The three major liver trematodes (flukes) that infect humans are Clonorchis sinensis, Opisthorchis species, and Fasciola hepatica. Clonorchiasis, opisthorchiasis, and the North American liver fluke, Metorchis conjunctus, will be reviewed here. Fascioliasis is discussed separately. (See "Liver flukes: Fascioliasis".)
CLONORCHIS SINENSIS AND OPISTHORCHIS SPECIES
Epidemiology — C. sinensis (or Opisthorchis sinensis), also known as the Chinese liver fluke, is endemic in the Far East, particularly in China, Japan, Taiwan, Vietnam, and Korea; it is also endemic in far eastern Russia [1,2]. Clonorchis is a parasite of fish-eating mammals; dogs and cats are the most common reservoirs. It is estimated that more than 35 million people are infected worldwide, with 600 million at risk . Prevalence rates in endemic areas vary widely; in different provinces of China, for example, the prevalence ranges from <1 to 57 percent . One study in Korea noted a prevalence of 16 percent .
Opisthorchiasis is generally caused by Opisthorchis felineus or O. viverrini. These are liver flukes of cats, dogs, and other fish-eating mammals, which serve as reservoir hosts. O. felineus occurs in Southeast Asia and in Central and Eastern Europe, particularly in Siberia and other parts of the former Soviet Union. Prevalence rates are 40 to 95 percent in some areas, and it is estimated that more than 16 million people are infected . O. viverrini is endemic in Thailand, Vietnam and Cambodia, and Laos. Prevalence rates of over 24 to 90 percent in villages in Thailand and 40 to 80 percent in Laos have been reported, and it is estimated that worldwide over 23 million people are infected, including 15 million in China [6-9].
Adult flukes can remain in the bile ducts for 26 years . Protective immunity does not develop and repeat infection can occur, so the cumulative worm burden can increase with age among infected individuals in endemic areas. As a result, symptomatic infection is most common in older adults and may be delayed for many years following initial infection . In endemic areas, the incidence of cholangiocarcinoma is very high. (See 'Chronic symptoms and complications' below.)
Travelers to or immigrants from endemic areas may harbor the infection and import it to nonendemic areas. Surveys in the United States have suggested that up to 26 percent of Southeast Asian immigrants have an active liver fluke infection [12,13]. In addition, frozen, dried, or pickled fish that contain surviving metacercariae can be exported to nonendemic areas, so occasionally infection occurs in individuals who have never traveled to endemic areas [14-16].