Human biliary infection with the trematodes Clonorchis sinensis and Opisthorchis viverrini (oriental liver flukes) affects an estimated 20 million people worldwide and is especially prevalent throughout areas of China and southeast Asia. Seven million infections may exist in Thailand alone where 80% of selected populations are infested.1 United States immigration officials report that 1.4 million Asians entered the United States between 1971 and 1979, and, in 1981, 121,300 Indochinese refugees emigrated to the United States.2 This influx of patients from countries with endemic clonorchiasis and opisthorchiasis increases the likelihood that American physicians will encounter such parasitic infections.
Humans acquire parasites through ingestion of viable metacercariae (120 to 150 μm) encysted in the subcutaneous tissues of carp and other freshwater fish. Fish become infectious by penetration and maturation of cercariae, released from snails acting as the first intermediate host. Snails acquire infection through ingestion of infective ova passed in
Clement C. S. Hsu, Michael A. Kron. Clonorchiasis and Praziquantel. Arch Intern Med. 1985;145(6):1002–1003. doi:10.1001/archinte.1985.00360060058005