Effects of different roadway deicing salts on host-parasite interactions: The importance of salt type.

The application of roadway deicing salts is increasing the salinity of freshwater systems. Increased salinization from salts, such as NaCl, CaCl and MgCl can have direct, negative impacts on freshwater organisms at concentrations found in nature. Yet, our understanding of how these salts can indirectly impact freshwater organisms by altering important ecological interactions, such as those between hosts and their parasites, is limited. Using a larval amphibian and infectious free-living helminth (i.e. trematode) model, we examined whether exposure to environmentally relevant concentrations of NaCl, CaCl and MgCl 1) influence trematode mortality; 2) alter amphibian-trematode interactions; and 3) alter larval amphibian activity (a behavior associated with parasite avoidance). We found that exposure to CaCl greatly reduced trematode survival across all Cl concentrations (230, 500, 860 and 1000 mg Cl L) while NaCl and MgCl had no effect. When both host and parasites were exposed to the salts, exposure to NaCl, but not MgCl or CaCl, increased infection. The lack of effect of CaCl on infection was likely driven by CaCl reducing trematode survival. Exposure to NaCl increased infection at 500 mg Cl L, but not 230 or 860 mg Cl L. Increased infection was not due to salt exposure altering tadpole behavior. Our results suggest that NaCl can negatively impact amphibian populations indirectly by increasing trematode infections in tadpole hosts.