Lecture 6 Platyhelminthes (Flatworms)
Flukes other than Schistosoma, Cestodes
|LECTURE 1 Introduction and Intestinal Protozoa Notes||Lecture Powerpoint (.pdf)||Printable handout (.pdf)|
|LECTURE 2 Malaria Notes||Lecture Powerpoint (.pdf)||Printable handout (.pdf)|
|LECTURE 3 Systemic Protozoa Notes||Lecture Powerpoint (.pdf)||Printable handout (.pdf)|
|LECTURE 4 Intestinal Helminths Notes||Lecture Powerpoint (.pdf)||Printable handout (.pdf)|
|LECTURE 5 Filaria and schistosoma Notes||Lecture 5 Powerpoint (.pdf)||Printable 5 handout (.pdf)|
|LECTURE 6 Other trematodes and cestodes Notes||Lecture 6 Powerpoint (.pdf)||Printable 6 handout (.pdf)|
The phylum Platyhelminthes consists of two classes, the Trematoda (flukes) and the Cestodea (tapeworms). They are evolutionarily more primitive then the nematodes. The following characteristics are noteworthy.
- Most species are flattened dorsoventrally (hence flat worms)
- Trematodes are usually leaf shaped
- Size: microscopic to 100 ft long
- Covered by a cytoplasmic tegument (vs cuticle of nematodes)
- No body cavity
- Digestive system either absent or rudimentary.
- Excretory system containing ciliated cells extending into excretory tubules; in living state, motion of cilia reminiscent of flames and when seen under microscope indicates viability.
- Reproduction: Most species are hermaphroditic; schistosomes are an exception. Adult worms reproduce sexually. Subsequently, larvae can reproduce asexually (polyembryony)
Clonorchis sinensis, Opistorchis
spp., Metorchis conjunctus, Fasciola spp. (liver flukes)
transmission: consumption of infected freshwater fish
biology: adult flukes live in biliary radicles
clinical: ascending cholangitis, cholangiocarcinoma
Paragonimus spp. (lung flukes)
transmission: consumption of infected fresh water crabs
biology: adult flukes live in lung
clinical: pulmonary nodules/masses, hemoptysis
Diagnosis: - stool, urine or sputum for ova; serology
Treatment: - praziquantel (triclabendazole for Fasciola)
Diphyllobothrium spp. (fish tapeworm)
transmission: consumption of raw infected fish
adults live in human intestinal lumen
clinical: rare B12 deficiency, vague abdominal pains
Taenia saginata (beef tapeworm)
transmission: consumption of infected raw bee
biology: adult lives in human intestine lumen
clinical: vague abdominal pains
Taenia solium (pork tapeworm)
transmission: consumption of infected raw pork
biology: adult lives in human intestine
clinical: vague abdominal pains
Taenia solium (cysticercosis)
transmission: eating T. solium eggs in human stool contaminated garden produce
biology: cyst like larvae live in human tissu
clinical: subcutaneous tissue nodules, space occupying lesions (1.5 cm) in brain (epilepsy, hydrocephalus); basalar meningitis
Echinococcus granulosis (hydatid
transmission: ingestion of dog stool contaminated food
biology: cyst like larvae live in human tissue (usually lung or liver); adult is small dog tapeworm
clinical: large space occupy lesion (1-15 cm) in liver or lung with pain and at times rupture
Stool examination for eggs of adult forms
Ultrasound/CT scan/MRI for tissue larval forms
Treatment: praziquantel, albendizole, surgery
Flukes - Paragonomiasis
Epidemiology: Prevalent in the Far East; also areas in Central America and Africa. Transmission is related to the consumption of raw fresh water crabs and crayfish which contain the larvae (metacercaria) of Paragonimus
Biology: Many different species of Paragonimus all with some different levels of human host adaptability. Eggs expectorated in sputum into fresh water hatch as ciliated miracidia which penetrate specific snails and multiply within. Free-swimming cercaria leave the snail and penetrate fresh water crabs and crayfish, infecting any that consume them. The consumed metacercaria penetrate the human small intestine wall and migrate to the lungs where they mate and live for a number of years laying eggs which are expectorated or swallowed.
|adult Paragonimus 7-12 mm long||Paragonimus egg 80-120 Ám long|
Lung abscesses, chronic cough, rusty coloured sputum
At times (~10%) aberrant worms end up in brain or in subcutaneous tissues
Examination of sputum and stool for eggs
CT/MRI of head
Prevention: Cooking crab meat
Epidemiology: Clonorchis sinensis (China), Opisthorchis viverini (SE Asia) Opisthorchis felineus (Russia), Metorchis conjunctus (Canada). and Fasciola spp worldwide. Infect more then 20 million esp. in far east.
Acquired by eating infected freshwater fish.
Biology: The adult , living in the intra-hepatic bile ducts releases eggs that, in stool, if reach fresh water will hatch as ciliated swimming miracidia. These infect specific snails, multiply and are released as swimming cercaria which infect freshwater fish (or edible plants in the case of Fasciola). If ingested by humans the larvae climb up the biliary tree (or cross the peritoneal cavity and liver parenchyma in the case of Fasciola) and mature to adults in the intrahepatic biliary radicles.
Acute: abdominal pain, eosinophilia ( a couple of weeks normally except in Fasciola 2-3 months)
Chronic: Live for years in biliary tree producing obstruction and at times ascending cholangitis and abscesses and biliary endothelial metaplasia leading to cholangiocarcinoma.
Diagnosis Acute stage CT scan, serology
Chronic stage: serology, eggs in stool, ultra sound
Treatment: Praziquantel (Triclabendazole for Fasciola)
These are long segmented worms in which each segment is independent of the other except for neurological intercommunication. The adult stage always reside in the gut of the definitive host. All except for one have an intermediate host. Humans may serve as definite host, as intermediate host, or as both, depending on the species of tapeworm.
General Characteristics of Tapeworms
Proglottids (segment): body of adult worm is subdivided into segments; a segment usually corresponds to one proglottid which has male and female reproductive organs, nervous and excretory system. There is no intestine; nutrients are absorbed through the integument. New proglottids are produced near the anterior end, and are shunted posteriorly as they mature. Mature, gravid proglottids detach from the strobila.
Scolex (head): Anterior segment of the worm modified for attachment to host's digestive tract; equipped with suction apparatus and/or hooks.
|Taenia saginata||human gut||cow muscle|
|Taenia solium||human gut||pig muscle|
|Taenia solium (cysticercosis)||human gut||human brain etc.|
|Diphyllobothrium latum||human gut||fish|
|Echinococcus granulosis||dog gut||human/cow/sheep liver/lung|
Taenia saginata (Beef tapeworm)
Epidemiology: Most common tapeworm found world wide. Acquired from eating uncooked beef. Humans are definitive hosts. Very common in Lebanon and Ethiopia. Occasionally found in Canadian dairy cattle herds.
Biology: Eggs are released individually in human feces or while still within tapeworm proglotids. They are ingested by cows and hatch; the resultant larvae invade the gut and are hematogenously spread , and develop as small 1 cm cysts in muscle. When humans consume these cysts in raw beef the cyst grows into an adult tapeworm in the small intestine, holding onto the small intestine mucosa with suckers on the head (scolex).
|proglotid of T. saginata (note many uterine branches)||scolex of T. saginata|
Clinical: Few if any symptoms; non-specific intermittent pains or indigestion. Tapeworm segments (proglotids) or segment chains are seen in the stools.
Diagnosis: Stool examination for eggs or segments. Segments differentiated from pork tapeworm by having large number of uterine branches when viewed with a trans-illuminating light source.
Prevention: Sewage treatment; cook or freeze beef
Taenia solium (pork tapeworm)
Epidemiology: Not as widely disseminated as T. saginata. As with beef tapeworm humans are definitive hosts and pig the intermediate host.. Acquired by eating uncooked pork.
Biology: same as for the beef tapeworm (above) except for the intermediate host (pig). However humans can be infected with the larval cysts like in the pig (see cysticercosis below)
|Scolex of T. solium||Taenia egg||T. solium proglotid|
Clinical: The same as beef tapeworm (above)
Prevention: Sewage disposal; cook or freeze pork
Taenia solium (cysticercosis)
Epidemiology: The larval form of T. solium infects humans who ingest the eggs of this tapeworm passed in human stool. This occurs most frequently where human stool is used as fertilizer. This occurs most frequently in developing countries where pigs are bred.
Biology: T. solium eggs, on ingestion, hatch into larvae which penetrate small intestine mucosa and are carried throughout the body where they are deposited and grow in many different tissues (muscle, subcutaneous, brain, eye, heart). They form small (1-3 cm cysts with an invaginated scolex) and live for ~7 years.
Clinical: Pathology is
produced by the cysts as space occupying lesions (especially brain) and as foci of host
inflammatory response when the cyst eventually dies or is killed with anti-helminthics.
Frequent Presentations are seisures or hydrocephalus.
|cerebral cysticercosis||cysticercus containing inverted scolex|
Treatment: Praziquantel, albendizole
Diphyllobotrium latum (fish tapeworm)
Epidemiology: World wide - particularly frequent in Scandinavia and northern Canada where raw freshwater fish are consumed.
Biology: Human is the definitive host with adult residing in the small intestine lumen. Eggs or proglotids containing eggs are passed in stool and if reaching freshwater hatch (as coricidium) and are consumed by copepods which are then ingested by fish. The resulting larva (plerocercoid) in fish flesh, following human consumption, grow into a tapeworms several meters long that attach to small intestine mucosa by sucking grooves on the head (scolex).
There are several other species of Diphyllobothrium in Canada especially in the North (D. dendriticum. D. ursi etc.) with the same life cycle.
life cycle of Diphyllobothrium
|proglotids (segments) of Diphyllobothrium||egg of Diphyllobothrium|
Clinical: As with the Taenia the Diphyllobothrium species have
few if any symptoms. In Finland B12 deficiency occurs due to consumption by the
tapeworm. This is not seen in the Americas
Diagnosis: Stool examination for eggs (typical) and proglotids with characteristic uteri.
Prevention: Freezing or cooking fish. Sewage treatment.
Echinococcus granulosis: (Hydatid disease)
Epidemiology: This is a dog tapeworm, the larval (cyst) stage of which infects humans who eat food contaminated with dog feces containing tapeworm eggs. The infection occurs most frequently in geographic regions where dogs and the usual non-human intermediate host live together (eg cattle raising Argentina, sheep raising Australia, sheep and goat raising Mediterranean, caribou sled dog Arctic North America.
Biology: Eggs are passed in dog feces and when consumed by humans (or bovines) hatch and penetrate the small intestine mucosa where they enter the portal venous system to eventually lodge in liver or lungs. There thy slowly grow over years to large size (2-15 cm diameter). Within each cyst the germinal epithelium at the periphery of the cyst buds off, within the host cyst, new protoscolices and daughter cysts which themselves produce protoscolices. These hydatid cysts if consumed by dogs will produce a new tapeworm from each scolex.
|adult of Echinococcus granulosis from dog intestine||protoscolices from human hydatid cyst|
Clinical: The enlarging cysts can produce symptoms and signs related to their space occupation or can rupture with resultant anaphylactic shock. Leakage of daughter cysts produce hydatid cysts in new locations.
|Hydatid cyst in left lobe of liver; note
membrane split off the outer wall.
|Hydatid cyst in lung||Hydatid cyst of liver containing daughter cysts|
Treatment: surgical removal; albendizole; PAIR (percutaneous aspiration, instillation of scolicide, reaspiration)