A Common Agrochemical Mixture Increases Snail Hosts and Ecological Drivers of Schistosomiasis Risk

ABSTRACT Agricultural intensification is accelerating across tropical regions, increasing fertilizer and pesticide inputs into freshwater ecosystems that regulate the transmission of many environmentally mediated diseases. For schistosomiasis, a major neglected tropical disease, transmission risk is strongly governed by the abundance of freshwater snail intermediate hosts and their aquatic habitat, yet most agrochemical studies have focused on single compounds rather than realistic mixtures. Using replicated semi-natural aquatic microcosms, we experimentally tested how nitrogen– urea fertilizer and the widely used herbicide atrazine, applied alone and in combination at environmentally relevant concentrations, alter snail hosts ( Biomphalaria glabrata ), submerged aquatic vegetation ( Ceratophyllum demersum ), and algal resources that underpin schistosomiasis transmission. Fertilizer and atrazine, both singly and together, increased snail habitat ( C. demersum ) and adult snail abundance. By reducing phytoplankton, atrazine likely alleviated shading on vegetation, thereby promoting habitat that supported snails through indirect bottom-up pathways. Agrochemical mixtures produced non-additive effects on host and habitat dynamics, indicating that transmission-relevant ecological responses cannot be predicted from single-compound exposures alone. Although parasite infection levels were low, the observed increases in snail hosts and habitat represent reliable ecological drivers of schistosomiasis risk. Synthesis and applications. Our findings demonstrate that common agrochemical mixtures can restructure freshwater food webs in ways likely to elevate disease risk, highlighting the need to incorporate mixture-driven ecological responses into agrochemical risk assessment and agricultural management strategies in disease-endemic regions.