Print PageSchistosomiasis (bilharzia), a disease caused by trematode helminths, is considered to be the second most economically debilitating disease 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 atrazine (herbicide), chlorpyrifos (insecticide), and fertilizer additions, alone and in all combinations, 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 and cercarial shedding rates.
Results/Conclusions
There was a significant effect of chlorpyrifos on community composition at the end of the experiment, showing a top-down effect where snail abundance increased after predators (crayfish and Belostomatid bugs) declined. However, all agrochemicals had significant main effects that increased snail reproduction, and all combinations of agrochemicals exhibited additive effects, with the combination of chlorpyrifos and fertilizer having a nearly significant interaction. Fertilizer increased phytoplankton and periphyton abundance, and there was a positive interaction of the combination of fertilizer and atrazine on periphyton abundance. This suggests that there may have been bottom-up effects of increased periphyton increasing snail reproduction, although top-down control in treatments that did not receive insecticides may have kept snail abundance similar to water and solvent controls. Chlorpyrifos significantly increased infection prevalence, and fertilizer also caused a nearly significant increase, with a nearly significant interaction between fertilizer and chlorpyrifos. Our results suggest that increasing insecticide contamination in wetlands will increase the abundance of the intermediate hosts of schistosomiasis, and will increase exposure of humans to schistosome cercariae. Furthermore, there is evidence that herbicides and fertilizer may also increase snail abundance and infection rates, especially in the presence of insecticides.
