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.

The influence of physical factors on the behaviour and infectivity of miracidia of Schistosoma mansoni and S. haematobium. II. Effect of light and depth.

A striking difference was found in the responses of Schistosoma mansoni and S. haematobium miracidia to the intensity and direction of light and to gravity. S. mansoni miracidia were found to be positively phototactic and sensitive to small changes in light intensities. They were indifferent to gravity. S. haematobium miracidia however were negatively phototactic and unable to distinguish low light intensities from darkness. They showed a strong positive geotaxis. Both S. mansoni and S. haematobium miracidia could successfully find and infect snails to a depth of 2 metres of water: the significance of these findings in relation to transmission of schistosomiasis in the field is discussed.

The influence of physical factors on the survival and infectivity of miracidia of Schistosoma manosni and S. haematobium I. Effect of temperature and ultra-violet light.

The influence of temperature and ultra-violet radiation on the degree of activity, survival and infectivity of schistosome miracidia is profound. Miracidia of Schistosoma mansoni and S. haematobium were affect eaually. Only miracidia classified as "active" or "slow" were capable of penetration, a capacity they retained for about 17 hours at 19 degrees C. Miracidia that were "lethargic" as a result of low temperature, old age or ultra-violet radiation lost their infective capacity. The conclusion, however, is that neither the temperatures encountered in the field nor the solar ultra-violet radiation penetrating turbid waters are likely to be harmful to miracidia and thus have no effect on the level of transmission.