Studies of the relationships between Schistosoma nasale and S. spindale and their snail host Indoplanorbis exustus.

Infectivity and cercarial production of Indoplanorbis exustus related to variation of miracidial dose (1, 4, 10 or 20) with Schistosoma nasale and S. spindale from Sri Lanka were studied. The intermediate host-parasite relationships of the two schistosome species showed marked differences under the conditions of observation recorded in this study. Prepatent death rates (PDR) were on average higher for S. spindale (30%) than for S. nasale (10%). The size of the miracidial dose to which snails had been exposed had no effect on PDR. The infection rates (IR) were on average higher for S. nasale (41%) compared with S. spindale (27%). Highest IR occurred after exposure to 4 miracidia in S. nasale infections (79%) and after exposure to 10 miracidia in S. spindale infections (60%). The highest daily average cercarial production per snail was recorded for S. nasale at a level of 4 miracidia (1311), and for S. spindale at a level of 10 miracidia (1615). At low level (1 or 4 miracidia) of exposure, I. exustus showed a better compatibility with S. nasale than with S. spindale. An opposite tendency was observed at higher levels (10 or 20 miracidia) of exposure. Unsuccessful infections of Lymnaea luteola with either S. nasale or S. spindale indicate that this species is not involved in transmission.