Trematode life cycle stages

Trematodes or flukes are small parasitic flatworms that use vertebrates as their definitive host, and molluscs (usually freshwater snails or land snails) as their intermediate host. To accomplish this, they have several varied life cycle stages.

The life cycle of a typical digenean trematode can be thought to begin when its egg is immersed in water. Following this, a miracidium hatches, which swims to find a mollusc host. The miracidia go through several stages in the mollusc host, eventually emerging as motile cercarial larvae. The cercaria either infects vertebrates through the skin or is ingested. In its vertebrate host, the cercaria matures to an adult form, the fluke, and lays eggs that are discharged with the host feces or urine. In the presence of open water, the eggs hatch and the miracidium stage of life is reached again.