Life cycle of the trematode parasite Heterophyes heterophyes
Adults release embryonated eggs, each with a fully-developed miracidium, and eggs are passed in the host's feces (1). After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia which penetrate the snail’s intestine (2). Genera Cerithidia and Pironella are important snail hosts in Asia and the Middle East, respectively. The miracidia pass through several developmental stages in the snail: sporocysts (2a), rediae (2b), and cercariae (2c). Many cercariae are produced from each redia. The cercariae are released from the snail (3) and encyst as metacercariae in the tissues of a suitable fresh/brackish water fish (second intermediate host) (4). The definitive host becomes infected by ingesting undercooked or salted fish containing metacercariae (5). After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine (6) and mature into adults (measuring 1.0 to 1.7 mm by 0.3 to 0.4 mm) (7). In addition to humans, various fish-eating mammals (e.g., cats and dogs) and birds can be infected by H. heterophyes.
From Centers for Disease Control Parasites and Health website.