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
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 70 million years ago and 120 million years ago. 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.
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. 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.
New species As of 2012, four additional species have been transferred to this genus., previously classified as species in the genus Orientobilharzia. 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
Schistosoma bomfordi Schistosoma datta Schistosoma harinasutai Schistosoma turkestanicum
… excerpt ends here. Continue reading the full article.
