Heterophyes: A minute parasite that lives in the small intestine of humans, cats, dogs and other mammals who acquire it by eating raw or uncooked contaminated fish. Heterophyes is one of the smallest trematodes (parasitic flatworms) infecting humans. The main symptoms are diarrhea and abdominal pain. The lining of the small intestine may break down, permitting the parasite's eggs to enter the bloodstream and be carried to other organs, especially to the heart, liver, and brain, where they can cause serious disease, The diagnosis is based on the microscopic identification of eggs in the stool. Praziquantel (Biltricide) is the drug of choice.