ABSTRACT
Trematodes are digenetic parasitic flatworms with unique life cycles involving sexual reproduction in mammalian and other vertebrate definitive hosts and asexual reproduction in the intermediate snail hosts. Trematodes may be divided into four groups based upon their final habitat in human hosts: the hermaphroditic liver flukes, which reside in bile ducts and infect humans on ingestion of watercress (Fasciola) or raw fish (Clonorchis and Opisthoricis); the hermaphroditic intestinal fluke, which infects humans by ingestion of water chestnuts (Fasciolopsis); the hermaphroditic lung fluke, which infects humans on ingestion of raw crabs or crayfish (Paragonimus); and the bisexual blood flukes (Schistosoma), which live in intestinal venules or urinary venules in the bladder and infect humans by cercarial penetration of the skin. Flukes do not multiply in humans, so the intensity of infection is related to the degree of exposure to the infectious larvae. Tissue (liver, lung) trematodes are the most important of the helminths causing human infection. Freshwater snails serve as the intermediate host for all trematode infections. The geographic distribution of the specific snail species generally delineates the location of human disease.
