Significance to humans

Trematodes pose a significant health threat to humans, particularly those living in developing countries. A common illness in developing countries is schistosomiasis. This condition, caused by three species of Schistosoma, affects more than 40 million individuals who live in tropical and subtropical countries, causing weakness, diarrhea, hemorrhage, fever, enlargement of the spleen, and other severe symptoms.

Other trematodes also infect humans, including the trematode Opisthorchis, which is transmitted to humans through eating infected fishes. Prevalent in parts of Russia, the fluke currently infects 1.2 million people, which is more than 4 percent of the region's population.

The cost for treatment of human fluke infections ranges into billions of dollars, in part because these conditions are frequently misdiagnosed. Treatment for infection by such organisms as Paragonimus species costs about $1, but the patient's illness is often misinterpreted as tuberculosis, which calls for years of expensive treatment.

Trematodes that infect such household pets as rabbits, dogs, and cats may cause gastrointestinal symptoms requiring veterinary treatment. In the case of dogs, the trematode Nanophyetus salmincola or so-called salmon-poisoning fluke, may cause a fatal disease resembling distemper because it carries a rickettsia (a type of bacterium) to which dogs are susceptible. The rickettsia, however, does not produce clinical disease in either humans or cats.

Trematodes can also infect livestock, sport and commercial fishes, and game mammals, which can have negative economic impacts on agriculture, sport fishing, and commercial fishing.

1. Rugogaster hydrolagi; 2. Black-spot flatworm (Uvulifer ambloplitis); 3. Human blood fluke (Schistosoma mansoni); 4. Liver fluke (Fasciola hepatica); 5. Lancet fluke (Dicrocoelium dendriticum); 6. Oriental liver fluke (Clonorchis sinensis); 7. Salmon-poisoning fluke (Nanophyetus salmin-cola); 8. Echinostome (Echinostoma revolutum); 9. Aspidogaster conchicola; 10. Nematobothrium texomensis. (Illustration by Bruce Worden)

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