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 Schistosoma mansoni 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, shorelineassociated Biomphalaria sudanica from PHS and RESP villages did not differ in relative abundance or prevalence of S. mansoni infection. Water hyacinth intrusions were associated with increased B. sudanica abundance. The deeper water snail Biomphalaria choanomphala was significantly more abundant in the PHS villages, and prevalence of S. mansoni among villages both before and after control was positively correlated with B. choanomphala 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 S. mansoni abundance. Biomphalaria choanomphala provides an alternative, deepwater mode of transmission that may favor greater persistence of S. mansoni in PHS villages. As we found evidence for ongoing S. mansoni 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.

Snail-Related Contributions from the Schistosomiasis Consortium for Operational Research and Evaluation Program Including Xenomonitoring, Focal Mollusciciding, Biological Control, and Modeling

“…Possible inherent differences in snail-related aspects of transmission were investigated in 10 of these villages, six of which were identified as PHS villages, and four of which were responders, locations of which along with survey times and methods are detailed in the associated publication. 46 Initial prevalence values were 60.5-91.8% for PHS villages and 52.9-81.6% for responder villages. 46 Villages that were PHSs were located along the west-facing shore of the lake and responders faced the Winam Gulf, suggesting the potential importance of ecological factors.…”

“…46 Initial prevalence values were 60.5-91.8% for PHS villages and 52.9-81.6% for responder villages. 46 Villages that were PHSs were located along the west-facing shore of the lake and responders faced the Winam Gulf, suggesting the potential importance of ecological factors. Biomphalaria sudanica did not differ in the relative abundance or prevalence of S. mansoni infection between PHSs and responding villages, when all sampling sites, times, and water hyacinth presence/absence were taken into account.…”

“…Recoveries of S. mansoni from sentinel mice occurred but did not differ between PHSs and responder villages. 46 All 10 villages we examined provided evidence for potential ongoing S. mansoni transmission (either because of infected snails or positive sentinel mice), leading to the conclusion that conditions conducive for transmission and reinfection occur throughout the study area. Owing to rapid reinfections facilitated by large snail populations and movements of infected people around the lake, sustained control of schistosomiasis in this area may require an integrated, persistent, basin-wide approach.…”

“…Two multiyear intervention studies-one in Zanzibar 42,43 and one in Côte d'Ivoire 44 -evaluated the impact of adding focal mollusciding with niclosamide to MDA on human prevalence and intensity of schistosomiasis. Two observational studies were conducted-one in Côte d'Ivoire 45 and one in Kenya 46 -to explore the possible relationships between crustaceans, snail populations, and human infection.…”

Challenges in Protocol Development and Interpretation of the Schistosomiasis Consortium for Operational Research and Evaluation Intervention Studies

“…Schistosomiasis Consortium for Operational Research and Evaluation has since performed post hoc analysis of community-level risk factors in some depth, including assessment of the effects of starting prevalence, treatment compliance, local water supply, hygiene factors, snail abundance, road access, landscape use, rainfall, and other factors, but have not found any consistent population-specific or environmental factors linked to local risk of persistent high transmission in our study communities. [25][26][27][28] Because heterogeneity in community-level response to treatment is not easily predictable on this basis, it is not clear how rerandomization could have been performed or whether it would have ultimately been effective.…”