Metagonimiasis life cycle

Adults release fully 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). Snails of the genus Semisulcospira are the most frequent intermediate host for Metagonimus yokogawai. The miracidia undergo several developmental stages in the snail [ie, sporocysts (2a), rediae (2b), and cercariae (2c)]. Many cercariae are produced from each rediae. The cercariae are released from the snail (3) and encyst as metacercariae in the tissues of a suitable fresh- or 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 mm to 2.5 mm by 0.4 mm to 0.75 mm) (7). In addition to humans, fish-eating mammals (eg, cats and dogs) and birds can also be infected by M. yokogawai (8).