A Search for Snail-Related Answers to Explain Differences in Response of Schistosoma mansoni to Praziquantel Treatment among Responding and Persistent Hotspot Villages along the Kenyan Shore of Lake Victoria

Abstract

Following a 4-year annual praziquantel (PZQ) treatment campaign, the resulting prevalence of <i>Schistosoma mansoni</i> was seen to differ among individual villages along the Kenyan shore of Lake Victoria. We have investigated possible inherent differences in snail-related aspects of transmission among such 10 villages, including six persistent hotspot (PHS) villages (≤ 30% reduction in prevalence following repeated treatments) located along the west-facing shore of the lake and four PZQ-responding (RESP) villages (> 30% prevalence reduction following repeated treatment) along the Winam Gulf. When taking into account all sampling sites, times, and water hyacinth presence/absence, shoreline-associated <i>Biomphalaria sudanica</i> from PHS and RESP villages did not differ in relative abundance or prevalence of <i>S. mansoni</i> infection. Water hyacinth intrusions were associated with increased <i>B. sudanica</i> abundance. The deeper water snail <i>Biomphalaria choanomphala</i> was significantly more abundant in the PHS villages, and prevalence of <i>S. mansoni</i> among villages both before and after control was positively correlated with <i>B. choanomphala</i> abundance. Worm recoveries from sentinel mice did not differ between PHS and RESP villages, and abundance of non-schistosome trematode species was not associated with <i>S. mansoni</i> abundance. <i>Biomphalaria choanomphala</i> provides an alternative, deepwater mode of transmission that may favor greater persistence of <i>S. mansoni</i> in PHS villages. As we found evidence for ongoing <i>S. mansoni</i> transmission in all 10 villages, we conclude that conditions conducive for transmission and reinfection occur ubiquitously. This argues for an integrated, basin-wide plan for schistosomiasis control to counteract rapid reinfections facilitated by large snail populations and movements of infected people around the lake.