Parasites play a significant role in shaping host populations of humans, wildlife, and plants. As human populations grow, anthropogenic activities (i.e. chemical use) will continue to alter the environment in which host-parasite interactions occur. For example, antibacterial compounds are commonly used in livestock to treat and prevent bacterial infections. Through runoff events, low doses of these chemicals commonly contaminate aquatic ecosystems. Toxicity assays suggest low concentrations of many antibacterials are nonhazardous to aquatic species. However, though environmentally-relevant concentrations of antibacterials may not have direct toxic effects, they may still indirectly interact with parasites to influence host species. Given this, we need to consider how anthropogenic chemicals alter host-parasite interactions. Towards this goal, using larval leopard frogs (Lithobates pipiens) and two amphibian parasites (the trematode Echinoparyphium and ranavirus), we examined the influence of the antibacterial Sulfadimethoxine (SDM) on tadpole growth and susceptibility to parasites. In the first experiment, we conducted a factorial experiment with two trematode exposure treatments (0 or 50 Echinoparyphium) and two SDM treatments (0 or 1 µg/L SDM). In the second experiment, we conducted a factorial experiment with two ranavirus exposure treatments (0 or 103 plaque forming units [PFUs]/mL) and two SDM treatments (0 or 1 µg/L SDM).
When exposed to Echinoparyphium, we found that SDM exposure had no effect on tadpole growth. However, tadpole susceptibility to Echinoparyphium increased when exposed to SDM. Tadpoles exposed to SDM had 180% more Echinoparyphium encysted when compared to treatments not exposed to SDM. These results suggest that SDM may be able to alter tadpole-trematode interactions. When exposed to ranavirus, tadpoles exposed to SDM were 20% smaller than tadpoles not exposed to SDM. However, we saw no effect on tadpole susceptibility to ranavirus. Our ranavirus results suggest that while there was no direct effect on tadpole susceptibility to the virus itself, it is possible that there were indirect effects on host growth. Collectively, our results suggest that antibacterials can affect tadpole size and potentially alter disease dynamics within hosts, underscoring the importance of integrating disease ecology with ecotoxicology.