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.
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