Schistosoma

Electron micrograph of an adult male Schistosoma parasite worm. The bar (bottom left) represents a length of 500 μm.

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.[3] The original hosts for the South East Asian species were probably rodents.[4]

Based on the phylogenetics of the host snails it seems likely that the genus evolved in Gondwana between 70 million years ago and 120 million years ago.[5]

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.[6][7] This latter species has since been transferred to the genus Schistosoma.[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. mansoni and S. 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 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.[10][11]

S. mekongi appears to have invaded South East Asia in the mid-Pleistocene.[4]

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.[4]

Schistosoma turkestanicum is found infecting red deer in Hungary. These strains appear to have diverged from those found in China and Iran.[12] The date of divergence appears to be 270,000 years before present.

The genus Schistosoma as currently defined is paraphyletic, so revisions are likely. Over twenty species are recognised within this genus.

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).

• 
  Schistosoma japonicum
• 
  Schistosoma haematobium
• 
  Schistosoma mansoni
• 
  Schistosoma mekongi
• 
  Schistosoma intercalatum

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.[28] There had been no cases in Europe since 1965, until an outbreak occurred on Corsica.[25]

The parasitic flatworms of Schistosoma cause a group of chronic infections called schistosomiasis known also as bilharziasis.[29] An anti-schistosome drug is a schistosomicide.

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 syncytial tegument, a blind-ending digestive system consisting of mouth, esophagus and bifurcated caeca; the area between the tegument and alimentary canal filled with a loose network of mesoderm cells, and an excretory or osmoregulatory system based on flame cells. Adult worms tend to be 10–20 mm (0.39–0.79 in) long and use globins from their hosts' hemoglobin for their own circulatory system.

Unlike most flatworms, schistosomes are gonochoristic. The narrow female can be seen emerging from the thicker male's gynecophoral canal below his ventral sucker.

Unlike other trematodes, the schistosomes are dioecious, 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.[31]

The genomes of Schistosoma haematobium, S. japonicum and S. mansoni have been reported.[32][33][34][35]

The eggs of these parasites were first seen by Theodor Maximilian Bilharz, a German pathologist working in Egypt in 1851 who found the eggs of Schistosoma haematobium during the course of a post mortem. 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.[36] Bilharz wrote a paper in 1856 describing the worms more fully.[37] 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.[38] In 1858 David Friedrich Weinland proposed the name Schistosoma (Greek: "split body") because the worms were not hermaphroditic but had separate sexes.[39] 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.

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.[40]

In 1898, all then known species were placed in a subfamily by Stiles and Hassel. This was elevated to family status by Looss in 1899. Poche in 1907 corrected a grammatical error in the family name. The life cycle of Schistosoma mansoni was determined by the Brazilian parasitologist Pirajá da Silva (1873-1961) in 1908.[41]

In 2009, the genomes of Schistosoma mansoni and Schistosoma japonicum were decoded [32][33] opening the way for new targeted treatments. In particular, the study discovered that the genome of S. mansoni contained 11,809 genes, including many that produce enzymes for breaking down proteins, enabling the parasite to bore through tissue. Also, S. mansoni does not have an enzyme to make certain fats, so it must rely on its host to produce these.[42]

Praziquantel[43]

Wikimedia Commons has media related to Schistosoma.

• British Department for International Development Control of Schistosomiasis
• The World Health Organisation page on Schistosomiasis
• University of Cambridge Schistosome Laboratory
• Schistosoma parasites overview, biology, life cycle image at MetaPathogen