Evaluating agrochemical contributions to increased intermediate host density and infection prevalence of schistosome parasites

Background/Question/Methods

Schistosomiasis, a disease caused by trematode helminths, is considered to be among the second most economically debilitating diseases in developing tropical and subtropical countries by the World Health Organization. Many people in schistosomiasis-endemic countries are stricken by poverty and malnourishment that also affects their health.  To reduce poverty and improve human health, these countries are expected to enhance their agriculture, an approach supported by the Millenium Development Goals. This will almost certainly entail an increase in the use of agrochemicals, such as fertilizer and pesticides. However, agrochemical pollution has been associated with increased trematode burdens in amphibians by indirectly increasing populations of intermediate hosts (snails). We hypothesized that agrochemicals would have a similar effect on the risk of schistosomiasis. We used freshwater mesocosms to determine the community-level effects of thirteen agricultural chemicals – chloroacetanilide herbicides (acetochlor, alachlor, metolachlor), triazine herbicides (atrazine, propazine, simazine), organophosphate insecticides (chlorpyrifos, malathion, terbufos), and pyrethroid insecticides (esfenvalerate, lambda-cyhalothrin, permethrin), and fertilizer – on wetland communities including the intermediate hosts for two schistosome species, Schistosoma mansoni and S. haematobium. We added live eggs of each schistosome species during the experiment to measure infection prevalence.

Results/Conclusions

Fertilizers and insecticides increased snail intermediate host densities indirectly through either bottom-up or top-down effects, respectively. Both organophosphate and pyrethroid insecticides induced a top-down effect in which snail density increased after predators (crayfish and belostomatid bugs) declined. Fertilizer increased periphyton biomass, and subsequently increased snail densities. Herbicides reduced periphyton growth and decreased snail densities. Insecticides increased infection prevalence in both snail species, but fertilizer increased infection prevalence only in Bulinus truncatus. Individual pesticides within a chemical class had similar effects on snail abundance. However, malathion decreased predator densities (and increased snail densities) less than the other organophosphates. Likewise, lambda-cyhalothrin, did not reduce predator densities as much as the other pyrethroid insecticides, and thus the subsequent increase in snail densities was also relatively lower. Our results suggest that both top-down and bottom-up regulation of snail intermediate hosts play important roles in schistosomiasis dynamics. Furthermore, the indirect effects of pesticides on snail densities can be generally well predicted by the type (e.g., herbicide or insecticide) of pesticide. Finally, identifying specific pesticides that are less toxic to predators of snails might reduce the risk of human exposure to schistosome cercariae.