The snail intermediate hosts are three species

of Robertsiella, again snails of the family Pomatiopsidae (see Attwood et al., Selleck MS 275 2005). Humans and rats are the only known natural hosts for S. malayensis (see Ambu et al., 1984), with Rattus muelleri and Rattus tiomanicus recorded as the main definitive hosts ( Greer et al., 1988 and Attwood et al., 2005); however, the low prevalence in humans, combined with the failure to recover eggs from the stool of a biopsy-positive patient ( Murugasu et al., 1978) or from serologically positive patients ( Greer and Anuar, 1984), suggests that humans are not an important host for this parasite. A small number of experimental infections also indicated that dogs are not permissive hosts ( Ambu et al., 1984). Consideration of the data currently available suggests varying but significant animal reservoirs of infection for all three species of Asian Schistosoma infecting humans. The major zoonotic element in transmission of human disease is attributable to S. japonicum; however, it is not clear if differences in host group

utilization (e.g., the differences in the involvement of dogs, bovines and rodents between the Philippines and China) result from small sample sizes (in some cases only two villages were sampled), differences in land form (highland or flat marshland), different definitive host population sizes and behaviour, or different parasite strains. The parasite in the Philippines is transmitted by Oncomelania hupensis quadrasi whilst that in the lower Yangtze basin in China is transmitted by O. h. hupensis; aminophylline these two PLX3397 mw snails may have different ecological habit and such differences could affect definitive host usage. One most obvious difference is that transmission (and snail activity) in China is much more seasonal than in the Philippines. Clearly more villages, host animals and ecological situations must

be sampled in order to determine which host groups are most epidemiologically significant for disease in humans so that these can be targeted by control programs (the China National Control Program currently includes only cattle and water buffalo, Wang et al., 2005). In this way inter-village variation can be assessed and any important and stable patterns identified. In the case of Mekong schistosomiasis there is indirect evidence for a major animal reservoir (i.e., prevalences in, and densities of, snail populations remain stable in the face of marked reductions in human infections), but prevalences in dogs in Cambodia are relatively low (e.g., one dog in 310 sampled in 2001 was found to be infected) and it is likely that additional species are involved. Successful control of S. mekongi is unlikely to be achieved until all reservoir host species are known and their roles characterized. Of the three species, S.