PROJECT SUMMARY Soil-transmitted helminth (STH) infections (intestinal worms) affect 1.5 billion individuals globally. Recent evidence from trials and modeling studies suggests that community-wide mass drug administration (cMDA) of deworming medication with sufficient coverage and adherence can eliminate STH transmission. However, in settings with ongoing environmental transmission and low coverage of networked sanitation, persistent environmental reservoirs of STH eggs result in high reinfection rates, and hinder progress towards elimination. STH control programs use human stool-based methods to assess STH prevalence and intensity in endemic settings. However, individual stool sampling is expensive and logistically difficult, particularly when human infection prevalence is low. If collecting and analyzing soil from locations in communities with high human activity (e.g. home entrances, water collection points, schools) were established to be equally or more sensitive than human stool diagnostics, this approach may represent a less invasive and more cost-effective surveillance tool for MDA program monitoring and evaluation. This study will leverage a multi-country cluster-randomized controlled trial delivering three-years of biannual cMDA in Benin and India (DeWorm3). The trial will collect and analyze by qPCR, human stool samples 24 months (n=80,000 in total) after the final round of cMDA, which will yield highly accurate human STH infection prevalence estimates in the study areas. Through extensive laboratory studies and field testing in India and Benin, our team has developed a sensitive and specific molecular method for detecting STH environmental DNA (eDNA) in large volumes of soil. We will nest soil sampling within the trial at the same time point as human stool collection and one year later with the specific aims to: 1) Quantify the effect of a biannual cMDA intervention on the soil STH reservoir; 2) Determine whether soil STH eDNA levels can predict community-level human STH infection prevalence; and 3) Develop the optimal soil sampling strategy and compare costs to human stool-based surveillance. Pairing soil STH eDNA assessments with human infection prevalence data already being collected by the DeWorm3 trial is a unique and time sensitive opportunity to validate and test the utility of environmental STH surveillance. Our findings will also contribute to understanding the conditions under which MDA program integration with improved sanitation interventions is needed for achieving sustained reductions in STH infection prevalence.