Effect of agrochemical exposure onSchistosoma mansonicercariae survival and activity
Authors: Jones Devin K. , Davila David D. , Nguyen Karena H. , Rohr Jason R. 
Published: July 2019

Land conversion and agrochemical use has altered freshwater systems worldwide, introducing chemicals and pathogens (e.g., helminths) that threaten human health. In developing countries where stringent pesticide use and water treatment is limited, understanding how contaminants and pathogens interact is of particular importance. Schistosomiasis, a neglected tropical disease, is caused by the free-swimming cercariae ofSchistosoma mansoni, a flatworm (trematode) that is transmitted from snails to humans. Schistosomiasis afflicts over 200 million people, reinforces poverty, and has an enormous impact on children. To investigate the effects of pesticide exposure onS. mansoni, we exposed cercariae to four insecticides (cypermethrin, deltamethrin, dimethoate, and methamidophos) at five concentrations above estimated environmental concentrations, and recorded survival and activity during a 24-hr time-to-death assay. To identify live, but paralyzed, cercariae from dead cercariae, we used Trypan blue dye, which is only expelled from live cells. We found no effect of cypermethrin, deltamethrin, or dimethoate exposure on the survival and activity ofS. mansonicercariae. Surprisingly, methamidophos exposure decreased activity and increased survival of cercariae compared to those in control treatments. This result is likely due to methamidophos causing paralysis of cercariae, which reduced energy consumption lengthening lifespan. Although methamidophos exposure increased survival time, the pesticide-induced paralysis left cercariae functionally dead, which could influence overall disease prevalence and thus human health. Future studies that examine the influence of agrochemicals on waterborne disease prevalence and transmission need to consider both the lethal and sublethal effects of exposure to fully understand the complexity of host-parasite interactions.

Author Summary

Previous methods used to investigate the effects of pesticide exposure on free-swimming life stages of trematode pathogens include 1) normal activity, 2) movement following stimuli, or 3) staining dyes. As pesticides commonly target motor function, the use of an individual metric to assign trematode survival might misidentify pesticide-induced paralysis as mortality, therefore underestimating trematode tolerance. In this study, we used activity assays in tandem with Trypan blue staining dye to assess the effects of four pesticides onSchistosoma mansonicercariae. We found that cercariae are highly tolerant to pesticide levels far beyond environmentally relevant concentrations. Surprisingly, exposure to methamidophos increased the survival and decreased the activity of cercariae compared to those in control treatments. Reduced activity was presumably caused by methamidophos-induced paralysis of cercariae. Although we observed increased survival following methamidophos exposure, the pesticide-induced paralysis rendered cercariae functionally dead. Our results highlight the need for future assays examining trematode tolerance to contaminants to employ both activity assays and staining dye to discern cercarial paralysis from mortality. Understanding the effects of pesticide exposure on disease transmission is of vital importance as pesticide use and agricultural activities intensify in developing nations endemic to waterborne pathogens.