Persistent hotspots of Schistosoma mansoni transmission in the Kenyan waters of Lake Victoria: a search for snail-related answers beneath the waves

Background: Schistosoma mansoni transmission in Lake Victoria is often intense and hard to interrupt with praziquantel (PZQ) treatment alone. Following termination of a five-year annual treatment campaign, we sought to reveal underlying explanations by investigating snail-related aspects of transmission in six PZQ non-responsive villages that lie within a transmission hotspot located along the west-facing Kenyan shore of the lake, and in four PZQ-responding villages facing the more protected waters of Winam Gulf. Methodology/Principal Findings: We examined on four occasions over a two-year period the relative abundance and S. mansoni prevalence for Biomphalaria sudanica along the lake shore and for B. choanomphala mostly from deeper waters, and acquisition of S. mansoni worms by sentinel mice, at 20 sampling sites (two per village). Biomphalaria sudanica was found at all sites and abundance did not differ significantly between hotspot and responding villages. By contrast, B. choanomphala was found at all six hotspot villages and only one responding village. Overall, more B. sudanica (83/10,249, 0.81%) were positive for S. mansoni than B. choanomphala (3/2,120, 0.14%). S. mansoni prevalence among B. sudanica in hotspot villages (60/5,346, 1.12%) was higher than in responding villages (23/4,903, 0.47%), and sentinel mice worm recoveries were also higher in hotspot villages, though variability among sampling times and locations precluded statistical significance in either case. The ratio of predatory echinostome to schistosome infections in snails was significantly higher at responding than hotspot sites. Conclusions/Significance: All ten villages offer considerable opportunities for transmission because of abundant Biomphalaria populations. Hot spot villages had higher prevalence of snails positive for S. mansoni suggesting transmission trends higher than in responding villages. Although recovery of B. choanomphala including individuals infected with S. mansoni was relatively low, these snails offer a different, potentially underestimated, deeper water avenue for transmission largely unavailable in the responding villages.