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, japonicum, 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.
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