Schistosoma

Taxobox
name = "Schistosoma"


image_width = 250px
image_caption = "Schistsoma mansoni" egg
regnum = Animalia
phylum = Platyhelminthes
classis = Trematoda
subclassis = Digenea
ordo = Strigeidida
familia = Schistosomatidae
genus = "Schistosoma"
genus_authority = Weinland, 1858
subdivision_ranks = Species
subdivision = "S. mansoni"
"S. japonicum"
"S. mekongi"
"S. intercalatum"
"S. haematobium"
"S. indicum"
"S. nasale"
"S. leiperi"
"S. malayensis"
"S. rodhaini"

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.

Taxonomy

There are four species of schistosome which are infective to humans:

*"Schistosoma 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 host for this trematode.
*"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 host for "S. japonicum".

*"S. mekongi" is related to "S. japonicum" and affects both superior and inferior mesenteric veins. "S. mekongi" differs in that it has smaller eggs, a different intermediate host, and longer prepatent period in the mammalian host.

*"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 host for this parasite.

"S. indicum", "S. nasale", "S. leiperi" are all parasites of ruminants.

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 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.

ee also

For a full discussion of life cycles, symptoms and control measures, see the main article on schistosomiasis.gonochoristic not dioceous

External links

* [http://www.dfid-kar-water.net/w5outputs/output_summary_sheets/control_schistosomiasis_os.html British Department for International Development Control of Schistosomiasis]
* [http://www.who.org The World Health Organisation]
* [http://www.path.cam.ac.uk/~schisto/ University of Cambridge Schistosome Laboratory]


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