Distribution in Kenya of intermediate hosts of Schistosoma

A major factor in the geographical distribution of endemic schistosomiasis in Kenya is the discontinuous distribution of intermediate hosts. Biomphalaria pfeifferi is the main transmitter of S. mansoni and is found in the altitude zone 300-2,300 m; its absence from the costal region results in the absence of endemic schistosomiasis mansoni amongst the large human population in the coastal region. Snails belonging to the Bulinus africanus group appear to be responsible for all transmission of S. haematobium; they occur from near sea-level to an altitude of about 1,800 but are discontinuously distributed. There appears to be considerable potential for increase in the areas in which each form of schistosomiasis is endemic, taking into account existing patterns of snail distribution and likely developments in irrigation and water conservation. New localities reported here considerably extend the known distribution in western Kenya of B. truncatus, which could serve as an effective intermediate host for S. haematobium if a suitable strain were to become established in Kenya.