Along with the cestodes, the trematodes (Class Trematoda) belong to the Phylum Platyhelminthes, the flatworms. Trematodes generally have flattened bodies, a blind alimentary tract, suckers for attachment to their hosts, and are hermaphrodites. The Class is split into the Monogenea, which includes important ectoparasites of fish, and the Digenea, including species parasitic in the bile ducts, alimentary and respiratory tracts, and blood vessels of vertebrates.
The most important trematodes from a veterinary point of view are the liver flukes, which cause severe disease and production loss in farmed ruminants in particular. Flukes generally use snail intermediate hosts, and are more of a problem where conditions favour high snail densities, e.g. on wet land or in wet years. One species uses ants as a second intermediate host and manipulates the ant's behaviour such that it climbs onto the tips of grass blades and grasps them, increasing the chances of ingestion by a passing grazer. The schistosomes are widely represented in vertebrates but are especially important in humans, in which they cause bilharzia. Another family, the Opisthorchiidae, use fish as second intermediate hosts, and can cause liver disease in people who inadvertently eat poorly cooked infected fish. The life cycles of the digenetic trematodes, and factors involved in transmission and pathogenesis, are invariably complex and in many species are still poorly known.