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Opisthorchis life cycle

1. In the human host, the adult fluke releases embryonated eggs that are passed in feces. 2. After ingestion by a snail (first intermediate host), eggs release miracidia, which develop into sporocysts, then rediae, and then cercariae. 3. Cercariae are released from the snail and penetrate freshwater fish (2nd intermediate host). 4. The cercariae encyst as metacercariae in muscles or under the scales. The mammalian primary (definitive) host (cats, dogs, various fish-eating mammals including humans) becomes infected by ingesting undercooked fish containing metacercariae. 5. After ingestion, the metacercariae excyst in the duodenum. 6. They ascend through the ampulla of Vater into the biliary ducts, where they attach and develop into adults. They lay eggs after 3 to 4 wk.

Image from the Centers for Disease Control and Prevention Image Library.