Schistosoma

Schistosoma
Schistosoma mansoni egg
Scientific classification
Kingdom:	Animalia
Phylum:	Platyhelminthes
Class:	Trematoda
Subclass:	Digenea
Order:	Strigeidida
Family:	Schistosomatidae
Genus:	Schistosoma Weinland, 1858
Species
Schistosoma bovis Schistosoma curassoni Schistosoma edwardiense Schistosoma guineensis Schistosoma haematobium Schistosoma hippotami Schistosoma incognitum Schistosoma indicum Schistosoma intercalatum Schistosoma japonicum Schistosoma leiperi Schistosoma malayensis Schistosoma mansoni Schistosoma margrebowiei Schistosoma mattheei Schistosoma mekongi Schistosoma ovuncatum Schistosoma nasale Schistosoma rodhaini Schistosoma sinensium Schistosoma spindale

A genus of trematodes, Schistosoma spp., commonly known as blood-flukes and bilharzia, cause the most significant infection of humans by flatworms (schistosomiasis) and are considered by the World Health Organization as second in importance only to malaria, with hundreds of millions infected worldwide. Adult worms parasitize mesenteric blood vessels. Eggs are passed through urine or feces to fresh water, where larval stages can infect a new host by penetrating the skin.

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 wrote a paper (published in 1852) summarizing Bilharz's findings. Bilhart's 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.

Bilharz also described Schistosomum mansoni but this species was redecribed by Louis Westenra Sambon in 1907 at the London School of Tropical Medicine who named it after his teacher Patrick Manson.

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 Schistosoma mansoni was made of 11,809 genes including many which produce enzymes for breaking down proteins which enable the parasite to bore through tissue. Also, Schistosoma mansoni does not have an enyzme to make certain fats so that it must rely on its host to produce these.^[1]

Evolution

Electron micrograph of an adult male Schistosoma parasite worm. The bar (bottom left) represents a magnification 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 hippo (Hippopotamus amphibius) species (S. edwardiense and S. hippopotami) may be basal. Since hippos were present in both Africa and Asia during the Cenozoic era the genus may have originated as parasites of hippos.^[2] 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. Another mammalian genus - Orientobilharzia is also closely related.

The cattle, sheep, goat and cashmere goat parasite Orientobilharzia turkestanicum appears to be related to the African 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.

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.

Taxonomy

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

The genus has been divided into four groups - indicum, japonicam, haematobium and mansoni. The affinities of the remaining three species are still being clarified.

Twelve species are found in Africa. Eleven 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).

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

The eight haematobium group species are: S. bovis, S. curassoni, S. intercalatum, S. guineensis, S. haematobium, S. leiperi, S. margrebowiei and S. matthei.

S. spindale is widely distributed in Asia but is also found in Africa.

The other species occur in Asia and India

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. indicum is found in India and Thailand.

The japonicum group has three species: S. japonicum, S. malayensis and S. mekongi.

S. sinensium is a sister clade to the S. japonicum group and is found in China.

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.

S. incognitum appears to be basal in this genus. It may be more closely related to the African/Indian species than to the South East Asian group. This species uses pulmonate snails as hosts.

Schistosomiasis

Schistosoma cause parasitic disease called schistosomiasis. Drug used to treat schistosomiasis is schistosomicide.

Species infecting humans

Parasitism of humans by Schistosoma appears to have evolved 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 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.^[3].

• S. intercalatum. The usual final hosts are humans. Other animals can be infected experimentally.^[3]

• 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.^[3]

• S. malayensis This species appears to be a rare infection in humans and is considered to be a zooenosis. The natural vertebrate host is von Muller's rat (Rattus muelleri). The snail host(s) are not yet known.

• 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.^[3]

• 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.^[3] The snail Tricula aperta can also be experimentally infected with this species.

Species infecting animals other than humans

Schistosoma indicum, Schistosoma nasale, Schistosoma leiperi are all parasites of ruminants.

Schistosoma edwardiense and Schistosoma hippopotami are parasites of the hippo.

Morphology

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.

Reproduction

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.

References

External links

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
• Schistostoma parasites overview, biology, life cycle image at MetaPathogen