Hypersaline roadside conditions influence larval amphibian susceptibility to ranavirus infection
With environmental change occurring at an unprecedented rate, studies suggesting links between human activity and the emergence of infectious diseases in wildlife are on the rise. Roads are a major anthropogenic disturbance covering around 1% of the area of the US and affecting nearly one fifth. Moreover, de-icing salt run-off is a cause of chronic salinization of freshwater wetlands, and is associated with decreased survival, growth and development rates in larval amphibians. Because hypersaline concentrations in roadside ponds are likely an osmoregulatory stressor for freshwater amphibians, we hypothesized animals living in roadside ponds will be more susceptible to infection. We tested this hypothesis with a two-part study. First, we monitored twenty ponds for ranavirus-associated die-offs to examine correlations with landscape features. Second, we conducted a dose-response experiment in which we exposed wood frog (Lithobates sylvaticus) tadpoles and metamorphs from ponds varying in distance to a paved road to a ranavirus (FV3; emerging pathogen of amphibians). Animals (N=10/dose, 4 roadside, 5 woodland ponds) were individually exposed to one of three doses of ranavirus or a control (unexposed) and monitored for survival. Susceptibility was measured in terms of mortality rate, proportion infected with a clinical infection, and liver viral titer (pathogen burden).
First, animals collected were not naive to ranavirus infection, and prevalence of this pathogen is high in this system. Interestingly, ponds with ranavirus-associated die-offs were more likely to occur near roads and had an eDNA viral concentration three orders of magnitude higher. Second, infection intensity at death was over a magnitude of order higher in larvae from roadside ponds in the control group; and at the low ranavirus dose (3 x 104 pfu) animals from roadside ponds had similar viral titer but two-thirds lower survival than those from woodland ponds, suggesting roadside animals are more susceptible to a secondary ranavirus infection. We found metamorphs migrating out of the pond with a ranavirus infection, suggesting a source of year-to-year reservoir in these ephemeral ponds. Furthermore, roadside larvae that survived infection through metamorphosis have a faster developmental rate at high pathogen burdens compared to woodland, suggesting a possible “escape the pond” response in highly infected roadside animals. Overall, roads may contribute to population declines by serving as a source of ranavirus propagation in low quality individuals in this matrix of ponds. Understanding the interaction between environment quality and wildlife disease dynamics is critical to predict population-level consequences of this anthropogenic disturbance.