Effects of water velocity on snails and cercariae

Abstract

Snail vectors of schistosomiasis occur in lakes and ponds, rivers and streams, and in the supply and drainage systems of irrigation schemes. The survival of snails and of schistosome cercariae in any localised system is greatly affected by water velocity, and this could be used to advantage, particularly in the design of new irrigation systems, to limit the spread of schistosomiasis and its vectors. Use of high velocity water in endemic areas could play a useful role in the focal control of schistosomiasis, whereas neglect of this factor by water development engineers may have contributed to the spread of schistosomiasis in some areas, for example in the S~o Francisco River development in Bahia, Brazil. Slow flow through field irrigation canals contributed substantially to the spread of schistosomiasis mansoni in the Gezira Scheme in the Sudan I, but in fiat areas such as this it may not always be practical to increase the flow rates due to high pumping costs. Most studies on the effect of water velocity on snails and cercariae have been carried out on Biomphalaria snails and cercariae of Schistasama mansoni, but there is no obvious reason to suggest that these results cannot be applied in general terms to Bulinus snails and cercariae of S. haematobium. Oncomelania snail vectors of S. ]aponicum, are amphibious and in their aquatic phase prefer very sluggish streams and swamps.