1. Resistance and tolerance are unique host defense strategies that can limit the impacts of a pathogen on a host. However, for most wildlife-pathogen systems there are still fundamental uncertainties regarding 1) how changes in resistance and tolerance can affect disease outcomes and 2) the mechanisms underlying resistance and tolerance in host populations.

2. 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.

3. 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.

4. 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 defense.

5. 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 defense strategies.

We performed multi-dose Bsal challenge experiments on four Salamandridae species found throughout North America. The species tested included Notophthalmus viridescens (n=147 individuals), N. meridionalis (n=40), N. perstriatus (n=50) and Taricha granulosa (n=47). N. viridescens were collected from six geographically separated populations: three in Tennessee (subsequently labeled as Middle, Ijams, and Roan) and one each in Pennsylvania, Vermont and Michigan. T. granulosa were captured by the California Fish and Wildlife Department. N. meridionalis and N. perstriatus were captive bred at the Fort Worth and Jacksonville Zoos, respectively, and transferred to the University of Tennessee with authorization by U.S. Fish and Wildlife Service. Individuals were acclimated for between 2-4 weeks before the experiment began. 

All animals were housed in environmental chambers set at 15 C and 12 hour light and dark cycles. For each challenge experiment, we randomly assigned animals to one of four exposure doses ranging from 5x10³ - 5x10⁶  Bsal zoospores/mL or to a control group where we inoculated animals with autoclaved dechlorinated water (n=5-10 individuals per treatment per species). We exposed an animal to Bsal by placing it in a 237 mL container with 9 mL of autoclaved dechlorinated water and 1mL of the desired exposure dose. We mock inoculated control animals by placing them in the same size container with 10 mL dechlorinated water. All animals were exposed for 24 hrs.  Following exposure, each animal was housed individually during the experiment inside 710 cm³ enclosures, containing a moist paper towel and a 7.6 cm PVC cover object.  We replaced each animal’s container and water every three days. We swabbed each animal every six days beginning four days post exposure until the end of each experiment (40-70 days, depending on the species).  We chose six days as this time step allowed us to capture Bsal growth dynamics on individual hosts while minimizing the amount of time we handled the animals.  During each swabbing period, we examined the animals for signs of skin lesions and skin sloughing. Skin lesions were counted grossly along the body during each swabbing period. We counted lesions as visible skin erosions and ulcers, occasionally with the aid of a magnifying lamp. We also recorded presence or absence of skin sloughing during each check by visually examining the animal and the animal’s container for signs of skin sloughing. 

We extracted genomic DNA from skin swabs using a Qiagen DNeasy Blood and Tissue kits (Qiagen, Hilden, Germany). We then performed Bsal quantitative polymerase chain reaction (qPCR) using methods similar to Blooi et al. 2013. All qPCR reactions were performed on an Applied Biosystems Quantstudio 6 Flex qPCR instrument (Thermo Fisher Scientific, USA). Using qPCR we determined the infection status of each animal and estimated the Bsal zoospore copies/microL recovered from each swab. Each qPCR reaction was run in duplicate and samples were considered positive if both reactions amplified prior to 50 cycles.

See README.txt file for data description.