A genus of trematodes, Schistosoma, commonly known as blood-flukes and bilharzia, includes flatworms which are responsible for a highly significant parasitic infection of humans by causing the disease schistosomiasis, and is considered by the World Health Organization as the second most socioeconomically devastating parasitic disease, next only to malaria, with hundreds of millions infected worldwide.
Adult worms parasitize mesenteric blood vessels. They are unique among trematodes or any other flatworms in that they are dioecious with distinct sexual dimorphism between male and female. Eggs are passed through urine or feces to fresh water, where larva must pass through an intermediate snail host, before a different larval stage of the parasite emerges that can infect a new mammalian host by directly penetrating the skin.
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 wrote a paper (published 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 genus Bilharzia for them. In 1858 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 the then known species were placed in a subfamily by Stiles and Hassel. This was then 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 da Silva 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 which produce enzymes for breaking down proteins which enable the parasite to bore through tissue. Also, S. mansoni does not have an enzyme to make certain fats so that it must rely on its host to produce these.
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.
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. 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.
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.
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.
This 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 the rat (Rattus rattus) and the intermediate host is the snail Tricula bollingi. This species is known to use snails of the family Pomatiopsidae as hosts.
Four additional species have been transferred to this genus. These were 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 that have been transferred to this genus are
- Schistosoma bomfordi
- Schistosoma datta
- Schistosoma harinasutai
- Schistosoma turkestanicum
Examination of the mitochondria suggests that Schistosoma incognitum may be a species complex.
Comparison of eggs
Species infecting humans
- 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 Bulinus genus 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 found widely spread in Eastern Asia and the southwestern Pacific region. In Taiwan this species only affects animals, not humans. Freshwater snails of the Oncomelania genus 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. mansoni, found in Africa, Brazil, Venezuela, Suriname, the lesser Antilles, Puerto Rico, and the Dominican Republic. It is also known as Manson's blood fluke or swamp fever. Freshwater snails of the Biomphalaria genus are an important intermediate host for this trematode. Among final hosts humans are most important. Other final hosts are baboons, rodents and raccoons.
- 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.
|Scientific Name||First Intermediate Host||Endemic Area|
|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||Not known||South East Asia|
|Schistosoma mansoni||Biomphalaria spp.||Africa, South America, Caribbean, Middle East|
|Schistosoma mekongi||Neotricula aperta||South East Asia|
Species infecting animals other than humans
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, oesophagus 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 (0.4–0.8 in) long and use globins from their hosts' hemoglobin for their own circulatory system.
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, where they reproduce sexually.
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