This study was carried out in five sites along a small perennial river
system in south-central Tanzania, which had been identified as the focus
for transmission of intestinal schistosomiasis in the area. Malacological
surveys preceding the study showed a focal distribution of Biomphalaria
pfeifferi, intermediate host snail of Schistosoma mansoni, the snails being
present in three sites but absent from the other two sites. The objective
of this study was to evaluate to what extent chemical and/or
physical-morphological factors determine the distribution of B. pfeifferi
between these five sites.
It was found that none of the chemical constituents in the waters examined
were outside the tolerance range of B. pfeifferi snails. Moreover, the
composition of water from B. pfeifferi-free sites was not different from
that in those sites where snails occurred. Furthermore, none of the
physical-morphological 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 be associated with a
single environmental factor and is rather the result of more complex
interactions of habitat factors.