Background Schistosomiasis is a neglected tropical disease caused by trematodes of the genus Schistosoma. The pathogen is transmitted via freshwater snails. These snails indirectly benefit from agricultural pesticides which affect their enemy species. Pesticide exposure of surface waters may thus increase the risk of schistosomiasis transmission unless it also affects the pathogen.

Methodology We tested the tolerance of the free-swimming infective life stages (miracidia and cercariae) of Schistosoma mansoni to the commonly applied insecticides diazinon and imidacloprid. Additionally, we investigated whether these pesticides decrease the ability of miracidia to infect and further develop as sporocysts within the host snail Biomphalaria pfeifferi.

Principal findings Exposure to imidacloprid for 6 and 12 hours immobilized 50% of miracidia at 150 and 16 μg/L, respectively (nominal EC50); 50% of cercariae were immobilized at 403 and 284 μg/L. Diazinon immobilized 50% of miracidia at 51 and 21 μg/L after 6 and 12 hours; 50% of cercariae were immobilized at 25 and 13 μg/L. This insecticide tolerance is lower than those of the host snail B. pfeifferi but comparable to those of other commonly tested freshwater invertebrates. Exposure for up to 6 hours decreased the infectivity of miracidia at high sublethal concentrations (48.8 μg imidacloprid/L and 10.5 μg diazinon/L, i.e. 20 - 33 % of EC50) but not at lower concentrations commonly observed in the field (4.88 μg imidacloprid/L and 1.05 μg diazinon/L). The development of sporocysts within the snail host was not affected at any of these test concentrations.

Conclusions Insecticides did not affect the performance of S. mansoni at environmentally relevant concentrations. Accordingly, pesticide exposure is likely to increase the risk of schistosomiasis transmission by increasing host snail abundance without affecting the pathogen. Our results illustrate how the ecological side effects of pesticides are linked to human health, emphasizing the need for appropriate mitigation measures.