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The trematode flatworm Clonorchis sinensis (the Chinese River Fluke) lives in the liver of humans and is the cause of clonorchiasis. Clonorchiasis is endemic in East Asia (east Russia and Manchuria, South Korea, mainland China [except the northwest], Taiwan, and northern Vietnam), where tens of millions of people are infected (Lim 2011). It has also been reported in non-endemic areas (including the United States), where it occurs in Asian immigrants or following the ingestion of imported undercooked or pickled freshwater fish containing metacercariae.
Infection by liver flukes can cause chronic parasitic inflammatory disease of the bile ducts. Infection occurs through ingestion of raw or undercooked fluke-infested freshwater fish. The best known species causing human infection are Clonorchis sinensis, Opisthorchis felineus, and O. viverrini. Adult flukes settle in the small intrahepatic bile ducts of the host, then live there for 20 to 30 years. The long-lived flukes cause long-lasting chronic inflammation of the bile ducts and this produces epithelial hyperplasia, periductal fibrosis, and bile duct dilatation. The vast majority of human hosts are asymptomatic, but individuals with heavy infections may suffer from lassitude and nonspecific abdominal complaints. Complications include stone formation, recurrent pyogenic cholangitis, and cholangiocarcinoma. Approximately 35 million people are infected with liver flukes throughout the world and the exceptionally high incidence of cholangiocarcinoma in some endemic areas (e.g., in Korea and Thailand) is closely associated with a high prevalence of liver fluke infection. Lim (2011) reviewed the parasitology, epidemiology, and clinical findings and complications of liver fluke infection. (Lim 2011 and references therein)
The life cycle of C. sinensis involves both a first intermediate snail host and a second intermediate fish host. Embryonated eggs are discharged in the biliary ducts and stool of a human host. An adult fluke lays 2000 to 4000 eggs each day. If these eggs are ingested by a suitable intermediate snail host, the eggs release miracidia, which go through several developmental stages: sporocyst, redia, and cercaria. The cercariae are released from the snail and after a short period of free-swimming time in water, they may come in contact with and penetrate the flesh of a freshwater fish (13 genera are reported to be the most common intermediate fish hosts, all in the family Cyprinidae: Pseudorasbora, Ctenopharyngodon, Mylopharyngodon, Abbottina, Rhodeus, Hemiculter, Sarcocheilichthys, Hemibarbus, Acanthorhodeus, Puntungia, Pseudogobio, Gnathopogon, and Acheilognathus). Here they encyst as metacercariae. Infection of humans occurs by ingestion of undercooked, salted, pickled, or smoked freshwater fish. After ingestion, the metacercariae excyst in the duodenum (first portion of the small intestine) and ascend the biliary tract through the ampulla of Vater (=hepatopancreatic ampulla, where the pancreatic and bile ducts come together). Maturation takes approximately one month. The adult flukes (which measure 10 to 25 mm by 3 to 5 mm) reside in small and medium-sized biliary ducts. In addition to humans, carnivorous animals can serve as reservoir hosts.
(Centers for Disease Control Parasites and Health Website; Kaewkes 2003 and references therein)