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Schistosoma is agenus oftrematodes, commonly known asblood flukes. They areparasiticflatworms responsible for a highly significant group ofinfections inhumans termedschistosomiasis, which is considered by theWorld Health Organization to be the second-most socioeconomically devastating parasiticdisease (aftermalaria), infecting millions worldwide.[1][2]
Adult flatworms parasitize blood capillaries of either themesenteries orplexus of the bladder, depending on the infecting species. They are unique among trematodes and any other flatworms in that they aredioecious with distinctsexual dimorphism betweenmale andfemale. Thousands of eggs are released and reach either the bladder or the intestine (according to the infecting species), and these are then excreted inurine orfeces tofresh water.Larvae must then pass through an intermediate snailhost before the next larval stage of the parasite emerges that can infect a new mammalian host by directly penetrating the skin.
The origins of thisgenus remain unclear. For many years it was believed that this genus had an African origin, butDNA sequencing suggests that thespecies (S. edwardiense andS. hippopotami) that infect the hippo (Hippopotamus amphibius) could be basal. Since hippos were present in both Africa and Asia during theCenozoic era, the genus might have originated asparasites of hippos.[3] The original hosts for the South East Asian species were probablyrodents.[4]
Based on thephylogenetics of the host snails it seems likely that the genus evolved inGondwana between70 million years ago and120 million years ago.[5]
Thesister group toSchistosoma is a genus ofelephant-infecting schistosomes —Bivitellobilharzia.
Thecattle,sheep,goat and cashmeregoat parasiteOrientobilharzia turkestanicum appears to be related to the African schistosomes.[6][7] This latter species has since been transferred to the genusSchistosoma.[8]
Within thehaematobium groupS. bovis andS. curassoni appear to be closely related as doS. leiperi andS. mattheei.[citation needed]
S. mansoni appears to have evolved inEast Africa 0.43–0.30 million years ago.[citation needed]
S. mansoni andS. rodhaini appear to have shared a common ancestor between 107.5 and 147.6 thousand years ago.[9] This period overlaps with the earliest archaeological evidence for fishing in Africa. It appears thatS. mansoni originated in East Africa and experienced a decline in effective population size 20-90 thousand years ago before dispersing across the continent during theHolocene. This species was later transmitted to the Americas by the slave trade.
S. incognitum andS. nasale are more closely related to the African species rather than thejaponicum group.[citation needed]
S. sinensium appears to have radiated during thePliocene.[10][11]
S. mekongi appears to have invaded South EastAsia in the mid-Pleistocene.[4]
Estimated speciation dates for thejaponicum group: ~3.8 million years ago forS. japonicum/South East Asian schistosoma and ~2.5 million years ago forS. malayensis/S. mekongi.[4]
Schistosoma turkestanicum is found infecting red deer inHungary. These strains appear to have diverged from those found inChina andIran.[12] The date of divergence appears to be 270,000 years before present.
The genusSchistosoma as currently[when?] defined isparaphyletic,[13] so revisions are likely. Over twenty species are recognised within this genus.
The genus has been divided[citation needed] into four groups:indicum,japonicum,haematobium andmansoni. The affinities of the remaining species are still being clarified.
Thirteen species are found in Africa. Twelve of these are divided into two groups—those with a lateral spine on the egg (mansoni group) and those with a terminal spine (haematobium group).
The fourmansoni group species are:S. edwardiense,S. hippotami,S. mansoni andS. rodhaini.
The ninehaematobium group species are:S. bovis,S. curassoni,S. guineensis,S. haematobium,S. intercalatum,S. kisumuensis,S. leiperi,S. margrebowiei andS. mattheei.
S. leiperi andS. matthei appear to be related.[14]S. margrebowiei is basal in this group.[15]S. guineensis is the sister species to theS. bovis andS. curassoni grouping.S. intercalatum may actually be a species complex of at least two species.[16][17]
Theindicum group has three species:S. indicum,S. nasale andS. spindale. This group appears to have evolved during the Pleistocene. All use pulmonatesnails as hosts.[18]S. spindale is widely distributed in Asia, Africa, and India.[citation needed].[19]
S. indicum is found in India andThailand.[citation needed]
The indicum group appears to be the sister clade to the African species.[20]
Thejaponicum group has five species:S. japonicum,S. malayensis andS. mekongi,S. ovuncatum andS. sinensium and these species are found inChina and Southeast Asia.[21]
S. ovuncatum forms aclade withS. sinensium and is found in northern Thailand. The definitive host is unknown and the intermediate host is the snailTricula bollingi. This species is known to use snails of thefamilyPomatiopsidae as hosts.[21]
S. incognitum appears to be basal in this genus. It may be more closely related to the African-Indian species than to the Southeast Asian group. This species uses pulmonate snails as hosts.[citation needed] Examination of the mitochondria suggests thatSchistosoma incognitum may be a species complex.[22]
As of 2012, four additional species have been transferred to this genus.,[8] previously classified as species in the genusOrientobilharzia. Orientobilharzia differs from Schistosoma morphologically only on the basis of the number of testes. A review of the morphological and molecular data has shown that the differences between these genera are too small to justify their separation. The four species are
The hybridS. haematobium-S.guineenis was observed in Cameroon in 1996.S. haematobium could establish itself only after deforestation of the tropical rainforest inLoum next to the endemicS. guineensis; hybridization led to competitive exclusion ofS. guineensis.[23]
In 2003, aS. mansoni-S. rodhaini hybrid was found in snails in westernKenya,[24] As of 2009, it had not been found in humans.[25]
In 2009,S. haematobium–S. bovis hybrids were described in northern Senegalese children. TheSenegal River Basin had changed very much since the 1980s after theDiama Dam in Senegal and theManantali Dam in Mali had been built. The Diama dam prevented ocean water to enter and allowed new forms of agriculture. Human migration, increasing number of livestock and sites where human and cattle both contaminate the water facilitated mixing between the different schistosomes inN'Der, for example.[25] The same hybrid was identified during the 2015 investigation of a schistosomiasis outbreak onCorsica, traced to theCavu river.[26]
In 2019, aS. haematobium–S. mansoni hybrid was described in a 14-year-old patient withhematuria fromCôte d'Ivoire.[27]
Acladogram based on18S ribosomal RNA,28S ribosomal RNA, and partialcytochrome c oxidase subunit I (COI) genes shows phylogenic relations of species in the genusSchistosoma:[28]
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Geographical areas associated with schistosomiasis by the World Health Organization as of January 2017 include in alphabetical order: Africa, Brazil, Cambodia, the Caribbean, China, Corsica, Indonesia, Laos, the Middle East, the Philippines, Suriname, and Venezuela.[29] There had been no cases in Europe since 1965, until an outbreak occurred on Corsica.[26]
The parasitic flatworms ofSchistosoma cause a group of chronic infections calledschistosomiasis known also as bilharziasis.[30] An anti-schistosome drug is aschistosomicide.
Parasitism of humans bySchistosoma appears to have evolved at least three occasions in bothAsia andAfrica.
| Scientific Name | First Intermediate Host | Endemic Area |
|---|---|---|
| Schistosoma guineensis | Bulinus forskalii | West Africa |
| Schistosoma intercalatum | Bulinus spp | Africa |
| Schistosoma haematobium | Bulinus spp. | Africa,Middle East |
| Schistosoma japonicum | Oncomelania spp. | China,East Asia,Philippines |
| Schistosoma malayensis | Robertsiella spp. | Southeast Asia |
| Schistosoma mansoni | Biomphalaria spp. | Africa,South America,Caribbean,Middle East |
| Schistosoma mekongi | Neotricula aperta | Southeast Asia |
Schistosoma indicum,Schistosoma nasale,Schistosoma spindale,Schistosoma leiperi are all parasites ofruminants.[citation needed]
Schistosoma edwardiense andSchistosoma hippopotami are parasites of the hippo.[citation needed]
Schistosoma ovuncatum andSchistosoma sinensium are parasites of rodents.[citation needed]
Adult schistosomes share all the fundamental features of the digenea. They have a basicbilateral symmetry, oral and ventral suckers, a body covering of asyncytialtegument, a blind-endingdigestive system consisting ofmouth,esophagus and bifurcatedcaeca; the area between the tegument and alimentary canal filled with a loose network ofmesodermcells, and an excretory or osmoregulatory system based onflame cells. Adult worms tend to be 10–20 mm (0.39–0.79 in) long and useglobins from their hosts'hemoglobin for their own circulatory system.
Unlike other trematodes and basically all other flatworms, the schistosomes aredioecious,i.e., the sexes are separate. The two sexes display a strong degree ofsexual dimorphism, and the male is considerably larger than the female. The male surrounds the female and encloses her within hisgynacophoric canal for the entire adult lives of the worms. As the male feeds on the host's blood, he passes some of it to the female. The male also passes on chemicals which complete the female's development, whereupon they will reproduce sexually. Although rare, sometimes mated schistosomes will "divorce", wherein the female will leave the male for another male. The exact reason is not understood, although it is thought that females will leave their partners to mate with more genetically distant males. Such a biological mechanism would serve to decrease inbreeding, and may be a factor behind the unusually high genetic diversity of schistosomes.[32]
The genomes ofSchistosoma haematobium,S. japonicum andS. mansonihave been reported.[33][34][35][36]
The eggs of theseparasites were first seen byTheodor Maximilian Bilharz, aGermanpathologist working inEgypt in 1851 who found the eggs ofSchistosoma haematobium during the course of apost mortem. He wrote two letters to his former teachervon Siebold in May and August 1851 describing his findings. Von Siebold published a paper in 1852 summarizing Bilharz's findings and naming the wormsDistoma haematobium.[37] Bilharz wrote a paper in 1856 describing theworms more fully.[38] Their unusualmorphology meant that they could not be comfortably included inDistoma. So in 1856 Meckel von Helmsback (de) created thegenusBilharzia for them.[39] In 1858David Friedrich Weinland proposed the nameSchistosoma (Greek: "split body") because the worms were not hermaphroditic but had separate sexes.[40] DespiteBilharzia having precedence, thegenus nameSchistosoma was officially adopted by theInternational Commission on Zoological Nomenclature. The termBilharzia to describe infection with these parasites is still in use in medical circles.[citation needed]
Bilharz also describedSchistosoma mansoni, but this species was redescribed byLouis Westenra Sambon in 1907 at theLondon School of Tropical Medicine who named it after his teacherPatrick Manson.[41]
In 1898, all then known species were placed in asubfamily by Stiles and Hassel. This was elevated to family status byLooss in 1899. Poche in 1907 corrected agrammatical error in the family name. Thelife cycle ofSchistosoma mansoni was determined by the Brazilian parasitologistPirajá da Silva (1873-1961) in 1908.[42]
In 2009, the genomes ofSchistosoma mansoni andSchistosoma japonicum were decoded[33][34] opening the way for new targeted treatments. In particular, the study discovered that the genome ofS. mansoni contained 11,809genes, including many that produceenzymes for breaking downproteins, enabling the parasite to bore through tissue. Also,S. mansoni does not have an enzyme to make certainfats, so it must rely on its host to produce these.[43]
Praziquantel is the current drug of choice against schistosomiasis. It is effective against all schistosome species that infect humans[44][45]. Oxamniquine is effective againstSchistosoma mansoni infections, but relatively ineffective againstSchistosoma haematobium andSchistosoma japonicum.[44]
{{cite book}}:ISBN / Date incompatibility (help) From p. 114:"Bilharz beschrieb zuerst in v. Siebold u. Kölliker's Zeitschr. f. Zoologie 1852. einen neuen Eingeweidewurm des Menschen, sehr den Distomen ähnlich und deshalb von ihmDistomum haematobium genannt. Der Art-Name ist sehr bezeichnend, der Gattungs-Name darf nicht füglich Distoma bleiben, ist durch Bilharzia zu ersetzen." (Bilharz first described in von Siebold and Kölliker'sJournal for [Scientific] Zoology of 1852 a new intestinal worm of humans, [which is] very similar to the Distoma and therefore was named by himDistomum haematobium. The species name is very characteristic; the genus name may not justifiably remain Distoma; [it] is to be replaced by Bilharzia.){{citation}}: CS1 maint: work parameter with ISBN (link)
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