Schistosomes and Other Trematodes

Excerpt

Trematodes, or flukes, are parasitic flatworms with unique life cycles involving sexual reproduction in mammalian and other vertebrate definitive hosts and asexual reproduction in snail intermediate hosts. These organisms are divided into four groups on the basis of their final habitats in humans: (1) the hermaphroditic liver flukes which reside in the bile ducts and infect humans on ingestion of watercress (Fasciola) or raw fish (Clonorchis and Opisthorchis); (2) the hermaphroditic intestinal fluke (Fasciolopsis), which infects humans on ingestion of water chestnuts; (3) the hermaphroditic lung fluke (Paragonimus), which infects humans on ingestion of raw crabs or crayfish; and (4) the bisexual blood flukes (Schistosoma), which live in the intestinal or vesical (urinary bladder) venules and infect humans by direct penetration through the skin.

Fascioliasis is a cosmopolitan zoonosis; sporadic cases in humans have appeared in most parts of the world. The remaining hermaphroditic fluke infections of humans are confined largely to Asia. Schistosomiasis occurs in South America, the Caribbean, Africa, the Middle East, and Asia, and is spreading in many areas due to the introduction of dams and irrigation systems.

Flukes do not multiply in humans, so the intensity of infection is related to the degree of exposure to the infective larvae. In most endemic areas, the majority of infected individuals have light or moderate worm burdens. Overt disease occurs largely in the relatively small proportion of the population with a heavy worm burden, although genetic predisposition may also play a role. Pathology may be caused by the worms themselves (as with liver flukes, which damage the bile ducts) or by their eggs (for example, schistosome eggs, which induce granulomatous inflammation in the venules or tissues). Treatment of all fluke infections has been greatly improved by the introduction of the anthelminthic drug praziquantel.