Magnetic resonance image of a patient with neurocysticercosis demonstrating multiple cysticerci within the brain.
Cysticercosis is a parasitic disease caused by infection by the pork tapeworm, Taenia solium, in which the tapeworm enters the body and forms cysticerci (larval form of the tapeworm enclosed in a cyst, or fluid-filled protective capsule). When the infection results in cysticerci in the brain and spinal cord, the condition is known as neurocysticercosis. Cysticercosis is the most common parasitic infestation of the central nervous system worldwide.
Humans develop cysticercosis when they ingest eggs of the pork tapeworm, which hatch in the stomach, and the larva enter the bloodstream, travel to various tissues, and form cysticerci. These cysts may form in such areas as the muscles, eyes, and brain. Cysticerci often occur in the central nervous system and if the infestation is heavy, it can cause major neurological problems like epilepsy and even death.
While the tapeworm is found worldwide, infection is most common in areas with poor hygiene. Tapeworm eggs are passed through the bowel movements of an infected person, and then spread when fecally contaminated water or food is consumed or when contaminated fingers are placed in the mouth. Pigs are an intermediate host and the pork tapeworm can spread to humans, to complete the life cycle, when pigs likewise consume fecally contaminated water or food and then the undercooked pork of infected pigs is eaten. A person can be infected with T. solium, with the adult tapeworms producing eggs in the intestine, without developing the cysticerci diagnostic for cysticercosis.
Cysticercosis is a preventable disease, through avoiding undercooked pork, washing hands thoroughly before handling food, drinking bottled or boiled water in areas likely to be contaminated, and getting treated for any infections that have not yet resulted in the condition. In addition to personal responsibility, there also is an issue of social responsibility in terms of proper treatment of human waste and informing the public how cysticercosis is contracted and transmitted.
Overview and the pork tapeworm life cycle
The pork tapeworm, Taenia solium, is a cyclophyllid cestode in the family Taeniidae. Tapeworms or cestodes (class Cestoda) are ribbon–like endoparasitic flatworms that live in the digestive tract of vertebrates as adults and often in the bodies of various animals (intermediate hosts) as juveniles. The pork tapeworm is one of the most common tapeworms to affect humans, along with the beef tapeworm and the fish tapeworm. The general term for infections involving the pork and beef tapeworms is taeniasis. Like all cyclophyllid cestodes, T. solium has four suckers on its scolex ("head"). T. solium also has two rows of hooks.
Humans are the definitive host for T. solium, which means that the adult tapeworms are found only in the intestine of humans. Pigs are the intermediate hosts of the parasite, and thus completion of the life cycle occurs in regions where humans live in close contact with pigs. The pork tapeworm infects pigs and humans in Asia, Africa, South America, parts of Southern Europe, and pockets of North America.
When humans ingest eggs, the eggs are capable of hatching in the stomach. The larvae of T. solium then are able to bore through the intestinal wall, enter the bloodstream, and migrate to striated muscles and to many organs, including the heart, liver, eye, brain, and spinal cord. There they form cysts in tissue called cysticerci. A cysticercus, also known as a bladder worm, is the larval stage of the tapeworm, where it is enclosed in a fluid-filled cyst or protective capsule. These larval forms cannot grow into adult worms in this state, and remain indefinitely encapsulated in tissue. A human is acting as an intermediate host in such a case.
In T. solium, cysticerci have three morpholocially distinct types. The common one is the ordinary "cellulose" cysticercus, which has a fluid filled bladder that is 0.5 centimeters to 1.5 centimeters in length and an invaginated scolex. The intermediate form has a scolex while the "racemose" has no evident scolex but are believed to be larger and much more dangerous. They are 20 centimeters in length and have 60 milliliters of fluid. Thirteen percent of patients might have all three types in the brain.
Cysticercosis (SIS-tuh-sir-KO-sis) is the term for infections of T. solium resulting in cysticerci. Cysticerci are commonly found at autopsy in asymptomatic inhabitants of endemic areas. In humans, cysts can cause serious sequelae if they localize in the brain, resulting in neurocysticercosis.
On the other hand, if a cysticercus is consumed alive by a person, such as when humans ingest undercooked pork containing cysticerci, the parasitic life cycle is completed. Cysts evaginate and attach to the host's small intestine by their scolex and develop directly into a mature adult. Adult tapeworms develop up to 2 to 7 meters in length and produce less than 1000 proglottids, each with approximately 50,000 eggs. These eggs do not have the capacity to invade tissue, and they are excreted with the rest of that person's feces.
Thus, it is possible for a human to be infested by T. solium (taeniasis) without having cysticercosis; in this case, the tapeworm lives in the jejunum and regularly lays its eggs. The adult tapeworms may reside in the small intestine for years.
In rural areas where cysticercosis is common, pigs ingest the eggs by contact with fecally contaminated food or water. When pigs eat the eggs, the larvae hatch and disseminate and form cysticerci in the striated muscle, which can be the infective source of the parasite for humans who later consume that pork. This describes why swine are the intermediate host of T. solium: Pigs eat the eggs laid by the tapeworms that live in the gut of infested humans.
When a human eats infected meat, fostering adult tapeworms in the intestine, and passing eggs through feces, sometimes a cysticercus develops in the human and the human acts like an intermediate host. This happens if eggs get to the stomach, usually as a result of contaminated hands, but also of vomiting. In the latter case, eggs laid by the infesting tapeworm are pushed back into the stomach. When these eggs hatch, the larva again passes into the bloodstream, and the infestation proceeds as usual.
In other words, humans may be infected either by ingestion of food, drink, or hands contaminated with feces containing eggs, or by autoinfection. In the case of autoinfection, a human infected with adult T. solium can ingest eggs produced by that tapeworm either through fecal contamination or, possibly, from proglottids being carried into the stomach by reverse peristalsis.
Cysticercosis is often seen in areas where poor hygiene allows for contamination of food, soil, or water supplies. Prevalence rates in the United States have shown that immigrants from Mexico, Central and South America and Southeast Asia account for most of the domestic cases of cysticercosis. Taeniasis and cysticercosis are very rare in predominantly Muslim countries, as Islam forbids the consumption of pork. It is important to note that human cysticercosis is acquired by ingesting T. solium eggs shed in the feces of a human T. solium tapeworm carrier, and thus can occur in populations that neither eat pork nor share environments with pigs.
In muscles, cysts cause painless swelling or create nodules under the skin. If cysts form in the eye, they can impair vision by floating in the eye and can cause blindness by causing swelling and detachment of the retina. Heart lesions can lead to abnormal rhythms or heart failure (rare).
The most dangerous symptoms are a result of encystment in the central nervous system. According to a Centers for Disease Control and Prevention Division of Parasitic Diseases in neurocysticercosis (cysticercosis of the brain), the most common symptoms are seizures and headaches, but other symptoms can include confusion, difficulty of balance, lack of attention to people and surroundings, and hydrocephalus (compression of the brain tissue due to obstruction of cerebrospinal fluid flow) (CDC 2008). With heavy infections, death can occur (CDC 2008).
When death occurs, it is most often due to involvement of the brain resulting in hydrocephalus, cerebral edema, cerebral compression, or epileptic seizures (Sorvillo et al. 2007).
Often, there are few symptoms until the parasite dies. When the parasite dies, the host's immune system detects the worm's remains and attacks them, causing swelling and scarring. This is what causes most of the symptoms. Spinal cord lesions can lead to partial loss of motor control, weakness, and even paralysis (CDC 2008).
Neurocysticercosis is difficult to diagnose in its early stage and may be apparent only when the first neurological symptoms start, or when a CT scan, or an MRI of the brain is performed for other reasons. Antibody tests or a biopsy of the affected area may be necessary to complete the diagnosis.
The anti-parasitic drugs Praziquantel and Albendazole may be used to treat neurocysticercosis. Steroid anti-inflammatory medication is also often used in conjunction to reduce the swelling (brain edema) that results from immune system attacks on dead worms. It is still controversial whether patients benefit from treatment, because live cysticerci do not provoke seizures; only dead or dying parasites invoke an inflammatory response and seizures. In theory, therefore, treating a patient with drugs that kill living parasites can induce seizures in someone who is otherwise well and seizure-free; likewise, treating someone with seizures may not have any effect on outcome as the parasites are already dead and no improvement can be expected. A meta-analysis of 11 trials suggests that there is probably some small benefit to patients who have active lesions, but no benefit to those with only dead or inactive lesions (Del Brutto et al. 2006).
If the cyst is in certain locations, such as the eye or the brain, steroids may be started a few days before the anti-parasitic, in order to avoid problems caused by swelling. If swelling and immune response are not controlled, the treatment itself can be lethal, so the medication is given in low dosages over several days. Sometimes surgery may be needed to remove the infected area or cysts, but this may be impossible when they are located in areas of difficult or dangerous surgical access. Also, some medications may treat symptoms, such as seizures or irregular heartbeat without affecting the worms.
If the cysticerci have calcified in the brain, or if there is only one lesion, treatment is not considered beneficial (CDC 2008).
It is possible to avoid infection with T. solium by avoiding undercooked pork and food and water contaminated with human feces. Extra care should be taken in places with poor hygiene or poor meat inspection laws. Freezing infested pork for a prolonged period will also kill cysticerci.
If a person is already infected with T. solium, they can avoid cysticercosis by treating the infection in the small intestine early, by not ingesting their own feces, and by not vomiting, as this brings eggs to the stomach so they form cysticerci.
- Centers for Disease Control and Prevention (CDC), National Center for Zoonotic, Vector-Borne, and Enteric Diseases, Division of Parasitic Diseases. 2008. Cysticercosis. Centers for Disease Control and Prevention. Retrieved February 15, 2009.
- Del Brutto, O. H., K. L. Roos, C. S. Coffey, and H. H. Garcia. 2006. Meta-analysis: Cysticidal drugs for neurocysticercosis: Albendazole and praziquantel. Ann Intern Med 145(1): 43-51. PMID 16818928. Retrieved February 15, 2009.
- Sorvillo, F. J., C. DeGiorgio, and S. H. Waterman. 2007. Deaths from cysticercosis, United States. Emerg Infect Dis 13(2): 230–5. PMID 17479884. Retrieved February 15, 2009.
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