Water, sanitation, and hygiene in schistosomiasis control: A scoping review of evidence gaps across transmission pathways

Schistosomiasis is a water-based disease transmitted through contact with freshwater contaminated by cercariae from infected snails. Transmission is shaped by access to water, sanitation, and hygiene (WASH) services, human behaviour, and environmental factors. While several systematic reviews have examined WASH-schistosomiasis links, a synthesis of emerging evidence and identification of knowledge gaps across the transmission pathways is needed. This scoping review aimed to map WASH-related risk factors linked to schistosomiasis transmission, to assess alignment with a schistosomiasis-specific conceptual framework, and to characterise methodological features of the evidence base, including diagnostics and WASH measurement. We searched five databases, supplemented by a call for papers, yielding 2915 de-duplicated records; 181 were included (160 observational, 21 interventional studies). We developed a conceptual framework to map WASH-related transmission pathways and identify evidence gaps. Our analysis reveals a growing body of research on WASH access and practices, yet key gaps persist. Observational studies mostly examined domestic, recreational, and occupational water contact, with limited attention to transmission pathways such as agricultural use of faecal waste; most relied on simple access measures, rarely assessing WASH quality, functionality, or sustained use, limiting the ability to translate findings into effective service delivery strategies. Among interventional studies, 43% focussed on water access, 24% implemented integrated WASH and health education, and 19% sanitation, hygiene, and education. Over 90% reported reduced infection, but few assessed sustained uptake or environmental impacts. Limitations include lack of species-specific reporting, inconsistent WASH definitions, and insensitive diagnostics. Future studies would benefit from adopting standardised WASH definitions and transmission-specific frameworks, report outcomes by species and infection intensity, and design interventions with explicit theories of change addressing transmission pathways. Intervention studies should assess fidelity, sustained uptake, and community-level coverage alongside infection outcomes, with follow-up to capture reinfection dynamics, strengthening evidence for integrating WASH with preventive chemotherapy in elimination programmes.