1. In the host, the adult worm releases embryonated eggs; each egg contains a fully developed miracidium. The eggs are passed in the host's feces. 2. After ingestion by a snail (first intermediate host), the eggs hatch and release miracidia, which penetrate the snail's intestine. Miracidia develop into sporocysts, then rediae, and then cercariae. Many cercariae are produced from each redia. 3. Cercariae are released from the snail. 4. The cercariae encyst as metacercariae in tissues of a freshwater or brackish-water fish (2nd intermediate host). 5. The definitive host becomes infected by ingesting raw, undercooked, or salted fish containing metacercariae. 6. After ingestion, the metacercariae excyst and attach to the mucosa of the small intestine. 7. There, they mature into adults. 8. In addition to humans, various fish-eating mammals (eg, cats, dogs) and birds can be infected by Heterophyes heterophyes .
Image from the Centers for Disease Control and Prevention Image Library.