Schistosoma ovuncatum n. sp (Digenea : Schistosomatidae) from northwest Thailand and the historical biogeography of Southeast Asian Schistosoma Weinland, 1858

Schistosoma sinensium Bao, 1958 was first isolated from an unidentified snail in Sichuan Province, PR China. This species was apparently rediscovered in Chiang Mai Province, northwest Thailand (Baidikul et al., 1984); the definitive host was the rat Rattus rattus and the intermediate host was the snail Tricula bollingi. In this paper S. sinensium is rediscovered in Sichuan Province and compared with worms recovered from experimentally infected mice, which had been exposed to cercariae shed by T. bollingi from Chiang Mai. Evidence is presented suggesting that the schistosome collected by Baidikul was not S. sinensium and that a new species is involved. The new species, named Schistosoma ovuncatum (etymology: ovum (egg) + uncatus (hooked)), is described and compared with related taxa. All previous papers on the Thai schistosome have used worms recovered from field-collected rodents only; this is the first account in which the life-cycle has been completed in the laboratory, using cercariae shed by T. bollingi, and the resulting worms described. S. ovuncatum differs from S. sinensium in terms of size and shape of body and egg, number of testes, size of ovary, length of vitellarium, intermediate host and biogeographical distribution. The relationships of the two taxa and their position with respect to the Schistosoma indicum- and S. japonicum-groups are discussed. The implications of the findings for the evolution of human schistosomiasis in the region are also commented upon.