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A genus of digenetic trematodes, including the important blood flukes of humans and domestic animals, that cause schistosomiasis; characterized by elongate shape, by separate sexes with marked sexual dimorphism, by their unusual location in the smaller blood vessel s of their host, and by utilization of water snails as intermediate hosts. [schisto- + G. soma, body]

- S. haematobium the vesical blood fluke, a species with terminally spined eggs that occurs as a parasite in the portal system and mesenteric veins of the bladder (causing human schistosomiasis haematobium) and rectum; common in the Nile delta but is found along waterways, irrigation ditches, or streams throughout Africa and in parts of the Middle East; the intermediate host is Bulinus truncatus in Egypt; elsewhere, other snails of the subfamily Bulininae (Bulinus, Physopsis, Pyrgophysa) are involved.

- S. intercalatum a blood fluke species related to S. haematobium locally distributed in Zaire and other areas of central Africa, causing mild dysentery and abdominal pains, with enlargement of the spleen and liver; a planorbid snail, Bulinus (Physopsis) africanus, serves as the intermediate host.

- S. japonicum the Oriental or Japanese blood fluke, a species having eggs with small lateral spines, usually only a small knob; causes schistosomiasis japonica, with extensive pathology from encapsulation of the eggs, particularly in the liver, and is the most pathogenic of the three common schistosome species afflicting man, possibly owing to greater egg production per female worm; it is also the most intractable to treatment and the most difficult to control, as the intermediate hosts are amphibious snails (species of Oncomelania, family Hydrobiidae) that can leave the water to avoid molluscicides, and also because many other animals, such as pigs, oxen, cattle, and dogs, serve as reservoir hosts.

- S. malayensis a member of the S. japonicum complex described from the rodent Rattus muelleri in peninsular Malaysia. The aquatic snail Robertsiella kaporensis and two other species of this genus were found to be naturally infected. S. malayensis is considered most closely related to S. mekongi. Human infections, based on serologic evidence, were reported among the indigenous people of central peninsular Malaysia.

- S. mansoni a common species of trematode characterized by large eggs with a strong lateral spine and transmitted by planorbid snails of the genus Biomphalaria; causes schistosomiasis mansoni in humans in Africa, parts of the Middle East, South America, and certain Caribbean islands.

- S. mattheei a species found in the portal and mesenteric veins of ruminants, primates (including humans), zebra, and rodents in Africa.

- S. mekongi the Mekong schistosome, a species described from the Mekong delta in southern Laos and northern Cambodia. Infection rates are highest for ages 7–15; dogs appear to be the chief reservoir host; the intermediate host snail is the operculid snail, Tricula aperta. Pathology is similar to but generally less severe than that of S. japonicum.

schis·to·so·ma .shis-tə-'sō-mə, .skis- n

1) cap a genus that is the type genus of the family Schistosomatidae and that includes elongated digenetic trematode worms parasitizing the blood vessels of birds and mammals and several forms (as S. haematobium, S. japonicum, and S. mansoni) causing human schistosomiasis

2) any digenetic trematode worm of the genus Schistosoma: SCHISTOSOME

a genus of blood fluke, three species of which are important parasites of humans causing one of the most serious of tropical diseases (see schistosomiasis). S. japonicum is common in the Far East; S. mansoni is widespread in Africa, the West Indies, and South and Central America; S. haematobium occurs in Africa and the Middle East.