Water, sanitation, and hygiene in soil-transmitted helminth control: A scoping review of evidence gaps across transmission pathways.

Soil-transmitted helminths (STH) are parasitic intestinal worms infecting humans through contact with, or ingestion of, soil contaminated with eggs shed by infected individuals. STHs include roundworms (Ascaris lumbricoides, Strongyloides stercoralis), whipworm (Trichuris trichiura) and hookworms (Ancylostoma duodenale, Necator americanus, Ancylostoma ceylanicum). Their transmission is influenced by water, sanitation and hygiene (WASH) services and practices. Unlike past reviews, this scoping review did not assess effect sizes or strength of associations. Instead, it aimed to: map WASH-related risk factors in relation to STH infection; assess whether these align with plausible transmission pathways and conceptual frameworks; and characterise methodological features of the evidence base, including diagnostic approaches, that affect comparability of findings. We analysed 197 studies, of which 174 were observational and 23 were intervention studies. We found a substantial and growing body of evidence examining the relationship between WASH and other environmental exposures and the prevalence and intensity of STH infection. Nonetheless, important aspects such as food chain pathways, environmental surveillance, under-studied species, and measures of service and infrastructure uptake and use, quality, and community coverage over time, remain under-investigated, hampering drawing practical conclusions for improvement of service delivery. Existing and new research can be enhanced in several areas. Routine investigations like prevalence surveys should apply standard WASH definitions to reduce heterogeneity, and apply a conceptual framework of transmission in study design. Intervention studies should be based on theories of change that map actions disrupting transmission pathways, implementing a comprehensive intervention package and examining factors affecting behaviours or intervention compliance. Evaluation of intervention uptake should also be included. New research should address the evidence gaps identified, and incorporate timelines with realistic likelihood of impact into study planning and proposals, given the environmental persistence of certain STH species and the fact that developing lasting and sustained behaviour change can be slow.