Biomphalaria species distribution and its effect on human Schistosoma mansoni infection in an irrigated area used for rice cultivation in northeast Brazil

The role of irrigated areas for the spread of schistosomiasis is of worldwide concern. The aim of the present study was to investigate the spatial distribution of the intermediate snail host <em>Biomphalaria</em> in an area highly endemic for schistosomiasis due to <em>Schistosoma mansoni</em>, evaluating the relationship between irrigation and types of natural water sources on one hand, and the influence of place and time of water exposure on the intensity of human infection on the other. A geographical information system (GIS) was used to map the distribution of the intermediate snail hosts in Ilha das Flores, Sergipe, Brazil, combined with a clinical/epidemiological survey. We observed a direct correlation between the intensity of human infection with <em>S. mansoni</em> and irrigation projects. Malacological studies to identify snail species and infection rates showed that <em>B. glabrata</em> is the main species responsible for human schistosomiasis in the municipality, but that <em>B. straminea</em> also plays a role. Our results provide evidence for a competitive selection between the two snail species in rice fields with a predominance of <em>B. glabrata</em> in irrigation systems and <em>B. straminea</em> in natural water sources