Effects of road salt on a free-living trematode infectious stage

Many temperate freshwater habitats are at risk for contamination by run-off associated with the application of road de-icing salts. Elevated salinity can have various detrimental effects on freshwater organisms, including greater susceptibility to infection by parasites and pathogens. However, to better understand the net effects of road salt exposure on host-parasite dynamics, it is necessary to consider the impacts on free-living parasite infectious stages, such as the motile aquatic cercariae of trematodes. Here, we examined the longevity and activity of cercariae from four different freshwater trematodes (Ribeiroia ondatrae, Echinostoma sp., Cephalogonimus sp. and an unidentified strigeid-type) that were exposed to road salt at five different environmentally relevant concentrations (160, 360, 560, 760 and 960 mg/ml of sodium chloride). Exposure to road salt had minimal detrimental effects, with cercariae activity and survival often greatest at intermediate concentrations. Only the cercariae of Cephalogonimus sp. showed reduced longevity at the highest salt concentration, with those of both R. ondatrae and the unidentified strigeid-type exhibiting diminished activity, indicating interspecific variation in response. Importantly, cercariae seem to be relatively unaffected by salt concentrations known to increase infection susceptibility in some of their hosts. More studies will be needed to examine this potential dichotomy in road salt effects between hosts and trematodes, including influences on parasite infectivity.