Trematodes are small parasitic flatworms that use vertebrates as their definitive host, and molluscs as their intermediate host. In order to accomplish this, they have several varied lifecyle stages.
The lifecycle 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 cercaria larvae.[1] The cercaria infect vertebrates either through the skin or are ingested. In its vertebrate host, the cercaria matures to an adult form, and lays eggs that are discharged with the host faeces or urine. In the presence of open water, the eggs will hatch and the miracidium stage of life is reached again.
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While the details vary with each species, the general lifecycle stages are as follows:
The typical sequence (eggs, miracidia, sporocysts, rediae, ceracariae, and adults) is by no means the rule for all trematode species. In some species the redia stage is omitted and sporocysts produce cercariae. In some species, the cercaria develops into an adult within the same host.
Many digenean trematodes require two hosts, one (typically a snail) where asexual reproduction occurs in sporocysts, the other a vertebrate (typically a fish) where the adult form engages in sexual reproduction to produce eggs. In some species (for example Ribeiroia) the cercaria encysts, and waits until the host is eaten by a third host, in whose gut it emerges and develops into an adult.
Most trematodes are hermaphroditic, but members of the family Schistosomatidae are dioecious. Males are shorter and stouter than the females.[2]