The spatial and seasonal distribution of Bulinus truncatus, Bulinus forskalii and Biomphalaria pfeifferi, the intermediate host snails of schistosomiasis, in N'Djamena, Chad

There is a paucity of epidemiological and malacological data pertaining to schistosomiasis in Chad. In view of a recently articulated elimination agenda, a deeper understanding of the spatio-temporal distribution of schistosomiasis intermediate host snails is pivotal. We conducted cross-sectional malacological surveys during the dry season (April/May 2013) and after the short rainy season (October 2013) in N'Djamena, the capital of Chad. Snails were identified at the genus and species level using morphological keys and molecular DNA barcoding approaches. Those belonging to Bulinus and Biomphalaria were examined for cercarial shedding. Snail habitats were characterised and their predictive potential for the presence of schistosomiasis intermediate host snails explored. Seasonal patterns were studied using geographical information system and kriging in order to interpolate snail abundance data to make predictions at non-sampled locations across N'Djamena. Overall, 413 Bulinus truncatus, 369 Bulinus forskalii and 108 Biomphalaria pfeifferi snails were collected and subjected to cercarial shedding. During the dry season, one Bu. truncatus of 119 snails collected shed Schistosoma spp. cercariae (0.84%), while S. mansoni was shed by one of 108 Bi. pfeifferi snails (0.93%). None of the snails collected after the rainy season shed Schistosoma spp. cercariae. The abundance of Bu. truncatus and Bu. forskalii showed an inverse U-shape relationship with the square term of conductivity, i.e. low abundance at the lowest and highest levels of conductivity and high abundance at intermediate levels. Bi. pfeifferi showed a negative, linear association with pH in the dry seasons. It is planned to link these intermediate host snail data to infection data in human populations with the goal to draw a predictive risk map that can be utilised for control and elimination of schistosomiasis in N'Djamena.