Role of temperature, transmission pathways, and pathogen load in Batrachochytrium salamandrivorans epidemiology

Amphibian populations are experiencing dramatic population declines. Amphibian diseases are one of the major reasons for these declines. A newly described fungal pathogen, Batrachochytrium salamandrivorans (Bsal), has been recently introduced to Europe and is currently decimating European salamander populations. Sampling efforts have not detected Bsal yet in the United States in captive or wild populations. Disease management hinges on knowing how environmental, host, and pathogen interactions influence disease occurrence and severity. Understanding routes of pathogen transmission is also critical as knowledge of these routes can elucidate management strategies. In Chapter I of my dissertation, I measured how environmental temperature influenced host susceptibility and Bsal pathogenicity by exposing two life stages (eft and adult) of N. viridescens to Bsal at three environmentally relevant temperatures (6, 14 and 22°C). Adult N. viridescens were resistant to Bsal infections and limited infection and mortality were observed for efts exposed at 22°C. In contrast, high rates of mortality were observed for both life stages at the optimal in vitro growth temperature for Bsal (14°C) and at the coldest exposure temperature (6°C). In Chapter II, I evaluated the effectiveness of direct contact as a Bsal transmission pathway. I found that the probability of transmission given contact increased as Bsal loads on infected N. viridescens increased. Similarly, individuals with greater Bsal loads on their skin, shed greater quantities of Bsal zoospores into an aquatic environment. These results suggest that Bsal is a load-dependent pathogen and that management strategies aimed at increasing host resistance might reduce both direct and indirect transmission probability by reducing pathogen loads on infected individuals. In another transmission experiment, I explored whether infected carcasses could play a role in Bsal transmission. We determined that infected carcasses increase the length of infectiousness for Bsal-infected animals by up to at least three days. Infected carcasses likely play an important role in Bsal epidemiology and should be included in future disease modeling efforts. Results from my research suggest that management of Bsal should take a holistic approach, incorporating strategies that reduce host contact rates, decrease Bsal persistence in the environment, and increase host resistance to infection.