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The trematode parasite known as the Cat Liver Fluke (Opisthorchis felineus) is found mainly in Eastern Europe and the former Soviet Union 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 liver flukes causing human infection are Opisthorchis felineus, O. viverrini, and Clonorchis sinensis. 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 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)
Kaewkes (2003) reviewed the taxonomy and biology of Opisthorchis felineus, O. viverrini, 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. Six fish genera are reportedly the most important secondary intermediate hosts for O. felineus: Abramis, Aspius, Leuciscus, Rutilus, Tinca and Phoxinus (all family Cyprinidae). The mammalian definitive host (cats, dogs, and various fish-eating mammals, including humans) become infected by ingesting 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 7 to 12 mm by 2 to 3 mm) reside in the biliary and pancreatic ducts of the mammalian host, where they attach to the mucosa.