Diarrhea causes approximately 500,000 deaths annually among children under five globally, with sub-Saharan Africa disproportionately affected. In Rwanda, prevalence remains 12–15% among under-fives despite improvements in water and sanitation, and evidence on the population-level effectiveness of household water treatment (HWT) remains inconsistent. We analyzed RDHS 2019–2020 data from 7,756 weighted children under five using descriptive statistics, chi-square tests, and multivariable logistic regression, with results reported as adjusted odds ratios (AORs) and 95% confidence intervals (CIs). The overall prevalence of diarrhea was 14.1%; HWT coverage was 33.6%. Diarrhea prevalence was lower in households practicing water treatment (12.5% vs. 15.0%; OR = 0.81; 95% CI: 0.70–0.93; p = 0.003); however, this association was not significant after multivariable adjustment (AOR = 1.02; 95% CI: 0.87–1.20; p = 0.786). Soap availability was the only WASH variable independently protective (AOR = 0.75; 95% CI: 0.63–0.89; p = 0.001). Child age 12–23 months (AOR = 2.086; 95% CI: 1.66–2.61, p < 0.001) and maternal education were strong predictors. Regionally, protective HWT effects were observed only in Kigali and the Southern Province. HWT was not independently associated with childhood diarrhea nationally. Soap availability, maternal education, and child age were the strongest modifiable predictors. Integrated WASH interventions combining handwashing promotion, maternal education, and region-targeted strategies are recommended.

Background: Diarrheal disease remains a major and persistent cause of morbidity and mortality in Rwanda, with substantial seasonal surges that strain routine services; however, transparent and operationally interpretable national forecasting has been underused for age-stratified burden. Methods: We analyzed the Rwanda Health Management Information System (HMIS) monthly diarrhea case counts (January 2015–December 2025), stratified by age group (under-five and five-and-above), and developed validated Seasonal Autoregressive Integrated Moving Average (SARIMA) forecasts for January 2026–December 2027. Stationarity was assessed using the Augmented Dickey–Fuller test and addressed through differencing. Candidate models were selected via rolling 5-fold cross-validation: Root Mean Square Error (RMSE), Mean Absolute Error (MAE), Akaike Information Criterion (AIC), Bayesian Information Criterion (BIC), and Mean Absolute Percentage Error (MAPE) and confirmed via Ljung–Box residual diagnostics, and benchmarked against seasonal naïve, Exponential Smoothing State-Space (ETS), and Seasonal-Trend decomposition using Loess (STL) + drift reference models. Results: Rwanda recorded 6,309,098 diarrhea cases during 2015–2025, with 49.2% among under-fives; while absolute counts were higher in those aged ≥5 years, risk remained consistently higher in under-fives (91.7–229.5 per 1000) than in those ≥5 years (17.9–34.3 per 1000). Both series showed strong annual seasonality with recurrent peaks in August–November, and forecasts suggest this pattern will persist through 2026–2027. Conclusions: These findings suggest a provisional seasonal (pre-peak, peak, and post-peak) preparedness framework and age-differentiated planning signals, underscoring that burden and risk are not inter changeable across age groups.