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

Background Giardia 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. Methods We 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 cumulative seasonal exposure history. Results Giardia 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%). Improved WSH was associated with a consistent 20% relative reduction and a modest absolute reduction that was slightly greater during the dry season (-6.1%, 95% CI: -10.1% to -2.1%), although interaction test did not show strong evidence of effect modification by season. Prevalence remained lower in the WSH group across increasing dry season exposure, with the largest differences among children with more than 17 months of dry-season exposure. Conclusion We demonstrate how WSH provides resilience to seasonal variation in infection risk and mitigates climate-driven, seasonally varying Giardia transmission.