[Factors responsible for cercarial concentration of schistosoma mansoni in Guadeloupe].

Research carried out in Guadeloupe on the sites of transmission of Schistosoma mansoni shows that the circadian evolution of the cercarial concentration (rhythm of presence) differs slightly according to the biotope:--in a running water habitat (greater than 0,10 m/s) the rhythm of presence is superimposed on the rhythm of emergence, which leads to a risk of contamination reaching a maximum between 11 a.m. and noon; --in a slow running water habitat (around 0,01 m/s), the acrophase of the rhythm of presence is clearly retarded in comparison with that of the rhythm of emergence. In this case, the risk of contamination is greatest between 2 p.m. and 5 p.m. because of the accumulation of cercariae. The absolute value of the cercarial concentration in running water results from two antagonistic processes: production of cercariae depending on the amount of Biomphalaria glabrata infected and dilution of cercariae depending on the flow rate of water. Thus along one channel the cercarial concentration shows great variation depending upon which of the above processes predominates. These results demonstrate that the risk of human contamination can never be directly deduced from the demography of vectors.