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 sinensisOpistorchis spp., Metorchis conjunctus, Fasciola spp.    (liver flukes)
WB00938_.GIF (1017 bytes)  transmission: consumption of infected freshwater fish
WB00938_.GIF (1017 bytes)  biology: adult flukes live in biliary radicles
WB00938_.GIF (1017 bytes) clinical: ascending cholangitis, cholangiocarcinoma

Paragonimus spp(lung flukes)
WB00938_.GIF (1017 bytes)   transmission: consumption of infected fresh water crabs     
WB00938_.GIF (1017 bytes) biology: adult flukes live in lung
WB00938_.GIF (1017 bytes) clinical: pulmonary nodules/masses, hemoptysis

- stool, urine or sputum for ova; serology

Treatment: - praziquantel (triclabendazole for Fasciola)


Cestodes (tapeworms)

Diphyllobothrium spp.     (fish tapeworm)
WB00938_.GIF (1017 bytes) transmission: consumption of raw infected fish
WB00938_.GIF (1017 bytes) adults live in human intestinal lumen
WB00938_.GIF (1017 bytes) clinical: rare B12 deficiency, vague abdominal pains

Taenia saginata    (beef tapeworm)
WB00938_.GIF (1017 bytes) transmission: consumption of infected raw bee
WB00938_.GIF (1017 bytes) biology: adult lives in human intestine lumen
WB00938_.GIF (1017 bytes) clinical: vague abdominal pains

Taenia solium   (pork tapeworm)
WB00938_.GIF (1017 bytes) transmission: consumption of infected raw pork
WB00938_.GIF (1017 bytes) biology: adult lives in human intestine
WB00938_.GIF (1017 bytes) clinical: vague abdominal pains

Taenia solium
WB00938_.GIF (1017 bytes) transmission: eating T. solium eggs in human stool contaminated garden produce
WB00938_.GIF (1017 bytes) biology: cyst like larvae live in human tissu
WB00938_.GIF (1017 bytes) clinical:  subcutaneous tissue nodules, space occupying lesions (1.5 cm) in brain  (epilepsy, hydrocephalus); basalar meningitis

Echinococcus granulosis (hydatid cyst)
WB00938_.GIF (1017 bytes) transmission: ingestion of dog stool contaminated food
WB00938_.GIF (1017 bytes) biology: cyst like larvae live in human tissue (usually lung or liver); adult is small dog tapeworm
WB00938_.GIF (1017 bytes) clinical:  large space occupy lesion (1-15 cm) in liver or lung with pain and at times rupture


Cestode diagnosis
WB00938_.GIF (1017 bytes) Stool examination for eggs of adult forms
WB00938_.GIF (1017 bytes) Ultrasound/CT scan/MRI  for tissue larval forms
WB00938_.GIF (1017 bytes) Serology

Treatmentpraziquantel, 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.

paragonadult-chmai.jpg (15565 bytes) paragonegg-chmai.jpg (6151 bytes)
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

Treatment: Praziquantel

Prevention: Cooking crab meat
                   Sewage treatment







Liver Flukes

: 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.



opisthorccercar-chmai.jpg (7954 bytes) opisthorcadult-chmai.jpg (9810 bytes) Clo_sinensis_egg1_DPDx.JPG (21118 bytes)


Opisthorchis adult

ciliated Miricidium

Opisthorchis egg



: 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)

Cestodes (Tapeworms)

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.

lithotape2.jpeg (39941 bytes) lithotape.jpeg (52523 bytes)


Scolex (head): Anterior segment of the worm modified for attachment to host's digestive tract; equipped with suction apparatus and/or hooks.











                          GravProglott_cdc.GIF (34165 bytes)













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).

sagunataproglot-chmai.jpg (20372 bytes) saginatascolex-chmai.jpg (20540 bytes)
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.

Treatment: Praziquantel

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)

tsoliumscolex-chmai.jpg (7073 bytes) Taenia_egg1_DPDx.JPG (24279 bytes) T_solium_proglt_DPDx.JPG (49781 bytes)
  Scolex of T. solium       Taenia egg         T. solium proglotid

Clinical: The same as beef tapeworm (above)

Treatment: Praziquantel

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.

Pathology platyhelminths2.jpg (15137 bytes) Pathology platyhelminths.jpg (11984 bytes)
            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.

                                        d_latum_cdc_cycle.gif (6619 bytes)
                   life cycle of Diphyllobothrium

Diphyl_latum_proglottid2_DPDx.JPG (15395 bytes) Diphyl_latum_egg2_DPDx.JPG (24839 bytes)
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.

Treatment: Praziquantel

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.

echinococgranadultchmai.jpg (7652 bytes) echinococcprotoscol-chmai.jpg (9881 bytes)
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.

hydatidsljuly271999.jpg (50701 bytes) Pathology platyhelminths6.jpg (12698 bytes) Pathology platyhelminths3.jpg (28054 bytes)
Hydatid cyst in left lobe of liver; note the laminar
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)