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Background  Schistosomiasis is hyper-endemic in the Lake Victoria basin; with intestinal schistosomiasis plaguing communities adjacent to the lake, where the intermediate host snails live. The two intermediate host species of  Schistosoma mansoni  in the Mwanza region are  Biomphalaria sudanica , found on the banks of the lakes, and  B. choanomphala , found in the lake itself. There are few longitudinal surveys documenting changing abundance and differential transmission patterns of these  Biomphalaria  snails across seasons and years. We undertook 15 field surveys at 26 sites over four years to determine the parameters that influence  Biomphalaria  abundance, presence of  S. mansoni -shedding snails and impact of schistosomiasis treatment interventions on transmission potential in the Mwanza region.    Results  Statistical analysis revealed seasonal difference in the abundance of  B. sudanica  with the highest number of snails found in the dry season (Kruskal-Wallis  χ   2  = 37.231,  df  = 3,  P  < 0.0001). Water measurements were not associated with  B. sudanica  abundance; however, high levels of rainfall did have a negative effect on  B. sudanica  [coefficient effect -0.1405, 95% CI (-0.2666, -0.0144)] and  B. choanomphala  abundance [coefficient effect -0.4388, 95% CI (-0.8546, -0.0231)] potentially due to inundation of sites “diluting” the snails and influencing collection outcome.  Biomphalaria sudanica  snails were found at all sites whereas  B. choanomphala  were far more focal and only found in certain sites. Shedding  Biomphalaria  did not show any variation between dry and rainy seasons; however, a decrease in shedding snails was observed in year 4 of the study.    Conclusions    Biomphalaria sudanica  is uniformly present in the Mwanza region whereas  B. choanomphala  is far more focal. Seasonality plays a role for  B. sudanica  abundance, likely due to its habitat preference on the banks of the lake, but not for  B. choanomphala . The decrease in shedding  Biomphalaria  abundance in Year 4 could be linked to ongoing schistosomiasis treatment efforts in the neighbouring human populations. The highest number of shedding  Biomphalaria  was observed at sites with high levels of human movement. Prioritising snail control at such sites could greatly reduce transmission in these high-risk areas.    Electronic supplementary material  The online version of this article (doi:10.1186/s13071-017-2252-z) contains supplementary material, which is available to authorized users.

parasites and vectors

Longitudinal survey on the distribution of Biomphalaria sudanica and B. choanomophala in Mwanza region, on the shores of Lake Victoria, Tanzania: implications for schistosomiasis transmission and control