Cestodes & Trematodes

Flatworm basics

Two classes. Cestodes (tapeworms) are ribbons of egg-filled segments (proglottids) hanging from a scolex that grips the gut wall with suckers, hooks or grooves. Trematodes (flukes) are unsegmented, leaf-shaped worms whose life cycles pass through a freshwater snail, so infection follows skin contact with fresh water or eating raw fish or crustaceans. The definitive host carries the adult worm; the intermediate host carries larvae.

Eosinophilia. A raised eosinophil count points to helminths invading tissue: migrating schistosomes, liver and lung flukes, and larval cestode cysts. Adult tapeworms confined to the gut lumen raise it little, and an intact hydatid cyst may not raise it until it leaks.

Taenia and cysticercosis

T. solium (pork tapeworm). Eating undercooked pork containing cysticerci (larval cysts) lets the hooked scolex attach in the small intestine and grow into an adult worm (taeniasis). Humans are the only definitive host. When anyone swallows eggs shed in a carrier's feces, that person becomes an intermediate host: larvae cross the gut wall and lodge in brain, muscle, subcutaneous tissue and eye. This is cysticercosis, and it needs no pork, so a vegetarian can catch it from a carrier in the household.

T. saginata (beef tapeworm). Acquired from undercooked beef, it causes intestinal infection only, because humans never host its larval stage.

Neurocysticercosis. The commonest acquired cause of epilepsy worldwide, frequent in Latin America, Asia and sub-Saharan Africa. Seizures are the usual presentation; ventricular cysts can block CSF flow and cause hydrocephalus. Imaging follows each cyst's life: a viable cyst with an eccentric scolex (the "hole with a dot"), a dying cyst with ring enhancement and edema, and finally a punctate calcification. Serology (enzyme-linked immunoelectrotransfer blot) supports the diagnosis.

Fish tapeworm and hydatid disease

Diphyllobothrium latum. The broad fish tapeworm, from raw or undercooked freshwater fish such as pike and perch, is the longest tapeworm of humans. It competes for dietary vitamin B12 in the small intestine, causing megaloblastic anemia with hypersegmented neutrophils that mimics pernicious anemia and, if prolonged, subacute combined degeneration. Stool shows operculated eggs; treat with praziquantel and replace B12.

Echinococcus granulosus. Adults live in dog intestines. Sheep are the usual intermediate host, and dogs complete the cycle by eating infected offal; humans who swallow eggs from dog feces are accidental dead-end hosts. Larvae reach the liver through portal blood (about 70% of cysts) or pass on to the lung. The hydatid cyst grows slowly for years, buds daughter cysts and shows eggshell (rim) calcification on imaging; an amebic liver abscess has neither.

Hydatid fluid is intensely antigenic. Rupture from trauma, blind needle aspiration or careless surgery can cause anaphylactic shock, and spilled protoscolices ("hydatid sand") seed new cysts.

Treatment. Albendazole (a benzimidazole that binds worm β-tubulin) is given around surgical removal or PAIR, a percutaneous drainage avoided when the cyst communicates with the bile ducts:

Schistosomiasis

Life cycle. Snails release cercariae that penetrate intact skin during wading or swimming, sometimes leaving an itchy cercarial dermatitis. The worms mature, pair and live for years in venules. Eggs trapped in tissue, not the adult worms, provoke the granulomas and fibrosis that cause disease.

Katayama fever. Weeks after a first exposure, typically in travelers, a hypersensitivity reaction brings fever, urticaria, cough, hepatosplenomegaly and marked eosinophilia. Praziquantel acts poorly on immature worms, so it is repeated several weeks later.

Hepatic disease. S. mansoni and S. japonicum eggs lodge in portal venules, and granulomas produce periportal "pipestem" (Symmers) fibrosis. The block is presinusoidal, unlike cirrhosis, so varices and massive splenomegaly develop while albumin and INR stay normal.

Urinary disease. S. haematobium eggs, terminal-spined and passed in urine, cause terminal hematuria; chronic infection calcifies the bladder wall, obstructs the ureters (hydronephrosis) and drives squamous metaplasia to squamous cell carcinoma. Elsewhere, smoking-related urothelial carcinoma is the usual bladder cancer.

Liver and lung flukes; praziquantel

Clonorchis sinensis and Opisthorchis. Common in East and Southeast Asia (O. viverrini in Thailand and Laos), these flukes live in the bile ducts for decades, causing obstruction, recurrent cholangitis, brown pigment stones and cholangiocarcinoma. Operculated eggs appear in stool.

Paragonimus westermani. Larvae from raw or undercooked crab or crayfish cross the gut wall and diaphragm into the lung, producing chronic cough, rusty-brown hemoptysis, nodular or cavitary lesions and pleural effusion that mimic tuberculosis, with eosinophilia and operculated eggs in sputum.

Praziquantel. It increases the permeability of the worm's tegument to Ca²⁺, causing sustained contraction (spastic paralysis) and tegumental damage that exposes worm antigens to host immunity. It treats all schistosomes, liver and lung flukes and adult intestinal tapeworms. By contrast, albendazole binds β-tubulin, ivermectin opens glutamate-gated chloride channels, and pyrantel is a depolarizing nicotinic agonist.

The exception: Fasciola hepatica, the sheep liver fluke acquired from watercress, is treated with triclabendazole rather than praziquantel.