**Background:** Diarrhoea kills an estimated 525,000 children under 5 years annually. Water, sanitation, and hygiene (WASH) improvements aim to reduce enteric infections by interrupting faecal-oral transmission, but recent trials have shown mixed or null effects on child diarrhoea and growth. Measuring enteropathogens and host-specific faecal markers in environmental samples can help determine whether interventions actually reduce exposure to faecal contamination. Previous reviews focused on faecal indicator bacteria (FIB), which correlate poorly with pathogens and cannot distinguish human from animal sources. This systematic review and IPD meta-analysis assessed the effects of WASH interventions on enteropathogens and microbial source tracking (MST) markers in environmental samples.
**Methods:** The authors searched PubMed, Embase, CAB Direct Global Health, Agricultural and Environmental Science Database, Web of Science, and Scopus from Jan 1, 2000, to Jan 5, 2023, for prospective studies with WASH interventions and concurrent controls that measured pathogens or MST markers in environmental samples and child health outcomes. Individual participant data were obtained from five eligible trials (WASH Benefits Bangladesh, WASH Benefits Kenya, MapSan Mozambique, Gram Vikas India, Total Sanitation Campaign India). Environmental samples included stored and source drinking water, hand rinses, soil, and flies. The primary outcomes were composite measures: detection of any pathogen or any MST marker in any sample type. Study-specific prevalence ratios (PRs) were estimated using modified Poisson regression with robust standard errors, adjusted for potential confounders, and pooled using random-effects models. Secondary outcomes included pathogen class, host-specific MST markers, and abundance measures.
**Key Results:** The pooled dataset included 12,184 samples with 40,156 observations across nine environmental assessments from five studies. All interventions evaluated onsite sanitation (improved latrines with pits or septic tanks); none met the SDG standard of safely managed sanitation. Latrine access was higher in intervention groups (78–97% vs 18–45% in controls), but child latrine use was low and safe management of child and animal faeces was uncommon. Interventions were consistently associated with reduced pathogen detection, but most individual study estimates had confidence intervals crossing the null. Pooled across studies, there was a small reduction in the prevalence of any pathogen in any sample type (pooled PR 0.94, 95% CI 0.90–0.99). Interventions reduced bacterial pathogens (pooled PR 0.92, 95% CI 0.85–0.99) but not viruses (pooled PR 0.90, 95% CI 0.62–1.33). The largest pathogen-specific reductions were for adenovirus (PR 0.21, 95% CI 0.06–0.68) and Shigella (PR 0.28, 95% CI 0.10–0.78) in one study. Interventions had no effect on human MST markers (pooled PR 1.00, 95% CI 0.88–1.13) or animal MST markers (pooled PR 1.00, 95% CI 0.97–1.03). Abundance analyses were limited; only 8 of 45 target-sample combinations had sufficient data. The BacCow animal marker on mothers' hands was reduced in the intervention group (adjusted log10 difference –0.28, 95% CI –0.49 to –0.07). Subgroup analyses by season, animal ownership, and zoonotic potential showed no consistent differences. No significant heterogeneity was detected by setting, study design, follow-up duration, or intervention uptake.
**Clinical Implications:** The small reduction in pathogen prevalence and absence of effect on faecal markers are consistent with the small or null health effects previously reported in these trials. The findings suggest that basic onsite sanitation interventions did not adequately contain human waste or reduce environmental exposure to enteropathogens, likely because young children's faeces and animal faeces remained unsafely managed. Only the WASH Benefits Bangladesh trial, which included tools for child and animal faeces management (potties and sani-scoops), showed reduced animal markers and was the only trial with significant diarrhoea reduction. The authors conclude that more comprehensive WASH approaches—including safe excreta removal rather than mere containment, and interventions targeting child and animal faeces—are needed to interrupt environmental pathogen transmission and improve child health. Limitations include the small number of eligible studies (all focused on sanitation), heterogeneity in pathogen targets and laboratory methods, low pathogen prevalence limiting precision, and lack of studies on water treatment, hygiene, or sewered sanitation interventions.