Life cycle of Echinostoma
trematode flatworms, the agents causing echinostomiasis in humans
Many animals may serve as definitive hosts for various Echinostoma species, including aquatic birds, carnivores, rodents, and humans (a definitive host is the host in which the adult parasites occur). Unembryonated eggs are passed in feces (1) and develop in the water (2). On average, the ciliated miracidium larva takes around 10 days to mature before hatching (3) and penetrating the first intermediate host, a snail (4). Several genera of snails may serve as the first intermediate host. The intramolluscan stages include a sporocyst (4a), one or two generations of rediae (4b), and cercariae (4c). The cercariae may encyst as metacercariae within the same first intermediate host or leave the host and penetrate a new second intermediate host (5). Depending on the species, a range of animals may serve as the second intermediate host, including other snails, bivalve mollusks, fish, and tadpoles. The definitive host becomes infected after eating a second intermediate host (6). Metacercariae excyst in the duodenum (7) and adults reside in the small intestine (8) of the definitive host.
From Centers for Disease Control Parasites and Health website.