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A genus of trematodes, ''Schistosoma'', commonly known as blood-flukes, are parasitic flatworms responsible for a highly significant group of infections in humans termed schistosomiasis. Schistosomiasis is considered by the World Health Organization as the second most socioeconomically devastating parasitic disease, (after malaria), with hundreds of millions infected worldwide. Adult flatworms parasitize blood capillaries of either the mesenteries or plexus of the bladder, depending on the infecting species. They are unique among trematodes and any other flatworms in that they are dioecious with distinct sexual dimorphism between male and female. Thousands of eggs are released and reach either the bladder or the intestine (according to the infecting species), and these are then excreted in urine or feces to fresh water. Larvae must then pass through an intermediate snail host, before the next larval stage of the parasite emerges that can infect a new mammalian host by directly penetrating the skin. == Evolution == The origins of this genus remain unclear. For many years it was believed that this genus had an African origin, but DNA sequencing suggests that the species (''S. edwardiense'' and ''S. hippopotami'') that infect the hippo (''Hippopotamus amphibius'') could be basal. Since hippos were present in both Africa and Asia during the Cenozoic era the genus might have originated as parasites of hippos. The original hosts for the South East Asian species were probably rodents. Based on the phylogenetics of the host snails it seems likely that the genus evolved in Gondwana between and .〔Beer SA, Voronin MV, Zazornova OP, Khrisanfova GG, Semenova SK (2010) Phylogenetic relationships among schistosomatidae. Med Parazitol (Mosk) 2010 (2):53-59〕 The sister group to ''Schistosoma'' is a genus of elephant-infecting schistosomes — ''Bivitellobilharzia''. The cattle, sheep, goat and cashmere goat parasite ''Orientobilharzia turkestanicum'' appears to be related to the African schistosomes. This latter species has since been transferred to the genus ''Schistosoma''.〔Aldhoun JA, Littlewood DT (2012) ''Orientobilharzia'' Dutt & Srivastava, 1955 (Trematoda: Schistosomatidae), a junior synonym of ''Schistosoma'' Weinland, 1858. Syst Parasitol 82(2):81-8. doi: 10.1007/s11230-012-9349-8〕 Within the ''haematobium'' group ''S. bovis'' and ''S. curassoni'' appear to be closely related as do ''S. leiperi'' and ''S. mattheei''. ''S. mansoni'' appears to have evolved in East Africa 0.43–0.30 million years ago. ''S. incognitum'' and ''S. nasale'' are more closely related to the African species rather than the ''japonicum'' group. ''S. sinensium'' appears to have radiated during the Pliocene. ''S. mekongi'' appears to have invaded South East Asia in the mid-Pleistocene. Estimated speciation dates for the ''japonicum'' group: ~3.8 million years ago for ''S. japonicum''/South East Asian schistosoma and ~2.5 million years ago for ''S. malayensis''/''S. mekongi''. ''Schistosoma turkestanicum'' is found infecting red deer in Hungary. These strains appear to have diverged from those found in China and Iran.〔Lawton SP, Majoros G (2013) A foreign invader or a reclusive native? DNA bar coding reveals a distinct European lineage of the zoonotic parasite ''Schistosoma turkestanicum'' (syn. ''Orientobilharzia turkestanicum''). Infect Genet Evol 14:186-93. doi: 10.1016/j.meegid.2012.11.013〕 The date of divergence appears to be 270,000 years before present. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「schistosoma」の詳細全文を読む スポンサード リンク
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