SOME FURTHER OBSERVATIONS ON SCHISTOSOME TRANSMISSION IN THE EASTERN TRANSVAAL.

By immersing groups of rodents into several types of natural waters at Malelane, Eastern Transvaal, and subsequently recovering schistosomes from them, the authors have shown that there is seasonal transmission, which is more marked with Schistosoma mansoni than with S. mattheei. Laboratory confirmation of this periodicity was obtained by infecting groups of snails with the appropriate parasite and studying the pattern of sporocyst development and cercarial shedding under outdoor conditions. It was also found that, apart from seasonal periodicity, the amount of transmission is dependent on the density of human population, the proximity of the definitive host to the immersion site, the degree to which the water is protected from pollution and the distance downstream from the polluting source.Negligible numbers of S. haematobium adults were recovered; possible reasons for this are discussed.The authors consider that an annual transmission cycle may be more common in bilharziasis than is generally supposed-although not exclusively or necessarily for the reasons given in this paper-and suggest that such a cycle be taken into consideration in the planning of control schemes.