The influence of physical factors on the behaviour and infectivity of miracidia of Schistosoma mansoni and S. haematobium. III. Effect of contact time and dispersion in static and flowing waters.

The scanning and infective capacity of Schistosoma mansoni and S. haematobium miracidia were tested under limited exposure times in small volumes of water, and under increasing mutual dispersion in vessels of different sizes also over increasing horizontal distances, and under different flow rates of water. It was found that the miracidia of both species retained their infective capacity for up to 1 hour in the presence of susceptible snails. The infection rates decreased with decreasing miracidial densities but was still over 50% at 0.4--0.8 miracidia per litre. Both species of miracidia produced high (36--44%) infection rates in host snails situated at a distance of 5 m and equally high infection rates (39--40%) in snails placed in water flowing at a rate of 12--15 cm/sec. The application of any of these physical factors in the control of schistosomiasis is discussed.