Snail vectors of Schistosoma mansoni in Alagoas, Northeast Brazil: spatial distribution in the hydrographic grid of the state

Schistosomiasis mansoni is a parasitic infectious disease of significant epidemiological importance caused by Schistosoma mansoni. The intermediate hosts are snails of the genus Biomphalaria spp. As gastropods are crucial for the spread of the disease, this study aimed to assess the influence of water bodies on the distribution of snails responsible for transmitting S. mansoni in the state of Alagoas. It is an ecological study with a spatial approach, carried out from 1997 to 2017. We considered all Biomphalaria snails captured and positive for S. mansoni in the state. Representative maps were created for better data visualization, having as unit of analysis the municipalities of Alagoas. We also calculated the natural infection rate of the snails (number of positives/number of captured). During the study period, 34 619 Biomphalaria snails were captured, consisting of Biomphalaria glabrata (30 132), Biomphalaria straminea (4445) and Biomphalaria tenagophila (42). The seventh Health Region (HR) of the state had the highest number of specimens collected (B. glabrata=15 676), while the sixth HR had the highest positivity rate (9.2%). Regarding hydrography, these snails were primarily found along the Jacuípe, Mundaú, Paraíba, São Miguel, Coruripe, Piauí, Perucaba and Boacica rivers. Our data reinforce the occurrence of the three medically significant snail species responsible for schistosomiasis transmission in Alagoas. Additionally, our findings can be used as a basis for malacological activities because we report the presence of these animals in the main rivers of the state.