Salt in the Wound: Assessing Pathogen Susceptibility in Amphibian Populations Across a Gradient of Salt Pollution

Freshwater salinization is an anthropogenic pollution issue that is receiving exponentially greater attention as we appreciate the global nature of the problem and the widespread lethal and sublethal impacts. A growing body of research is discovering that some freshwater taxa can evolve increased salt tolerance over generations, but such evolved tolerance often has tradeoffs in performance and fitness. We examined whether tadpoles collected from nine wood frog populations (Rana sylvatica), which have been exposed to generations of different elevated salt concentrations, differ in their susceptibility to common amphibian pathogens. When exposed to echinostomes, all tadpoles became infected, but there was no mortality and the populations had similar pathogen loads. When exposed to ranavirus (FV3), there were large population differences in survival, with populations from saltier ponds exhibiting lower survival. However, among the survivors, populations from saltier ponds had lower viral loads, suggesting higher resistance. Collectively, these results suggest that historic salt exposures in ponds can be associated with differences in susceptibility to some pathogens, although much more work needs to be conducted on additional populations, host species, and pathogen species.