Experimental epidemiology of schistosomiasis. II. Prepatency of Schistosoma mansoni in Biomphalaria glabrata at diurnally fluctuating temperatures.

The basic hyperbolic relationship established between constant water temperatures and the length of the prepatent period in the snail was tested as diurnally fluctuating temperatures, stimulating natural conditions. Diurnal temperature variations are generally summed up by S. mansoni according to the time-temperature product and are independent of the shape of the temperature curve. Using a simple hyperbolic formula, the time of prepatency can be calculated for temperatures ranging from 16 degrees C to 32 degrees C. The developmental rate (reciprocal of the prepatency period) within this range is a linear function of the mean environmental temperature. Biomphalaria glabrata and S. mansoni survive well at temperature peaks as high as 40.5 degrees C or as low as 11.5 degrees C, if they are given as part of a diurnal sinusoidal or trapezoidal temperature programme. At fluctuating temperatures decreasing to low values of about 11 degrees to 14 degrees C, however, the parasite's development was up to 27% faster than predicted. Apparently the development in the low temperature range does not follow the basic hyperbolic relationship, and even below the so-called "developmental null point" of 14.2 degrees C some development of S. mansoni takes place. For practical purposes, an empirical compensation of the deviation at low temperatures is proposed. Instead of the real mean of the diurnal temperature cycle, the mean between 14.2 degrees C and the daily maximum can be used as the basis for calculation, regardless of the time with temperatures below 14.2 degrees C.