Pesticide pollution can alter parasite transmission, but scientists are unaware if effects of pesticides<br>on parasite exposure and host susceptibility (i.e. infection risk given exposure) can be generalised<br>within a community context. Using replicated temperate pond communities, we evaluate effects<br>of 12 pesticides, nested in four pesticide classes (chloroacetanilides, triazines, carbamates<br>organophosphates) and two pesticide types (herbicides, insecticides) applied at standardised environmental<br>concentrations on larval amphibian exposure and susceptibility to trematode parasites. Most<br>of the variation in exposure and susceptibility occurred at the level of pesticide class and type, not<br>individual compounds. The organophosphate class of insecticides increased snail abundance (first<br>intermediate host) and thus trematode exposure by increasing mortality of snail predators (top–<br>down mechanism). While a similar pattern in snail abundance and trematode exposure was<br>observed with triazine herbicides, this effect was driven by increases in snail resources (periphytic<br>algae, bottom–up mechanism). Additionally, herbicides indirectly increased host susceptibility and<br>trematode infections by (1) increasing time spent in susceptible early developmental stages and (2)<br>suppressing tadpole immunity. Understanding generalisable effects associated with contaminant<br>class and type on transmission is critical in reducing complexities in predicting disease dynamics in<br>at-risk host populations.
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