Environmental temperature is a key factor driving various biological processes, including immune defenses and host-pathogen interactions. Here, we evaluated the effects of environmental temperature on the pathogenicity of the emerging fungal pathogen, Batrachochytrium salamandrivorans (Bsal), using controlled laboratory experiments, and measured components of host immune defense to identify regulating mechanisms. We found that adult and juvenile Notophthalmus viridescens died faster due to Bsal chytridiomycosis at 14°C than at 6 and 22°C. Pathogen replication rates, total available proteins on the skin, and microbiome composition likely drove these relationships. Temperature-dependent skin microbiome composition in our laboratory experiments matched seasonal trends in wild N. viridescens, adding validity to these results. We also found that hydrophobic peptide production after two months post-exposure to Bsal was reduced in infected animals compared to controls, perhaps due to peptide release earlier in infection or impaired granular gland function in diseased animals. Using our temperature-dependent susceptibility results, we performed a geographic analysis that revealed N. viridescens populations in the northeastern United States and southeastern Canada are at greatest risk for Bsal invasion, which shifted risk north compared to previous assessments. Our results indicate that environmental temperature will play a key role in the epidemiology of Bsal and provide evidence that temperature manipulations may be a viable disease management strategy.

Batrachochytrium salamandrivorans (Bsal) is a fungal pathogen of amphibians that is emerging in Europe and could be introduced to North America through international trade or other pathways. To evaluate the risk of Bsal invasion to amphibian biodiversity, we performed dose-response experiments on 35 North American species from 10 families, including larvae from five species. We discovered that Bsal caused infection in 74% and mortality in 35% of species tested. Both salamanders and frogs became infected and developed Bsal chytridiomycosis. Based on our host susceptibility results, environmental suitability conditions for Bsal, and geographic ranges of salamanders in the United States, predicted biodiversity loss is expected to be greatest in the Appalachian Region and along the West Coast. Indices of infection and disease susceptibility suggest that North American amphibian species span a spectrum of vulnerability to Bsal chytridiomycosis and most amphibian communities will include an assemblage of resistant, carrier, and amplification species. Predicted salamander losses could exceed 80 species in the United States and 140 species in North America.

Transmission is the fundamental process whereby pathogens infect their hosts and spread through populations, and can be characterized using mathematical functions. The functional form of transmission for emerging pathogens can determine pathogen impacts on host populations and can inform the efficacy of disease management strategies. By directly measuring transmission between infected and susceptible adult eastern newts (Notophthalmus viridescens) in aquatic mesocosms, we identified the most plausible transmission function for the emerging amphibian fungal pathogen Batrachochytrium salamandrivorans (Bsal). Although we considered a range of possible transmission functions, we found that Bsal transmission was best explained by pure frequency dependence. We observed that >90% of susceptible newts became infected within 17 days post‐exposure to an infected newt across a range of host densities and initial infection prevalence treatments. Under these conditions, we estimated R0 = 4.9 for Bsal in an eastern newt population. Our results suggest that Bsal has the capability of driving eastern newt populations to extinction and that managing host density may not be an effective management strategy. Intervention strategies that prevent Bsal introduction or increase host resistance or tolerance to infection may be more effective. Our results add to the growing empirical evidence that transmission of wildlife pathogens can saturate and be functionally frequency‐dependent.

Human-mediated disease outbreaks due to poor biosecurity practices when processing animals in wild populations have been suspected. We tested whether not changing nitrile gloves between processing wood frog (Lithobates sylvaticus) tadpoles and co-housing individuals increased pathogen transmission and subsequent diseased-induced mortality caused by the emerging pathogen, ranavirus. We found that not changing gloves between processing infected and uninfected tadpoles resulted in transmission of ranavirus and increased the risk of mortality of uninfected tadpoles by 30X. Co-housing tadpoles for only 15 minutes with 10% of individuals infected resulted in ranavirus transmission and 50% mortality of uninfected tadpoles. More extreme mortality was observed when the co-housing infection prevalence was >10%. Our results illustrate that human-induced disease outbreaks due to poor biosecurity practices are possible in wild animal populations.

Resistance and tolerance are unique host defence strategies that can limit the impacts of a pathogen on a host. However, for most wildlife–pathogen systems, there are still fundamental uncertainties regarding (a) how changes in resistance and tolerance can affect disease outcomes and (b) the mechanisms underlying resistance and tolerance in host populations. Here, we first compared observed patterns of resistance and tolerance and their effects on disease outcomes among salamander species that are susceptible to infection and mortality from the emerging fungal pathogen Batrachochytrium salamandrivorans (Bsal). We then tested whether two putative mechanisms that contribute to host resistance and tolerance, skin sloughing and skin lesion reduction, predicted reduced Bsal growth rate or increased host survival during infection, respectively. We performed multi‐dose Bsal challenge experiments on four species of Salamandridae found throughout North America. We combined the laboratory experiments with dynamic models and sensitivity analysis to examine how changes in load‐dependent resistance and tolerance functions affected Bsal‐induced mortality risk. Finally, we used our disease model to test whether skin sloughing and lesion reduction predicted variability in infection outcomes not described by Bsal infection intensity. We found that resistance and tolerance differed significantly among salamander species, with the most susceptible species being both less resistance and less tolerant of Bsal infection. Our dynamic model showed that the relative influence of resistance versus tolerance on host survival was species‐dependent—increasing resistance was only more influential than increasing tolerance for the least tolerant species where changes in pathogen load had a threshold‐like effect on host survival. Testing two candidate mechanisms of resistance and tolerance, skin sloughing and lesion reduction, respectively, we found limited support that either of these processes were strong mechanisms of host defence. Our study contributes to a broader understanding of resistance and tolerance in host–pathogen systems by showing that differences in host tolerance can significantly affect whether changes in resistance or tolerance have larger effects on disease outcomes, highlighting the need for species and even population‐specific management approaches that target host defence strategies. A free Plain Language Summary can be found within the Supporting Information of this article.