Schistosoma is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans 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 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 penetrating the skin.

Evolution

thumb|left|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. 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 .

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. The date of divergence appears to be 270,000 years before present.

Taxonomy

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).

Mansoni group

The four mansoni group species are: S. edwardiense, S. hippotami, S. mansoni and S. rodhaini.

Haematobium group

The nine haematobium group species are: S. bovis, S. curassoni, S. guineensis, S. haematobium, S. intercalatum, S. kisumuensis, S. leiperi, S. margrebowiei and S. mattheei.

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.

Indicum group

The indicum group has three species: S. indicum, S. nasale and S. spindale. This group appears to have evolved during the Pleistocene. All use pulmonate snails as hosts. S. spindale is widely distributed in Asia, Africa, and India..

S. indicum is found in India and Thailand.

The indicum group appears to be the sister clade to the African species.

Japonicum group

The japonicum group has five species: S. japonicum, S. malayensis and S. mekongi, S. ovuncatum and S. sinensium and these species are found in China and Southeast Asia.

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.

New species

As of 2012, four additional species have been transferred to this genus.,

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. 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.

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:

Comparison of eggs

<gallery>

File:Schistosoma haematobium egg 4842 lores.jpg|Schistosoma japonicum

File:Schistosoma japonicum egg 4843 lores.jpg|Schistosoma haematobium

File:Schistosoma mansoni egg 4841 lores.jpg|Schistosoma mansoni

File:S mekongi eggR.jpg|Schistosoma mekongi

File:S interculatum eggS.jpg|Schistosoma intercalatum

</gallery>

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. There had been no cases in Europe since 1965, until an outbreak occurred on Corsica. An anti-schistosome drug is a schistosomicide.

Species infecting humans

Parasitism of humans by Schistosoma appears to have evolved on at least three occasions in both Asia and Africa.

  • S. guineensis, a recently described species, is found in West Africa. Known snail intermediate hosts include Bulinus forskalii.
  • S. haematobium, commonly referred to as the bladder fluke, originally found in Africa, the Near East, and the Mediterranean basin, was introduced into India during World War II. Freshwater snails of the genus Bulinus are an important intermediate host for this parasite. Among final hosts humans are most important. Other final hosts are rarely baboons and monkeys.
  • S. intercalatum. The usual final hosts are humans. Other animals can be infected experimentally.

Genome

The genomes of Schistosoma haematobium, S. japonicum and S. mansoni have been reported.

History

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. Bilharz wrote a paper in 1856 describing the worms 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. In 1858 David Friedrich Weinland proposed the name Schistosoma (Greek: "split body") because the worms were not hermaphroditic but had separate sexes. 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.

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.

In 2009, the genomes of Schistosoma mansoni and Schistosoma japonicum were decoded

Treatment

Praziquantel is the current drug of choice against schistosomiasis. It is effective against all schistosome species that infect humans. Oxamniquine is effective against Schistosoma mansoni infections, but relatively ineffective against Schistosoma haematobium and Schistosoma japonicum.

References

Further reading

  • 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