Trematoda
Obligate internal parasites with complex, multi-host life cycles.
Trematodes, also called flukes, make up a class of flatworms that are obligate internal parasites. Their life cycle is complex and involves at least two hosts. A mollusk—usually a snail—serves as the intermediate host, where asexual reproduction takes place. A vertebrate acts as the definitive host, where sexual reproduction occurs. These parasites can cause disease in all five vertebrate groups: mammals, birds, amphibians, reptiles, and fish.
The common name "fluke" comes from the Old English word for flounder, referencing the organism’s flattened, rhomboidal shape. The scientific name *Trematoda* derives from the Greek *trēmatṓdēs*, meaning "pierced with holes," which describes the sucker that pierces the host during feeding.
Known species number between 18,000 and 24,000, split into two subclasses. Aspidogastrea is the smaller group, with 61 species that mainly infect bivalves and bony fishes. Digenea makes up the vast majority of trematodes and is found in certain mollusks and vertebrates.
Flukes that cause human disease are often grouped by the organ system they infect: blood, liver, lungs, or intestines.
Blood flukes live in the blood during some life stages. Human pathogens include *Trichobilharzia regenti* (causing swimmer’s itch) and seven *Schistosoma* species responsible for schistosomiasis: *S. guineensis*, *S. haematobium*, *S. intercalatum*, *S. japonicum*, *S. malayensis*, *S. mansoni*, and *S. mekongi*. Humans become definitive hosts when cercariae—larval forms—penetrate the skin during contact with infested water. Adult blood flukes can survive for years in human or animal reservoir hosts. *S. haematobium* and *S. japonicum* are carcinogenic. *S. haematobium*, which infects the urinary bladder, is a major cause of bladder cancer and is classified by the International Agency for Research on Cancer (IARC) as a Group 1 carcinogen (extensively proven). *S. japonicum* is linked to liver cancer and is classified as Group 2B (possibly carcinogenic to humans).
Liver flukes inhabit the bile ducts, liver, and gallbladder of certain mammals and birds. Species include *Clonorchis sinensis*, *Dicrocoelium dendriticum*, *Dicrocoelium hospes*, *Fasciola gigantica*, *Fasciola hepatica*, *Opisthorchis felineus*, and *Opisthorchis viverrini*. *Clonorchis* and *Opisthorchis* are carcinogenic and strongly associated with bile duct cancer.
Quick Facts
- Taxon
- Trematoda
Facts from the source article.
Lore & Background
Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although species as small as 1 millimeter are known. Their most distinctive external feature is the presence of two suckers, one close to the mouth, and the other on the underside of the animal. The body surface comprises a tough syncytial tegument, which helps protect against digestive enzymes in those species that inhabit the gut of larger animals. The mouth opens into a muscular pharynx, connecting to blind-ending caeca; there is no anus. The excretory system is mainly concerned with osmoregulation, consisting of protonephridia and a bladder. The brain consists of a pair of ganglia, with nerve cords running down the body. Body wall musculature has three layers: circular, longitudinal, and diagonal. Some species possess an acetabulum, a saucer-shaped organ lined with spines for attachment.
Reader's Guide
Trematodes are of major medical and veterinary significance. Blood flukes of the genus Schistosoma cause schistosomiasis, a disease affecting millions. S. haematobium is classified by the International Agency for Research on Cancer as a Group 1 carcinogen, strongly linked to bladder cancer. S. japonicum is associated with liver cancer and classified as Group 2B. Liver flukes such as Clonorchis sinensis and Opisthorchis viverrini are carcinogenic parasites strongly associated with bile duct cancer. Lung flukes, including Paragonimus westermani, cause paragonimiasis and require three hosts: a snail, a crab or crayfish, and a human or other animal. Intestinal flukes like Fasciolopsis buski cause fasciolopsiasis. Their complex life cycles, involving asexual reproduction in mollusks and sexual reproduction in vertebrates, make them challenging to control. The study of trematodes is essential for understanding parasitic diseases, their transmission, and their impact on human and animal health.
Did You Know?
- The term 'trematode' comes from the Greek word trēmatṓdēs, meaning 'pierced with holes', referring to the worm's sucker.
- Blood flukes (Schistosoma) are the only trematodes that are dioecious (separate male and female sexes).
- S. haematobium is classified by the IARC as a Group 1 carcinogen, a major cause of bladder cancer.
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