David Blair^1
1College of Science and Engineering, James Cook University, Australia

Trematodes are parasitic flatworms. All have a life cycle involving at least two hosts, one of them always being a mollusc. Humans are the definitive hosts for some species. Multiplication of adult worms does not occur within the human host: eggs must pass out of the body to the environment for the life cycle to continue.  There are two principal routes by which humans can be infected. Direct skin penetration by cercariae in freshwater allows schistosomes to enter the body. These worms mature in blood vessels of the intestine or bladder wall. Schistosomes that normally infect birds sometimes penetrate human skin but do not mature: this is “swimmers’ itch”.  The second route of infection is by ingestion of food containing infective metacercariae. These mature in different sites depending on species: liver (Fasciola, Opisthorchis and others with metacercariae on vegetation or in fish); lungs (with ectopic infections in other organs, Paragonimus species with metacercariae in freshwater crabs/crayfish); digestive tract (echinostomes, Fasciolopsis and others, with metacercaria on aquatic vegetation or various invertebrates). For an update on pathogenesis, see Chai and Jung. Pathogenesis of Trematode Infections (Blood, Liver and Lung Flukes). Molecular Medical Microbiology, 2024, 2965–3001.