Effects of Agrochemical Pollution on Schistosomiasis Transmission: A Systematic Review and Modeling Analysis

Abstract

Background: Agrochemical pollution of surface waters is a growing global environmental challenge, especially in areas where agriculture is rapidly expanding and intensifying. Recent work suggests that agrochemicals may affect schistosomiasis transmission through direct and indirect effects on Schistosoma parasites, their intermediate snail hosts, snail predators, and snail algal resources. Methods: We conducted a systematic review of agrochemical effects on the lifecycle of Schistosoma spp. and fitted dose-response models to data relating components of the lifecycle and agrochemical concentrations. We incorporated these dose-response functions and environmentally-relevant concentrations of agrochemicals into a mathematical model to estimate agrochemical effects on schistosomiasis transmission. Findings: We derived 120 dose-response functions describing effects of agrochemicals on schistosome lifecycle components. Agrochemical effects on estimates of the basic reproduction number, R0, for S. haematobium ranged from an increase of 306% (IQR: 246% – 362%) to transmission elimination (R0 -> 0). Simulations of transmission dynamics subject to interacting annual mass drug administration and agrochemical pollution yield a median estimate of 64·82 (IQR: 62·52 – 67·68) DALYs lost per 100,000 people per year attributable to atrazine use. In areas where aquatic arthropod predators of intermediate host snails suppress transmission, the insecticides chlorpyrifos and profenofos may also increase disability (6·82 [4·13 – 8·69] and 103·06 [89·63 – 104·90] DALYs lost per 100,000 people per year, respectively) through their toxic effects to arthropods. Interpretation: Expected environmental concentrations of agrochemicals alter schistosomiasis transmission through direct and indirect effects on intermediate host and parasite densities. As industrial agricultural practices expand in areas where schistosomiasis is endemic, strategies to limit increases in transmission due to agrochemical pollution should be developed and pursued. Funding Statement: Funding from the National Institutes of Health [1R01TW010286 and R01AI125842], the National Science Foundation Water, Sustainability and Climate program [1360330 and 1646708] and DS421 training program [1450053], and by the University of California Multicampus Research Programs and Initiatives [award MRP-17-446315]. Declaration of Interests: The authors report no competing interests.