Trematoda

Trematodes are commonly referred to as flukes. This term can be traced back to the Old English name for flounder, and refers to the flattened, rhomboidal shape of the organisms. The etymology of trematode stems from the Greek word trēmatṓdēs, which means "pierced with holes", and refers to the worm's sucker, which pierces a hole in the host while the worm is attached and feeding. ==Taxonomy==

There are 18,000 to 24,000 known species of trematodes, divided into two subclasses — the Aspidogastrea and the Digenea. Aspidogastrea is the smaller subclass, comprising 61 species. These flukes mainly infect bivalves and bony fishes. Digenea — which comprise the majority of trematodes — are found in certain mollusks and vertebrates. ==Trematodes of medical importance==

Flukes that cause disease in humans are often classified based on the organ system they infect such as the blood, liver, lungs, and intestines: Blood flukes Blood flukes inhabit the blood in some stages of their life cycle. Blood flukes that cause disease in humans include Trichobilharzia regenti, which causes swimmer's itch, and seven species of genus Schistosoma which cause schistosomiasis: S. guineensis, S.haematobium, S. intercalatum, S. japonicum, S. malayensis, S. mansoni, S. mekongi. As a definitive host, humans are infected when the cercariae (the larval forms of trematodes) penetrate the skin. Any contact with water containing these cercariae can potentially result in infection. Adult blood flukes can live for years in human or animal reservoir hosts. S. haematobium and S. japonicum are of particular importance, as these are carcinogenic parasites. S. haematobium, which infects the urinary bladder, is among the most important causes of bladder cancer in humans. This organism is classified by the International Agency for Research on Cancer (IARC) as a Group 1 (extensively proven) carcinogen. S. japonicum is associated with the development of liver cancer, and is classified as a Group 2B (possibly carcinogenic to humans) carcinogen. Lung flukes Lung flukes: there are ten species of lung flukes that infect humans, causing paragonimiasis. Of these, the most common cause of human paragonimiasis is Paragonimus westermani, the oriental lung fluke. Lung flukes require three different hosts in order to complete their life cycle. The first intermediate host is a snail, the second intermediate host is a crab or crayfish, and the definitive host for lung flukes is a human or other animal. Intestinal flukes Intestinal flukes inhabit the epithelium of the small intestine. These include Fasciolopsis buski (which causes fasciolopsiasis), Metagonimus miyatai, Metagonimus takahashii, Metagonimus yokogawai (which cause metagonimiasis), and Heterophyes heterophyes and Heterophyes nocens (which cause heterophyiasis). ==Anatomy==

Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although species as small as 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 of trematodes comprises a tough syncytial tegument, which helps protect against digestive enzymes in those species that inhabit the gut of larger animals. It is also the surface of gas exchange; there are no respiratory organs. Oral sucker and acetabulum: In some species of Trematoda, such as T. bragai, there is an acetabulum. This saucer-shaped organ is attached to the oral sucker in some Trematodes and other parasitic worms. This allows for parasitic worms to attach to their host by penetrating the host's tissue with spines lining the acetabulum organ. In trematodes, the oral sucker is linked to the pharynx via a canal composed of meridional, equatorial, and radial muscle fibers. Reproductive system of blood flukes Most trematodes are hermaphrodites, as are many internal parasites. Blood flukes (Schistosoma) are the only form of trematodes that are dioecious (have both a male and female sex). Blood flukes are unique in the way that they can undergo both asexual and sexual reproduction. Asexual reproduction occurs in the hepatopancreas of a freshwater snail, which serves as an intermediate host. Sexual reproduction occurs later in the life cycle, in the definitive (vertebrate) host. The male reproductive system usually includes two testes, though some species may have more. The testes are located posterior and dorsal to the ventral sucker. Spermatogenesis produces biflagellate sperm (sperm with two tails). Sperm is stored in the seminal vesicles, which are connected to the testes by the vas deferens. The male reproductive system varies considerably in structure between species; this can be very useful in species identification. The female reproductive system consists of one ovary connected to an elongated uterus by a ciliated oviduct. The uterus opens to the exterior at the genital pore (the common external opening of the male and female reproductive systems). The location of the ovary varies among different species, making the female reproductive system useful in species identification. At the base of the oviduct is a copulatory duct — termed ''Laurer's canal — which is analogous to a vagina. Oocytes are released from the ovary into the oocapt (the dilated proximal end of the oviduct). Sperm cells travel from the seminal vesicles through the uterus to reach the ootype (the dilated distal part of the oviduct), where fertilization occurs. The ootype is connected via a pair of ducts to a number of vitelline ducts that produce yolk. After the egg is surrounded by yolk, its shell is formed from the secretions of Mehlis' glands, the ducts of which also open into the ootype. From the ootype, the fertilized egg then travels back into the uterus, and is ultimately released from the genital atrium''. ==Life cycles==

Trematodes have a very complex life cycle and depending on what taxa they belong to, their life cycles can be completed with as little as one host compared to the typical three hosts.  When there is one host, this is normally a specific species of snail of the family Lymnaeidae. Almost all trematodes infect molluscs as the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes are monoecious and alternately reproduce sexually and asexually. The two main exceptions to this are the Aspidogastrea, which have no asexual reproduction, and the schistosomes, which are dioecious. Soldiers are smaller, more mobile, and develop along a different pathway than sexually mature reproductives. One big difference is their mouthparts (pharynx), which are five times as big as those of the reproductives. They make up nearly a quarter of the volume of the soldier. These soldiers don't have a germinal mass, can't metamorphose to be reproductive, and are, therefore, obligately sterile. Soldiers are readily distinguished from the immature and mature reproductive worms. Soldiers are more aggressive than reproductives, attacking heterospecific trematodes that infect their host in vitro. Interestingly, H. pumilio soldiers do not attack conspecifics from other colonies. The soldiers are not evenly distributed throughout the host body. They're found in the highest numbers in the basal visceral mass, where competing trematodes tend to multiply during the early phase of infection. This strategic positioning allows them to effectively defend against invaders, similar to how soldier distribution patterns are seen in other animals with defensive castes. They "appear to be an obligately sterile physical caste, akin to that of the most advanced social insects". ==Infections==

Trematodes can cause disease in many types of vertebrates, including mammals, birds, reptiles, and fish. Cattle and sheep can become infected by eating contaminated food. These infections lead to a reduction in milk or meat production, which can be of significant economic importance to the livestock industry. Treatment Albendazole can be used to treat clonorchiasis and opisthorchiasis. Triclabendazole is often used to treat fasciolosis, and may also be useful in the treatment of paragonimiasis and dicrocoeliasis. Praziquantel is effective in the treatment of all diseases caused by flukes (clonorchiasis, dicrocoeliasis, echinostomiasis, fasciolopsiasis, fasciolosis, gastrodiscoidiasis, heterophyiasis, metagonimiasis, opisthorchiasis, paragonimiasis, and schistosomiasis). ==References==