Effect of Agrochemical Exposure on Schistosoma mansoni Cercariae Survival and Activity

Abstract

Singular use of activity assays or staining dyes to assess pathogen agrochemical tolerance can underestimate tolerance if pesticides cause sublethal effects. We exposed Schistosoma mansoni cercariae, the aquatic life stage of this trematode that infects humans, to 4 insecticides at 5 concentrations using a 24-h time-to-death assay. We used Trypan blue dye, which stains dead tissue, and activity assays simultaneously to discriminate dead from live but paralyzed individuals. Whereas cypermethrin, deltamethrin, and dimethoate exposure did not affect cercariae at any ecologically relevant concentrations, methamidophos exposure increased survival of cercariae compared with those in the controls. This was because methamidophos-induced paralysis reduced cercarial activity and thus energy expenditures, extending the lifespan of this short-lived parasite that causes human schistosomiasis. These findings highlight that sublethal effects should be considered when pesticide effects on disease are under investigation. Environ Toxicol Chem 2020;39:1421-1428. © 2020 SETAC.

Keywords: Ecotoxicology; Organophosphate; Parasite; Pyrethroid; Trematode.

FIGURE 1:

Survival of Schistosoma mansoni cercariae following exposure to one of 6 methamidophos concentrations. Cercariae were exposed to 0, 100, 200, 300, 400, or 500 mg/L methamidophos for 24 h using a time-to-death assay. The pooled control treatment represents the combined survival of cercariae in the water and vehicle control treatments.

FIGURE 2:

Mean cercarial activity (%) of Schistosoma mansoni following exposure to one of 6 methamidophos concentrations. We recorded the number of cercariae active over 24 h following exposure to 0, 100, 200, 300, 400, or 500 mg/L methamidophos. We calculated cercarial activity (%) by dividing the number of trematodes observed moving by the total number of individuals in the well. We observed the main effect of methamidophos concentration (A) and time (B) on cercarial activity. Data points represents overall treatment mean values ±1 standard error.