Echinostomiasis is transmitted through snails and frogs.
A wide variety of domesticated and wild mammals and birds serve as reservoirs. Animals such as ducks, geese, pigeons, and chickens are known to harbor the parasite. All aquatic organisms that maintain the first and second intermediate hosts in the endemic area also serve as reservoirs. (Graczyk and Fried, 1998)
The echinostome adult fluke is attached to the intestinal wall of the human host and produces eggs that are passed in the feces. When the eggs reach water, miracidia hatch and penetrate the first intermediate hosts: snails. Over the course of 6-7 weeks, they develop into sporocysts, mother rediae, daughter rediae, and finally cercariae. At this stage, the cercariae leave the snails to find the second intermediate hosts, such as freshwater snails, fish, and tadpoles. Once a host is found, the cercariae encyst in the host to wait for an unwitting human to ingest the raw or undercooked second intermediate host (Go, 2003).
(Markell, John, Krotoski, Medical Parasitology, p. 195)