Crossroads of stress and disease:the influence of road runoff on amphibian disease susceptibility

Emily Marie Hall

One of the primary questions of disease ecology is how do environmental conditions influence host-pathogen dynamics. From a physiological perspective, disease susceptibility of hosts can be worsened by the immunosuppressive effects of chronic environmental stress. However, while environmental stress is implicated in the emergence of infectious diseases in wildlife, finding a causal relationship between an environmental stressor, disease susceptibility and outbreaks in wild populations still remains a challenge. Here, we took an integrative approach to test the hypothesis that elevated salinity caused by road runoff of de-icing salts increases the occurrence of disease outbreaks within the wood frog-ranavirus host-pathogen system in northeastern Connecticut, USA. First, we characterized the physiological effects of road runoff in natural populations and found larvae exhibited slower growth and sluggish behavior, and adult males showed severe water retention and elevated baseline stress hormones. Next, to assess pathogen prevalence in pond communities, we validated a technique to measure ranavirus DNA in pond water (environmental DNA). We found ranavirus was ubiquitous in our study region, but die-offs of larvae occurred more often in high salinity ponds. In addition, the timing of die-offs coincided with increases in temperature and the occurrence of susceptible developmental stages. Lastly, we conducted a set of mechanistic experiments to determine whether elevations in salinity causes increases in disease-induced mortality and ranavirus transmission. Ultimately, when dealing with the additional perturbation of infection, salt-stressed individuals had greater mortality. Further, road salt exposure increased the glucocorticoid response to infection, infection intensity, and viral shedding. Therefore, osmotic stress increases transmission and mortality from infection, leading to an exponential increase in prevalence and infection intensity in roadside ponds. Together these studies demonstrate a causal relationship between a sub-lethal stressor which deteriorates health to the point that infections are lethal, and the likelihood of die-offs in larval amphibian populations. This dissertation demonstrates the importance of combining physiology, ecology, and epidemiology frameworks in designing studies to identify causal relationships between environmental change and disease. With this information, we can better advise conservation efforts about the importance of monitoring ranaviruses in wildlife and to mitigate the negative effects of road salt runoff.

Crossroads of stress and disease:the influence of road runoff on amphibian disease susceptibility

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