. Many pathogens can set up autoinfection cycles within a host whereby they are able to maintain the infection of the host from within without having to shed eggs to the environment and re-infect again. The tables below indicate the main areas of the body which are affected by protozoan parasites. These are the areas affected both during the migration stage and at the final destination for growth and reproduction.
|Location in Body||Organisms Responsible|
|Central nervous system, Brain||Trypanosoma, Naegleria fowleri, Toxoplasma gondii, Plasmodium, Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Balamuthia mandrillaris, Hartmannella veriformis|
|Eye||Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Encephalitozoon intestinalis, Encephalitozoon hellem, Encephalitozoon cuniculi, Brachiola vesicularum, Brachiola connori, Vittaforma cornea, Nosema algerae, Nosema ocularum, Microsporidium ceylonensis, Microsporidium africanum, Microsporidium buyukmihcii|
|Genito-urinary system||Giardia lamblia, Entamoeba histolytica, Cryptosporidium parvum, Isospora belli, Balantidium coli, Blastocystis hominis, Cyclospora cayetanensis, Dientamoeba fragilis, Enterocytozoon bieneusi, Encephalitozoon intestinalis, Trichomonas vaginalis, Entamoeba histolytica|
|Kidney||Encephalitozoon intestinalis, Encephalitozoon hellem, Encephalitozoon cuniculi|
|Liver||Leishmania, Entamoeba histolytica, Encephalitozoon intestinalis|
|Lymphatic system||Wuchereria bancrofti|
|Mouth||amoebae, flagellates (usually non-pathogenic)|
|Respiratory tract, Lungs||Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Balamuthia mandrillaris, Hartmannella veriformis, Encephalitozoon intestinalis, Encephalitozoon hellem, Encephalitozoon cuniculi|
|Skin||Leishmania, Acanthamoeba culbertsoni, Acanthamoeba polyphaga, Acanthamoeba castellanii, Acanthamoeba rhysodes, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Balamuthia mandrillaris|
|Widespread||Encephalitozoon hellem, Encephalitozoon cuniculi, Thelohania apodemi|
Sources of Infections
While water borne transmission is important for many of these pathogens it is not the only, or in some cases the most important, means of spread. The anal/oral shortcut is faster and more efficient than discharge to the environment and infection or re-infection from contaminated water. This maintains a reservoir of pathogens from which the water is continuously re-contaminated and in some cases, where no alternate host is required in the life cycle, or currently available, may maintain a pathogen population in isolation from the water.
Understanding the means of acquisition, sources, and reservoirs of nosocomial pathogens is crucial for developing methods to reduce the incidence of nosocomial infections. Important water reservoirs in the hospital include potable water, sinks, faucet aerators, showers, tub immersion, toilets, dialysis water, ice and ice machines, water baths, flower vases, eyewash stations and dental-unit water stations. While virtually all nosocomial records, except for the protozoan Hartmannella at eyewash stations and Cryptosporidium in ice machines, document bacterial diseases it should be noted that contaminated water was the source and could well be the source for other parasites as well. Often there was not even an analysis carried out for protozoans, helminths, fungi and viruses.
Food processing operations
Few studies have been conducted on the effect of food processing operations on oocysts of Cryptosporidium, Cyclospora and Giardia cysts. Most studies in water have been conducted with Cryptosporidium, which is regarded as the most resistant of these organisms.
From limited studies it is probable that standard pasteurization procedures will inactivate these organisms. Heating Cryptosporidium cysts in water to 72.4°C for one minute or holding cysts in water at 64.2°C for two minutes rendered cysts non-infective to mice (International Journal of Food Microbiology 31, 1996, 1-26). The thermal death time point (the temperature at which organisms are destroyed) of Giardia cysts has been reported as 62°C (Journal of Food Protection 56, 1993, 451-456).
It must be stressed that the resistance of these cysts in a complex food medium such as milk may be different from that exhibited in water. Laberge and co-authors (International Journal of Food Microbiology 31, 1996, 1-26) in reviewing available data conclude it is unclear whether HTST pasteurization (72°C/15 sec) would lower the level of infective oocysts below the infective dose for humans. The level of contamination used in the study reported above (1 million oocysts/mL) is much higher than one would expect in raw milk.
There is one reported environmental/laboratory study of cryptosporidiosis attributed to an acid food, fresh pressed apple juice (Journal of the American Medical Association 272, 1994, 1592-1596). This is in fact the best-documented outbreak attributed to food. Unfortunately the pH of the juice involved is not recorded but it may be assumed to be around pH 4.0. Staff and students drank the juice the same afternoon it was prepared at a school agricultural fair in Maine, USA, in 1995. State Bureau of Health workers subsequently obtained a sample of the partially fermented apple juice ten days after the fair. The juice was then frozen and stored for six weeks before Cryptosporidium oocysts were counted. Up to 750 oocysts/L were recorded and the investigators believe the count was probably much higher. This indicates the ability of the oocysts to survive at the pH of the apple juice for several days at ambient temperature and weeks during frozen storage.
Two experimental studies confirm the relative tolerance of Cryptosporidium oocysts to low pH. Friedman and co-workers (Journal of Food Safety 17, 1997, 125-132) report a study in which oocysts were inoculated into beer, cola, orange juice and infant formula. The beer and cola were carbonated; the beer had a pH of 3.8 while the cola had a pH of 2.5. The pH of the orange juice was 3.9 and that of the infant formula 6.6. A pH 4.0 buffer as well as ethanol solutions were used as controls. Samples were incubated at 4°C and/or 22°C for 24 hours and viability of inoculated oocysts determined. The authors calculated that after this time there was a loss of >85% oocyst viability in beer or cola stored at 4°C while the loss of viability in water, orange juice and infant formula was equal to or less than 35 percent. The study did not permit estimation of the time required to achieve complete inactivation of Cryptosporidium oocysts.
The second study (FEMS Microbiology Letters 142, 1996, 203-208) used a laboratory medium with pH adjusted to 2.0 (hydrochloric acid), 4.0 (acetic acid), 6.0, 8.0 and 9.5. Cryptosporidium parvum, the common human pathogen, was one of two Cryptosporidium species tested. The initial population of cysts used was about 1 million/ml. These workers reported apparent zero viability of cysts after 60 minutes at pH 2.0 but 85 percent viability after the same length of time at pH 4.0. No other incubation time was studied.
The study reported with apple juice indicates that oocysts of Cryptosporidium are able to survive at least some weeks of frozen storage in a suitable medium. An experimental study (Applied and Environmental Microbiology 58, 1992, 3494-3500) using purified water as the medium showed that a small proportion of oocysts survived 750 hours at -22°C after slow freezing. Snap freezing using liquid nitrogen resulted in 100 per cent loss of viability. The initial population used in this study was again about one million cysts/mL. The case cited above where frozen tripe acted as a vehicle for infection is further confirmation that freezing cannot be relied upon to destroy all Cryptosporidium cysts.
No published studies have investigated the effect of drying of oocysts in or on foods. However experimental studies have shown that drying of cysts suspended in water on glass surfaces at ambient temperature resulted in 97 percent loss of viability after two hours and total loss after four hours.
It is clear from the above that we need to know a lot more about the behaviour of waterborne parasites especially Cryptosporidium and Giardia in food systems. While this knowledge is being accumulated, it is essential that food processors that use domestic drinking water, either for washing foods or as an ingredient in foods, for which there is no terminal heat process, develop contingency plans in case of similar incidents in the future.
Drinking water is not used just for drinking; in fact this is probably its most insignificant use in terms of volume used. It is used for cooking food, preserving food, washing food, washing people, as ice cubes in drinks, watering the garden, washing the house and objects in it, filling wading pools and filling humidifier reservoirs. People in a shower and kids playing under a sprinkler are exposed to aerosols.
There are a number of jobs in which the workers are exposed to feces, which may be contaminated with protozoan pathogens. The use of protective clothing, masks, rubber gloves and the practice of washing thoroughly with soap and using a nailbrush to clean under the fingernails will help to reduce the risk. A partial list of jobs and activities with high pathogen contact risk includes, sewer workers, sewage treatment plant workers, medical laboratory staff, hospital staff, child and adult daycare staff, caring for infants, farm workers where contaminated irrigation water is used, animal care staff in laboratories, zoos, feedlots, pet stores and organizations like the SPCA, commercial laundry staff, composting, recycling and garbage collection workers, abattoir workers, farm workers where animals are raised and ambulance and other emergency response staff. Everyone may be exposed daily when they go to the toilet.
Natural lakes and other bodies of water may be contaminated and continuously re-contaminated, particularly if heavily used by people. For most pathogens warmer waters are more of a risk and spas, hot tubs and hotsprings are pathogen reservoirs. Lack of flow and replacement of the water allows pathogens to build up. Surface waters are generally relatively clean and swimming from a dock or raft is a lower risk activity than wading in the shallows and stirring up the bottom sediments where pathogen concentrations are usually several orders of magnitude higher. Ironically, this puts toddlers at a higher risk than teens. For some organisms the risk is greatly increased by submersion of the head in diving or swimming since the infection route is through the nasal passages. Aerosols are also a problem and splashing increases the risk. The wading pool in the back yard that is freely accessible to cats and dogs and has been incubating for some weeks in the sun without a water change is a prime source of pathogens.
For artificial pools chlorination is not necessarily adequate as a treatment, especially if not properly maintained at high levels. Ozonation is also not adequate for some organisms. High intensity UV irradiation will control many spores including Giardia and Cryptosporidium. Sub-micron filtration is necessary to remove all the pathogens.
There are a number of sexual practices, which greatly increase the risk of anal/oral spread of many pathogens. Some ways to reduce, but not eliminate, such risk include use of protective devices such as condoms, washing with soap before and after such activities, avoiding such activities when one of the partners has an active infection and choosing a sexual partner with care.
Regulatory compliance is not good enough in an instance like this and improvement is possible. Blending of potentially oocyst-carrying, higher turbidity water with another lower turbidity water to meet turbidity guidelines may lead to an oocyst presence in the blended waters sufficient to initiate illness. With respect to potential oocyst contamination, operators need to view each treatment stream individually, rather than judging the quality of their product on the final, mixed water leaving the plant. The backwash process appears particularly critical with respect to oocyst presence in final waters. Some authorities recommend no recycling of backwash water and filtering to waste after backwashing until the filter restabilizes. Solids from backwashing may contain considerable numbers of oocysts, and should be handled appropriately as infectious waste.
Sewage treatment plants may be contributors of oocysts to raw water supplies depending upon where they are sited and they are certainly the primary repositories of oocysts produced by infected individuals within a community, monitoring data from sewage treatment facilities also fits into an improved communication network. Increasing oocyst numbers at the sewage treatment plant could serve as an early warning sign that a cryptosporidiosis problem is on the way. Water treatment facilities must gear to handling peak incidences of oocysts rather than normal levels, to ensure that unacceptably high risks of infection from Cryptosporidium, and other cyst forming pathogens never occur.
There are suggested yardsticks for gauging the significance of oocysts in finished waters, based upon monitoring studies and oocyst levels recorded or estimated during documented cryptosporidiosis outbreaks. When finished water concentrations are greater than 10-30 oocysts/100 L the possibility of an outbreak may exist. In the lower part of this range, or below, outbreaks may occur but not be detectable. If a single sample is at/above the action level, further sampling should be quickly initiated and supplemental performance data should be examined and/or collected, turbidity, particle data, individual filter performance data, fecal coliform counts, to support any subsequent decision making. The American Water Works Association guideline is that finished drinking water consistently measure 0.5 NTU, and that utilities set a goal of 0.1 NTU.
Implications for ground water management are not entirely clear at the present, but ground water sources are certainly of secondary importance relative to surface waters, with respect to contamination by cyst forming protozoans such as Cryptosporidium. In the US, a Groundwater Disinfection Rule is under development. The workgroup associated with this rule is not considering any requirements with respect to Giardia or Cryptosporidium. The conventional wisdom is that contamination of ground water with protozoa indicates surface water influence, and ground water-associated outbreaks have been associated with distribution system deficiencies. Treatment facilities can gauge the importance ofCryptosporidium to their product water quality by considering factors such as degree of watershed development, presence and location of upstream urban areas, animal husbandry operations, sewage treatment facilities, and the in-plant treatment train, especially the presence/absence of filtration. Process alterations may lead to changing circumstances, with respect to the risk of oocyst presence, or oocyst survivability, as may have been the case in Milwaukee. The outbreak in Kitchener/Waterloo took place after the area had shifted from complete ground water dependence to a mixed source of ground water augmented by some surface water, a change that increased the possibility of oocyst presence in the plant intake water. This may have been a factor in the outbreak.
The currently perceived waterborne protozoan threats to public health are Cryptosporidium and Giardia. However, many other protozoans are already emerging as the waterborne pathogens of the future. Cyclospora is one example; the only known, pre-1995, cyclosporiasis outbreak in the US was traced to contaminated water supply at a Chicago hospital. A second eastern US/Canada outbreak was recently associated with raspberries and possibly water used to irrigate the crop. A toxoplasmosis outbreak, Toxoplasma gondii, has been linked to a British Columbia water reservoir; and other microsporidia, Enterocytozoon, Septata, Encephalitozoon, Nosema and Pleistophora represent future waterborne threats.
When feeding very young infants, we carefully sterilize bottles and other equipment, to avoid pathogen exposure, because infant immune systems are not completely developed. Society views this practice as common sense. The same special care must be exercised in the case of other children or adults with imperfect or failing immune systems; this too is only common sense. Provision of point-of-use filtration devices, use of boiled drinking water, and other avoidance practices for high-risk individuals, seem more economically reasonable solutions to potential Cryptosporidium exposure than a massive overhaul of current water treatment facilities.
Even within the scientific literature, unreasonable statements appear. That...'outbreaks of cryptosporidiosis became more common during the next 5 years'... for example, is quite unlikely. It is far more likely that existing and previously un-recognized outbreaks, and their causes, became more commonly recognized.
The US Surface Water Treatment Rule states that all surface water that may potentially be used for drinking water must be filtered. Unfortunately, problems with Cryptosporidium, Giardia and protozoans like Cyclospor, which is larger than Cryptosporidium and thus more easily filtered, are still occurring, even in ground water sources. Also, because Cryptosporidium is pliable, it can fold up and pass through one-micron pores, thus slipping through most public utilities filtration systems. The only water treatment devices that can effectively filter Cryptosporidium are those certified for sub-micron filtration or less than one micron.
Sporocysts and oocysts may be quite different in size. The full range of reported values is given in the text for either kind of cyst. The trophozoite and amoeboid stages may be quite different in size from the cysts. It is the cysts which are generally the resistant and infective stage which need to be filtered out of drinking water. Many are not round but ovoid or other more complex shapes. In some cases the organisms are amoeboid and flexible and can pass through smaller pores than their spore size indicates. It is the minimum size of any water transmittable stage that is important for designing drinking water filtration processes so that is what is reported in the table below.
|Protozoan Parasites||Size of trophozoites/oocysts/sporocysts
smallest reported dimension in microns
Resistance to Disinfection
The resistance of many protozoan cysts to standard water disinfection procedures has been well reported. Cryptosporidium is resistant to the usual chlorine disinfection concentrations. Of the disinfectants commonly used to treat water, ozone and UV appear to be effective in destroying the oocysts of Cryptosporidium parvum, which is the most resistant of the protozoans. Notable outbreaks occurred in Milwaukee, WI in 1993, and Las Vegas, NV, in 1994. Milwaukee authorities installed ozone treatment plants to supplement filtration after the 1993 incident. Giardia can be killed only by long contact with high levels of chlorine. Recent outbreaks have occurred in Amsterdam, NY, Oregon, Georgia, and in Golden, CO.
Korich and co-workers (Applied and Environmental Microbiology 56 (5) 1990, 1423-1428) reported that 1 mg/L of ozone for 5 minutes achieved greater than 90 percent inactivation of Cryptosporidium parvum cysts. These authors concluded that with the possible exception of ozone, the use of disinfectants alone should not be expected to inactivate Cryptosporidium parvum in drinking water. However, 90% inactivation is inadequate for organisms where the initial concentration may be in the order or 104 to 107 or more, when the infectious dose is in the order of 1 to 10 spores. There are still 102 to 105 viable spores remaining after 90% inactivation, which is well over the infectious dose.
Many protozoans are relatively rare in the environment or, if common, very rarely cause disease in normal immunocompetent people. The risk of disease is extremely low and should be of little concern to normal healthy people, however, the consequences may be fatal for those few who do get sick. It is the immunoincompetent, such as AIDS and transplant patients, that are at greatest risk. One reason that many of these diseases have come into prominence recently is likely due to the recent AIDS epidemic, the prevalence of intravenous drug users and high risk sexual practices among such vulnerable people.
While this report is primarily concerned with water borne pathogens many of these organisms are also opportunistic and will spread directly from man to man, often via the fecal/oral route. Human behaviour is the weak link here; sexual practices and poor hygiene contribute to self-re-infection and person-to-person spread. Once in man they are then released again into the water to continue the cycle. It is important to break the cycle in as many places and as often as possible to reduce the pathogen load back to the water. Thus, many recommendations given here are not directly related to water transmission but are related to human behaviour.
Existing guidelines for many physical and chemical contaminants in water, and also for biological contaminants like bacteria, through the use of a surrogate measure like fecal coliforms or Escherichia coli, are based on numerical standards. Numerical standards may be appropriate for physical and chemical contaminants but not for biological pathogens where both the geographical and statistical distributions are intrinsically not normal, neither spatially nor temporally. For pathogens which can reproduce, a fixed number at a given place and time does not lead to predictable infectious doses at the host. For such contaminants, helminth worms, protozoans, bacteria and viruses it is more productive, and offers better protection from infection, to prescribe treatment regimes designed to remove the organisms rather than define numbers that have no defined risk factor attached to them. Measurement of numbers for such pathogens is often impossible, rarely reproducible, inaccurate, commonly rife with false positives and false negatives, changes constantly, correlates poorly if at all with disease or infection risk and does not necessarily determine whether or not the detected cyst is actually viable.
The risk of infection is a function of many factors including but not limited to: the numbers of spores or other infectious agents, the number of people in the habitat where the organisms are found, the behaviour of the people, sewage treatment processes, drinking water treatment processes, watershed conditions, the number of already infected people, the prevalence of alternate hosts and their infection rate, and climatic conditions. Only a few of these can be controlled to influence pathogen numbers and distribution. Drinking water and sewage treatment processes are the main influences under our direct and immediate control. Since such processes and infrastructures are already in place it is easiest, and likely the most economical, to modify them to the extent that pathogen control is achieved, rather than devise new control mechanisms.
There are a number of instances of co-infection or dependent species pairs involved in some diseases and in such cases control of the species of concern may actually necessitate control of the other species too. Dientamoeba fragilis is often linked to the pinworm, Enterobius vermicularis, and may gain access to the body in pinworm eggs or worms. Controlling pinworm spread may reduce the rate of Dientamoeba infections. The protozoan Hartmannella veriformis is a host protozoan in which the bacteria Legionella pneumophila multiplies to very large numbers. While Hartmannella may not cause a serious disease in man, Legionella does; one infectedHartmannella cell may introduce an infective dose of Legionella into the human body.
These techniques are all designed to reduce the number of pathogens below the infective dose, which in some cases is as low as one organism, ideally to zero. In order that the necessary sub-micron filters work and have a reasonable life span the bulk of the suspended materials in the water needs to be first removed by conventional sand filtration which may need to be preceded by coagulation and flocculation. This pre-filtration will also remove cyanophytes which release water soluble endotoxins when they lyse. These must be removed early in the water treatment process rather than killed later preventing the dead cells from releasing their toxins into the water supply. Sterilization techniques such as chlorination, ultraviolet light, ionizing radiation and ozone will not work effectively or efficiently unless such pre-filtration occurs to remove organic compounds and particulate materials. Disinfection must follow filtration since amoeboid protozoans; bacteria and viruses may penetrate filtration processes designed to remove spores and trophozoites.
Some of these techniques are not practical, economical, socially acceptable or useful for other reasons on a large scale, but do have some value for restricted uses of small quantities of water. On a practical and economical basis it is impossible to guarantee sterile water on a large scale with an extensive distribution system. What can be done is reduce the risk to an acceptable level and permit the immune system of healthy people to cope with the residual. There is an additional need to provide more expensive, smaller quantities of absolutely sterile water to people at special risk, for example kidney dialysis and extensive burn washing, as is currently done in hospitals.
Immunocompromised patients with diseases such as AIDS also need sterile water. Immunocompromised persons traveling to Latin America, Africa or other developing regions should exercise precautions when eating and drinking, in order to avoid infection with multiple gastrointestinal pathogens including Isospora, Cyclospora, microsporidium and Cryptosporidium. These precautions include the use of sub-micron filters for purifying water, eating well-cooked foods and avoiding recreational water activities.
Bottled water is not necessarily an alternative to boiled tap water. Bottled water suppliers rarely test for Cryptosporidium or other parasites and oocysts that can live for weeks in water, even refrigerated water. Bottled water from a ground water source, such as an artesian well, would have less likelihood of becoming contaminated with Cryptosporidium and other parasites than would surface water. Bottled waters with labels that specifically indicate treatment with ozone, ultraviolet light, sub-micron filtration or reverse osmosis are probably safest.
Other Vectors Many, but not all, of these are designed to break the anal/oral transmission cycle.
These precautions are for people with normal functional immune systems. Those whose immune systems are compromised, AIDS patients and those taking transplant anti-rejection drugs, should avoid all contacts completely and drink only sterile water since they are very vulnerable to these pathogens, which are often fatal if the immune system is not fully functional.
Drinking Water Treatment
All drinking should be disinfected. However, chlorine is known to be ineffective against many spores, including Cryptosporidium, and UV has been demonstrated to kill these spores. UV should be required as the main disinfectant with chlorine used for residual action against bacteria in distribution systems.
Administrative, Political and Regulatory
Given what science knows about these and other diseases and their spread some serious questions need to be addressed concerning political, administrative and regulatory inaction, complacency, non-enforcement, non-compliance and counter-productive laws and regulations in most jurisdictions. Science knows how to do a better job, not perfect but much better, but our responsible organizations are not doing it. We need a faster, more interactive and proactive way to enact water safety provisions as new information becomes available instead of a cumbersome, reactive process which only comes into play years after people have suffered and died. Our whole medical administrative system is geared towards treatment instead of prevention.