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 directly penetrating the skin.
Electron micrograph of an adult male Schistosoma parasite worm. The bar (bottom left) represents a length of 500 ?m.
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.
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 107.5-147.6 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 Schistosoma as currently[when?] 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).
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.
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, but is also found in Africa. They occur in Asia and India.
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.
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
The hybrid S. haematobium-S.guineenis was observed in Cameroon in 1996. S. haematobium could establish itself only after deforestation of the tropical rainforest in Loum next to the endemic S. guineensis; hybridization led to competitive exclusion of S. guineensis.
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 Nder e.g. The same hybrid was identified during the 2015 investigation of a schistosomiasis outbreak on Corsica, traced to the Cavu river.
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.
The parasitic flatworms of Schistosoma cause a group of chronic infections called schistosomiasis known also as bilharziasis. 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.
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.
S. japonicum, whose common name is simply blood fluke, is widespread in East Asia and the southwestern Pacific region. In Taiwan this species only affects animals, not humans. Freshwater snails of the genus Oncomelania are an important intermediate host for S. japonicum. Final hosts are humans and other mammals including cats, dogs, goats, horses, pigs, rats and water buffalo.
S. malayensis This species appears to be a rare infection in humans and is considered to be a zoonosis. The natural vertebrate host is von Muller's rat (Rattus muelleri). The snail host(s) are Robertsiella species (R. gismanni, R. kaporensis and R. silvicola (see Attwood et al. 2005 Journal of Molluscan Studies Volume 71, Issue 4 pp. 379-391).
S. mekongi is related to S. japonicum and affects both the superior and inferior mesenteric veins. S. mekongi differs in that it has smaller eggs, a different intermediate host (Neotricula aperta) and longer prepatent period in the mammalian host. Final hosts are humans and dogs. The snail Tricula aperta can also be experimentally infected with this species.
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.
The eggs of these parasites were first seen by Theodor Maximilian Bilharz, a Germanpathologist 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. Bilharz wrote a paper in 1856 describing the worms more fully and he named them Distoma haematobium. Their unusual morphology meant that they could not be comfortably included in Distoma. So in 1856 Meckel von Helmsback created the genusBilharzia for them. In 1858 David Friedrich Weinland proposed the name Schistosoma (Greek: "split body") after the male worms' morphology. 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.
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 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  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.
^Beer SA, Voronin MV, Zazornova OP, Khrisanfova GG, Semenova SK (2010) Phylogenetic relationships among schistosomatidae. Med Parazitol (Mosk) 2010 (2):53-59
^Wang CR, Li L, Ni HB, et al. (February 2009). "Orientobilharzia turkestanicum is a member of Schistosoma genus based on phylogenetic analysis using ribosomal DNA sequences". Exp. Parasitol. 121 (2): 193-7. doi:10.1016/j.exppara.2008.10.012. PMID19014940.
^Wang Y, Wang CR, Zhao GH, Gao JF, Li MW, Zhu XQ (December 2011). "The complete mitochondrial genome of Orientobilharzia turkestanicum supports its affinity with African Schistosoma spp". Infect. Genet. Evol. 11 (8): 1964-70. doi:10.1016/j.meegid.2011.08.030. PMID21930247.
^Crellen T, Allan F, David S, Durrant C, Huckvale T, Holroyd N, Emery AM, Rollinson D, Aanensen DM, Berriman M, Webster JP, Cotton JA (2016) Whole genome resequencing of the human parasite Schistosoma mansoni reveals population history and effects of selection. Sci Rep 6:20954. doi: 10.1038/srep20954
^Lawton SP, Majoros G (2013) A foreign invader or a reclusive native? DNA bar coding reveals a distinct European lineage of the zoonotic parasite Schistosoma turkestanicum (syn. Orientobilharzia turkestanicum). Infect Genet Evol 14:186-93. doi: 10.1016/j.meegid.2012.11.013
^Kaukas A, Dias Neto E, Simpson AJ, Southgate VR, Rollinson D (1994) A phylogenetic analysis of Schistosoma haematobium group species based on randomly amplified polymorphic DNA. Int J Parasitol 24(2):285-290
^Webster BL, Southgate VR, Littlewood DT (2006) A revision of the interrelationships of Schistosoma including the recently described Schistosoma guineensis. Int J Parasitol 36(8):947-955
^Kane RA, Southgate VR, Rollinson D, Littlewood DT, Lockyer AE, Pagès JR, Tchuem Tchuentè LA, Jourdane J (2003) A phylogeny based on three mitochondrial genes supports the division of Schistosoma intercalatum into two separate species. Parasitology 127(Pt 2):131-137
^Pagès JR, Durand P, Southgate VR, Tchuem Tchuenté LA, Jourdane J (2001) Molecular arguments for splitting of Schistosoma intercalatum, into two distinct species. Parasitol Res 87(1):57-62
^Agatsuma T, Iwagami M, Liu CX, Rajapakse RP, Mondal MM, Kitikoon V, Ambu S, Agatsuma Y, Blair D, Higuchi T (2002) Affinities between Asian non-human Schistosoma species, the S. indicum group, and the African human schistosomes. J Helminthol 76(1):7-19
^Webster BL, Littlewood DT (2012) Mitochondrial gene order change in Schistosoma (Platyhelminthes: Digenea: Schistosomatidae). Int J Parasitol 42(3):313-321
^Tchuem Tchuenté LA, Southgate VR, Njiokou F, Njine T, Kouemeni LE, et al. (1997) The evolution of schistosomiasis at Loum, Cameroon: replacement of Schistosoma intercalatum by S. haematobium through introgressive hybridization. Trans R Soc Trop Med Hyg 91: 664-665.DOI 10.1016/S0035-9203(97)90513-7
^Morgan JAT, DeJong RJ, Lwambo NJS, Mungai BN, Mkoji GM, et al. (2003) First report of a natural hybrid between Schistosoma mansoni and S. rodhaini. Journal of Parasitology 89: 416-418.
^ abT Huyse, BL Webster, S Geldof, et al. Bidirectional introgressive hybridization between a cattle and human schistosome species. PLoS Pathog, 5 (2009), p. e1000571. DOI 10.1371/journal.ppat.1000571
^ abJérôme Boissier; Sébastien Grech-Angelini; Bonnie L Webster; et al. (2016). "Outbreak of urogenital schistosomiasis in Corsica (France): an epidemiological case study". The Lancet Infectious Diseases. 16 (8): 971-979. doi:10.1016/S1473-3099(16)00175-4. PMID27197551.