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Background  Sound knowledge of the abundance and distribution of intermediate host snails is key to understanding schistosomiasis transmission and to inform effective interventions in endemic areas.    Methods  A longitudinal field survey of freshwater snails of biomedical importance was undertaken in the Niger River Valley (NRV) between July 2011 and January 2016, targeting  Bulinus  spp. and  Biomphalaria pfeifferi  (intermediate hosts of  Schistosoma  spp.), and  Radix natalensis  (intermediate host of  Fasciola  spp.). Monthly snail collections were carried out in 92 sites, near 20 localities endemic for  S. haematobium . All bulinids and  Bi. pfeifferi  were inspected for infection with  Schistosoma  spp., and  R. natalensis  for infection with  Fasciola  spp.    Results   Bulinus truncatus  was the most abundant species found, followed by  Bulinus forskalii ,  R. natalensis  and  Bi. pfeifferi . High abundance was associated with irrigation canals for all species with highest numbers of  Bulinus  spp. and  R. natalensis . Seasonality in abundance was statistically significant in all species, with greater numbers associated with dry season months in the first half of the year. Both  B. truncatus  and  R. natalensis  showed a negative association with some wet season months, particularly August. Prevalences of  Schistosoma  spp. within snails across the entire study were as follows:  Bi. pfeifferi : 3.45% (79/2290);  B. truncatus : 0.8% (342/42,500); and  B. forskalii : 0.2% (24/11,989). No  R. natalensis  ( n  = 2530) were infected. Seasonality of infection was evident for  B. truncatus , with highest proportions shedding in the middle of the dry season and lowest in the rainy season, and month being a significant predictor of infection.  Bulinus  spp. and  Bi. pfeifferi  showed a significant correlation of snail abundance with the number of snails shedding. In  B. truncatus , both prevalence of  Schistosoma  spp. infection, and abundance of shedding snails were significantly higher in pond habitats than in irrigation canals.    Conclusions  Evidence of seasonality in both overall snail abundance and infection with  Schistosoma  spp. in  B. truncatus , the main intermediate host in the region, has significant implications for monitoring and interrupting transmission of  Schistosoma  spp. in the NRV. Monthly longitudinal surveys, representing intensive sampling effort have provided the resolution needed to ascertain both temporal and spatial trends in this study. These data can inform planning of interventions and treatment within the region.

parasites and vectors

Freshwater snails of biomedical importance in the Niger River Valley: evidence of temporal and spatial patterns in abundance, distribution and infection with Schistosoma spp.