Seasonally varying effects of improved water, sanitation and handwashing interventions on Giardia infection in Bangladesh

BackgroundGiardia is the most common enteric parasite among children in low-resource settings, causing diarrhoea and leading to prolonged infection or asymptomatic carriage. We assessed whether the effect of water, sanitation and handwashing (WSH) interventions on Giardia infection among rural Bangladeshi children varies with seasonal conditions. MethodsWe conducted a secondary analysis of the WASH Benefits Bangladesh cluster-randomized trial, with 450 clusters assigned to four arms in a 2x2 factorial design (WSH: WSH, WSH+Nutrition; no WSH: Control, Nutrition). Giardia infection was measured by multiplex real-time PCR in stool samples after two years of intervention. Effects were estimated by marginal treatment and assessed for heterogeneity by season when Giardia was measured. We also assessed heterogeneity by cumulative exposure to dry and monsoon seasons from birth to measurement age to estimate the exposure history of Giardia. ResultsGiardia prevalence, measured among 2773 children (median age: 30 months, range: 22-38 months), was higher in the dry seasons (32%) than in the monsoon (21%). The effect of WSH was consistent on the relative (20% reduction) and absolute scales, with slightly greater absolute reduction during the dry season (dry: -6.1%, -10.1% to -2.1%). With increasing dry-season exposure, the period of highest risk, prevalence remained consistently lower in the WSH group, with the largest differences between study groups among children with more than 17 months of dry-season exposure by age over two years. ConclusionWe demonstrate how WSH provides resilience to seasonal variation in infection risk and mitigates climate-driven, seasonally varying Giardia transmission. Key MessagesO_LIWhat your research question was We investigated whether the effect of water, sanitation and handwashing (WSH) interventions on Giardia infection among rural Bangladeshi children varies with seasonal conditions. C_LIO_LIWhat you found We found that Giardia risk varied seasonally, with improved WSH providing consistent relative and absolute reductions -- slightly greater on the absolute scale during the dry season -- and larger reductions among children with greater cumulative exposure to dry months, reflecting higher baseline risk and demonstrating how WSH enhances resilience to seasonally varying infection risk. C_LIO_LIWhy it is important Since the dry and monsoon seasonal cycle is extreme in Bangladesh, this study demonstrates how WSH can mitigate seasonally varying Giardia infection risk. C_LI