Heterophyes. Light micrograph (LM). Heterophyiasis is a human disease caused by Heterophyes heterophyes a small parasitic fluke that infects humans who eat raw or undercooked fish infected with the metacercaria stage of the parasite. The adult parasite lives in the intestinal villi of humans (the definitive host). Symptoms include diarrhea, abdominal pain, and weight loss. The diagnosis is based on recovery of eggs in the feces. The first intermediate host of H. heterophyes are snails, Cerithidia sp. and Pironella sp. which are found in Asia and the Middle East respectively.