Soil-transmitted helminth reinfection after anthelmintic treatment in schools with improved versus unimproved WASH infrastructure in Mozambique: a prospective cohort study.

Background Soil-transmitted helminths (STH) infect more than a billion people worldwide, disproportionately affecting communities with poor water, sanitation, and hygiene (WASH). Mass drug administration is the primary control strategy, but reinfection frequently occurs within months of treatment, and the extent to which improved WASH reduces reinfection risk remains unclear. We conducted a prospective cohort study comparing STH reinfection rates over seven months after anthelmintic treatment among school-aged children attending schools with improved versus unimproved WASH in the Manhiça district, Mozambique, using reinfection dynamics as a proxy for the impact of WASH interventions. Methods Children were screened for STH using Kato-Katz microscopy, then dewormed with albendazole and praziquantel (plus ivermectin for those positive for T. trichiura and/or S. stercoralis). A follow-up sample was collected 21 days post-treatment from initially positive children to assess the cure rate (CR) and egg reduction rate (ERR). Cured participants were enrolled in the WASH-improved or WASH-unimproved cohort and followed monthly for seven months. Reinfection and incidence rates were estimated, and Cox proportional hazards regression was used to assess the association between WASH cohort and time to reinfection. Results STH prevalence was 17.5% (95% CI: 15.2–19.8; n=178/1018), comprising T. trichiura (9.5%), A. lumbricoides (6.3%), S. stercoralis (2.6%), and hookworms (1.9%). No heavy-intensity infections were observed; most T. trichiura (94.9%) and hookworm (94.7%) infections were light, whereas 43.8% of A. lumbricoides infections were moderate. CR/ERR were 88.4%/96.56% for T. trichiura (n=95) and 93.3%/93.36% for A. lumbricoides (n=50). Overall reinfection rate was 32.3% (n=54/167), higher in the improved-WASH cohort (39.3%) than in the unimproved-WASH cohort (18.2%), corresponding to 66.38 reinfections per 1,000 person-months at risk overall (82.27 vs. 35.89 by cohort). The unimproved-WASH cohort had a lower hazard of reinfection (HR=0.456; 95% CI: 0.229–0.908; p=0.0254). Conclusions Despite effective treatment, over a third of children were reinfected within seven months, with reinfection unexpectedly higher in the improved-WASH cohort. This suggests that infrastructure-based WASH classification may not capture true exposure or behavioural risk, underscoring the need for household-level monitoring to assess WASH impact alongside MDA.