Alaria

This parasite has one of the most complex of all parasite life cycles, requiring up to 4 different hosts. Adults live in the intestine of canids, often occurring in numbers so large that they appear to coat the lining of the intestine. They are hermaphrodites, and release unembryonated eggs which pass in the feces.

After a few weeks, the eggs have embryonated and a miracidium hatches from them. The miracidium prefers snails of the genus Helisoma. Within the snail, the miracidium transforms into a mother sporocyst that produces daughter sporocysts. The daughter sporocysts require about 1 year to mature and begin producing cercariae. The cercariae alternately swim upwards and sink slowly, until they contact a tadpole and penetrate the skin. Within the tissues of the tadpole the cercaria transforms into a mesocercaria, a life cycle stage unique to this parasite. The mesocercaria is intermediate in development between the cercaria and diplostomulum stage, and can survive the metamorphosis of the tadpole into an adult frog or toad.

If the tadpole or frog is eaten by a canid, the mesocercariae are digested out and penetrate through the intestine. They begin a migration through the abdominal cavity towards the diaphragm, and after a few weeks are found in the lungs. After another month they develop into a diplostomulum. The diplostomulae migrate up the trachea and are swallowed, re-entering the intestine where they develop into adults. This sort of migration, where a larval parasite leaves the intestine and undergoes further larval development before re-entering the intestine to mature, is quite common among a variety of parasites.

If a tadpole or frog is eaten by a non-canid (such as a rodent, snake or other frog) the mesocercariae will enter the tissues of the new host but remain as mesocercariae. These hosts thus serve as paratenic hosts. No development of the parasite occurs, and these hosts are not absolutely necessary for the parasite to complete its life cycle, but they greatly facilitate the transfer of the parasite from the obligate second intermediate host (tadpoles) to the obligate definitive host (canids). Canids do not normally feed on tadpoles, and the paratenic hosts serve to bioaccumulate the mesocercariae. Once the paratenic host is eaten by a definitive host, the life cycle proceeds normally.