EVALUATION OF CHEMICAL AND PHYSICAL-MORPHOLOGICAL FACTORS AS POTENTIAL DETERMINANTS OF BIOMPHALARIA-PFEIFFERI (KRAUSS, 1848) DISTRIBUTION

This study was carried out in five sites along a small perennial river system in south-central Tanzania, which had been identified as the fo cus for transmission of intestinal schistosomiasis in the area. Malaco logical surveys preceding the study showed a focal distribution of Bio mphalaria pfeifferi, intermediate host snail of Schistosoma mansoni, t he snails being present in three sites but absent from the other two s ites. The objective of this study was to evaluate to what extent chemi cal and/or physical-morphological factors determine the distribution o f B. pfeifferi between these Jive sites. It was found that none of the chemical constituents in the waters examined were outside the toleran ce range of B. pfeifferi snails. Moreover the composition of water fro m B. pfeifferi-free ee sites tvas not different from that in those sit es where snails occurred. Furthermore, none of the physical-morphologi cal constituents seemed likely to be a determinant for the absence of B. pfeifferi. In view of these findings, and those of previous studies , it is concluded that the focal distribution of B. pfeifferi cannot b e associated with a single environmental factor and is rather the resu lt of more complex interactions of habitat factors.