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The trematode parasite known as the Southeast Asian Liver Fluke (Opisthorchis viverrini) is found mainly in northeastern Thailand, Laos, and Kampuchea and can cause liver fluke disease in infected human hosts. Liver fluke disease is a 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 Opisthorchis viverrini, O. felineus, and Clonorchis sinensis. Adult flukes settle in the small intrahepatic bile ducts of the definitive 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. In one study, more than 90% of humans and up to 97% of individuals of second intermediate host fish species were found to be infected with O. viverrini, but prevalence in snail (first intermediate) hosts was only 0.11%, implying extremely efficient transmission of cercariae from snail to fish (Kaewkes 2003 and references therein). Lim (2011) reviewed the parasitology, epidemiology, and clinical findings and complications of liver fluke infection. (Lim 2011 and references therein)
Kaewkes (2003) reviewed the taxonomy and biology of Opisthorchis viverrini, O. felineus, Clonorchis sinensis, and related liver flukes. The adult flukes deposit fully developed eggs that are passed in the feces of the mammalian definitive host. After ingestion by a suitable snail (first intermediate host), the eggs release miracidia, which pass through several developmental stages within the snail: sporocyst, redia, and cercaria. Cercariae are released from the snail and penetrate freshwater fish (second intermediate host), encysting as metacercariae in the muscles or under the scales. Three fish genera are reportedly the most important secondary intermediate hosts for O. viverrini: Cyclocheilichthys, Puntius, and Hampala (all family Cyprinidae). The mammalian definitive hosts (cats, dogs, and various fish-eating mammals, including humans) become infected by ingesting raw or undercooked fish containing metacercariae. After ingestion, the metacercariae excyst in the duodenum (first portion of the small intestine) and ascend through the ampulla of Vater (=hepatopancreatic ampulla, where the pancreatic and bile ducts come together) into the biliary ducts, where they attach and develop into adults, which lay eggs after 3 to 4 weeks. The adult flukes (which measure 5 to 10 mm by 1 to 2 mm) reside in the biliary and pancreatic ducts of the mammalian host, where they attach to the mucosa.