Schistosomiasis in Kano State, Nigeria. I. Human infections near dam sites and the distribution and habitat preferences of potential snail intermediate hosts.

Stool and urine samples from 813 schoolchildren and adults from the Tomas and Rimin Gado dam areas of Kano State, Nigeria, showed Schistosoma haematobium to be present at both localities with prevalences of 26.6 and 36.8%, respectively. No cases of S. mansoni were found. The prevalence and intensity of S. haematobium was low and similar in both study areas. Statistical analysis revealed a correlation between prevalence and location, but no evident association with professed patterns of water contact. Investigation of 165 freshwater habitats throughout the state revealed the presence of a number of potential snail intermediate host species, namely Bulinus senegalensis, B. forskali, B. globosus, B. rohlfsi and Biomphalaria pfeifferi. The most widespread species was Bulinus senegalensis, which inhabited shallow pools and excavations on a variety of substrata. Its habitats were typically devoid of aquatic vegetation and included those with highly turbid waters and conductivities as low as 11 microseconds. Bulinus forskali was by contrast relatively rare, occurring in more permanent water courses, although it was often found in mixed populations with B. senegalensis. Bulinus globosus also occurred in seasonally rainfilled pools, but was confined to areas south of the 12 degrees N parallel, and its habitats tended to have a well-developed aquatic flora and clear water. The dominant species in man-made lakes was B. rohlfsi, which occurred in both the Tomas and Rimin Gado reservoirs. Biomphalaria pfeifferi was also primarily lake dwelling, although all species save B. senegalensis were found in irrigation canals.