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Schistosoma is a genus of , commonly known as blood flukes. They are Parasitism responsible for a highly significant group of in termed schistosomiasis, which is considered by the World Health Organization to be the second-most socioeconomically devastating parasitic disease (after malaria), infecting millions 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 Dioecy 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. 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.
Based on the phylogenetics of the host snails it seems likely that the genus evolved in Gondwana between and .
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.
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. mansoni and S. rodhaini appear to have shared a common ancestor between 107.5 and 147.6 thousand years ago. This period overlaps with the earliest archaeological evidence for fishing in Africa. It appears that S. mansoni originated in East Africa and experienced a decline in effective population size 20-90 thousand years ago before dispersing across the continent during the Holocene. This species was later transmitted to the Americas by the slave trade.
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. The date of divergence appears to be 270,000 years before present.
The genus has been divided into four groups: indicum, japonicum, haematobium and mansoni. 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).
S. leiperi and S. matthei appear to be related. S. margrebowiei is basal in this group. S. guineensis is the sister species to the S. bovis and S. curassoni grouping. S. intercalatum may actually be a species complex of at least two species.
S. indicum is found in India and Thailand.
The indicum group appears to be the sister clade to the African species.
S. ovuncatum forms a clade with S. sinensium and is found in northern Thailand. The definitive host is unknown and the intermediate host is the snail Tricula bollingi. This species is known to use snails of the family Pomatiopsidae as hosts.
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. Examination of the mitochondria suggests that Schistosoma incognitum may be a species complex.Webster BL, Littlewood DT (2012) Mitochondrial gene order change in Schistosoma (Platyhelminthes: Digenea: Schistosomatidae). Int J Parasitol 42(3):313-321
In 2003, a S. mansoni-S. rodhaini hybrid was found in snails in western Kenya, As of 2009, it had not been found in humans.
In 2009, S. haematobium–S. bovis hybrids were described in northern Senegalese children. The Senegal River Basin had changed very much since the 1980s after the Diama Dam in Senegal and the Manantali 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 in N'Der, for example. The same hybrid was identified during the 2015 investigation of a schistosomiasis outbreak on Corsica, traced to the Cavu river.
In 2019, a S. haematobium–S. mansoni hybrid was described in a 14-year-old patient with hematuria from Côte d'Ivoire.
==Comparison of eggs==
Schistosoma edwardiense and Schistosoma hippopotami are parasites of the hippo.
Schistosoma ovuncatum and Schistosoma sinensium are parasites of rodents.
Bilharz also described Schistosoma mansoni, but this species was redescribed by Louis Westenra Sambon in 1907 at the London School of Tropical Medicine who named it after his teacher Patrick Manson.See:
In 1898, all then known species were placed in a subfamily by Stiles and Hassel. This was elevated to family status by Arthur Looss in 1899. Poche in 1907 corrected a grammar error in the family name. The life cycle of Schistosoma mansoni was determined by the Brazilian parasitologist Pirajá da Silva (1873-1961) in 1908.See:
In 2009, the genomes of Schistosoma mansoni and Schistosoma japonicum were decoded opening the way for new targeted treatments. In particular, the study discovered that the genome of S. mansoni contained 11,809 , including many that produce for breaking down , enabling the parasite to bore through tissue. Also, S. mansoni does not have an enzyme to make certain , so it must rely on its host to produce these.
Cladogram
A cladogram based on 18S ribosomal RNA, 28S ribosomal RNA, and partial cytochrome c oxidase subunit I (COI) genes shows phylogenic relations of species in the genus Schistosoma:
Geographical distribution
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. WHO factsheet There had been no cases in Europe since 1965, until an outbreak occurred on Corsica.
Schistosomiasis
The parasitic flatworms of Schistosoma cause a group of chronic infections called schistosomiasis known also as bilharziasis.Britannica Concise Encyclopedia 2007 An anti-schistosome drug is a schistosomicide.
Species infecting humans
Parasitism of humans by Schistosoma appears to have evolved at least three occasions in both Asia and Africa.
+ Human Schistosomes 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
Species infecting other animals
Schistosoma indicum, Schistosoma nasale, Schistosoma spindale, Schistosoma leiperi are all parasites of .
Morphology
Adult schistosomes share all the fundamental features of the digenea. They have a basic bilateral symmetry, oral and ventral suckers, a body covering of a syncytium tegument, a blind-ending Digestion consisting of mouth, esophagus and bifurcated cecum; the area between the tegument and alimentary canal filled with a loose network of mesoderm cells, and an excretory or osmoregulatory system based on . Adult worms tend to be long and use from their hosts' hemoglobin for their own circulatory system.
Reproduction
Unlike other trematodes and basically all other flatworms, the schistosomes are Dioecy, i.e., the sexes are separate. The two sexes display a strong degree of sexual dimorphism, and the male is considerably larger than the female. The male surrounds the female and encloses her within his gynacophoric 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.
Genome
The genomes of Schistosoma haematobium, S. japonicum and S. mansoni have been reported.
History
The eggs of these parasitism were first seen by Theodor Bilharz, a Germany Pathology working in Egypt in 1851 who found the eggs of Schistosoma haematobium during the course of a Autopsy. He wrote two letters to his former teacher von Siebold in May and August 1851 describing his findings. Von Siebold published a paper in 1852 summarizing Bilharz's findings and naming the worms Distoma haematobium. See: "2. Distomum Haematobium Bilh.", pp. 59–62. Bilharz wrote a paper in 1856 describing the more fully. Their unusual morphology meant that they could not be comfortably included in Distoma. So in 1856 Meckel von Helmsback (de) created the genus Bilharzia for them. 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 ihm Distomum 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's Journal for Scientific Zoology of 1852 a new intestinal worm of humans, which very similar to the Distoma and therefore was named by him Distomum haematobium. The species name is very characteristic; the genus name may not justifiably remain Distoma; it is to be replaced by Bilharzia.) In 1858 David Friedrich Weinland proposed the name Schistosoma (Greek: "split body") because the worms were not hermaphroditic but had separate sexes. See footnote †. Despite Bilharzia having precedence, the genus name Schistosoma was officially adopted by the International Commission on Zoological Nomenclature. The term Bilharzia to describe infection with these parasites is still in use in medical circles.
Treatment
Praziquantel
Further reading
External links
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