trematodes

Trematodes, commonly known as flukes, are a class of parasitic flatworms within the phylum Platyhelminthes. They are obligate parasites of animals, including humans, and comprise numerous species of medical and veterinary importance. Most trematodes are digeneans with complex lifecycles that require mollusks as the first intermediate host and vertebrates as definitive hosts; some have three-host cycles.

Morphology and biology: Adults are dorsoventrally flattened and typically leaf-shaped, lacking a true digestive tract in the adult stage and possessing a tegument specialized for nutrient absorption. They usually bear anterior or ventral suckers and often have a complex arrangement of reproductive organs. Reproduction is frequently hermaphroditic, though some species, such as Schistosoma, are dioecious. Eggs produced by the adults hatch into free-swimming stages, initiating life cycles in snail hosts.

Lifecycle: In many digeneans, eggs exit the definitive host via feces or urine and release miracidia that infect freshwater snails. Inside the snail, they develop into sporocysts and rediae, producing cercariae that exit the snail and encyst as metacercaria on vegetation or within a second intermediate host. When the definitive host ingests the metacercariae, maturation occurs in organs such as the liver, lungs, or intestines, depending on the species.

Public health relevance: Several trematodes are major human pathogens, including Schistosoma spp. (blood flukes), Fasciola hepatica (liver fluke), Clonorchis sinensis and Opisthorchis spp. (liver flukes), and Paragonimus westermani (lung fluke). Infections cause a range of diseases such as schistosomiasis, fascioliasis, clonorchiasis, opisthorchiasis, and paragonimiasis. Diagnosis typically relies on finding eggs in stool or urine, with serology or imaging as adjuncts. Treatment commonly uses praziquantel, with triclabendazole reserved for fascioliasis.