Flukes Understanding Flukes (Trematodes) Flukes, scientifically known as Trematoda, are parasitic flatworms that require multiple hosts to complete their life cycle. They are internal parasites that infect a wide range of vertebrates, including mammals, birds, reptiles, amphibians, and fish. Their life cycle is complex. The first host is typically a freshwater snail, where the parasite reproduces asexually. The second host is usually a vertebrate, where sexual reproduction oc