Trematoda

Trematoda, commonly known as flukes, is a class of parasitic flatworms within the phylum Platyhelminthes. Most trematodes are endoparasites of vertebrates, including humans, and they possess complex life cycles that typically involve one or more intermediate hosts, usually mollusks such as freshwater snails, before reaching a definitive vertebrate host.

Morphology and anatomy: Adult trematodes are dorsoventrally flattened and leaf-shaped, with a syncytial tegument that protects them in host tissues. They have oral and ventral suckers for attachment, and many exhibit a reduced digestive system, absorbing nutrients across the tegument. Reproductive anatomy is highly developed; many species are hermaphroditic, though several groups, notably schistosomes (family Schistosomatidae), are dioecious with separate male and female individuals.

Life cycle: Trematodes typically have multipart life cycles. A molluscan first intermediate host disseminates larvae that develop from miracidia into sporocysts and then rediae, producing cercariae. Cercariae leave the snail and infect the next host, which may be a second intermediate host or the definitive host, where they mature into adults. In human pathogens, infection can occur via skin penetration by cercariae (Schistosoma spp.) or ingestion of metacercariae in undercooked fish or aquatic vegetation (Clonorchis sinensis, Opisthorchis spp., Paragonimus spp., Fasciola spp., among others).

Clinical and economic importance: Trematodes infect a wide range of vertebrates and are distributed globally, with major impacts on human health and livestock. Notable examples include Schistosoma mansoni, S. haematobium and S. japonicum; Clonorchis sinensis and Opisthorchis viverrini; Fasciola hepatica; Fasciolopsis buski; Paragonimus westermani. Diseases arise from migration, organ invasion, or obstruction by adult flukes in the biliary tract, lungs, or intestines, and from immune responses to larval stages.

Taxonomy: Trematoda comprises two major lineages, the primitive Aspidogastrea and the more diverse Digenea, which contains many families such as Schistosomatidae, Opisthorchiidae, Heterophyidae, Fasciolidae, Paragonimidae, and Diplostomatidae.