Mark Wilber: Resistance and tolerance of salamanders to an emerging fungal pathogen

Dr. Mark Wilber, Assistant Professor in the Department of Forestry, Wildlife, and Fisheries at the University of Tennessee, discusses his recently accepted article, “Putative resistance and tolerance mechanisms have little impact on disease progression for an emerging salamander pathogen”, describes his favourite part about being ecologist, and how he got into the field.

What is the background behind your paper?

Mark Wilber
Mark Wilber

Hosts can defend themselves against infection by avoiding contact with a pathogen, resisting infection when contact occurs, or tolerating the pathogen such that infection does not affect host fitness.  However, not all defence strategies are created equal and some strategies might be more effective for limiting the negative effects of pathogens on individuals and host populations. This is particularly relevant for some emerging infectious diseases of wildlife, where novel pathogens have led to dramatic population declines. In these situations, understanding how naturally evolved or purposefully managed changes in host defence strategies can mitigate disease-induced declines is critical for facilitating population recovery (or preventing declines in the first place).  A pathogen of particular concern, Batrachochytrium salamandrivorans (Bsal), is an emerging fungal pathogen that has led to drastic declines in salamander populations in Europe and is an imminent threat to salamander populations in North America.  In our study, we wanted to understand how 1) changes in resistance or tolerance would affect the negative effects of Bsal on North American salamanders and 2) whether putative mechanisms of resistance and tolerance, such as skin sloughing and skin lesion reduction, respectively, contributed to host defence.  We predicted that increased skin sloughing would reduce infection with Bsal and that reducing skin lesions would increase host survival.

What are the key messages of your article?

The bullseye-like lesions on the face of this roughed-skin newt (Taricha granulosa) are a result of infection with the emerging fungal pathogen Bsal (photo by Todd Amacker Conservation Visuals).
The bullseye-like lesions on the face of this roughed-skin newt (Taricha granulosa) are a result of infection with the emerging fungal pathogen Bsal (photo by Todd Amacker Conservation Visuals).

We found that resistance strategies that reduced pathogen infection were only more effective at increasing salamander survival when tolerance was low. Otherwise, increasing tolerance was a more effective strategy for increasing survival than increasing resistance. This suggests that different salamander species may invest in different defence strategies when faced with this fungal pathogen.

Surprisingly, we found little evidence that two putative mechanisms of resistance and tolerance in salamanders to Bsal, skin sloughing and skin lesion reduction, were strong mechanisms of host defence.  Thus, identifying the mechanisms contributing to differences in resistance and tolerance in salamander species susceptible to Bsal infection is still an open (and important!) question.

Does this article raise any new research questions?

A key question that this research raises is one we actually sought to answer with this research in the first place!  What are the primary mechanisms contributing to resistance and tolerance against Bsal? We were surprised by our result that that increasing skin sloughing and reducing skin lesion abundance had little effect of host resistance or tolerance, respectively. We used the presence of sloughed skin on the animal/in a tank through time and gross counts of skin lesions through time as practical, easy-to-measure proxies to test the resistance mechanism of skin sloughing and the tolerance mechanism of lesion reduction. However, these measures did have limitations. In the future, we are interested in exploring whether measurements of skin sloughing behaviour when Bsal infection intensity is low, histological assays of lesion severity, and temporal measurements of constituent salamander immune defences over the course of infection would help further uncover the mechanisms of resistance and tolerance to Bsal infection in salamander species.

About the Research

Why is it important?

Swabbing salamanders to detect Bsal infection and quantify Bsal infection intensity (photo by Todd Amacker Conservation Visuals).
Swabbing salamanders to detect Bsal infection and quantify Bsal infection intensity (photo by Todd Amacker Conservation Visuals).

Our results are important for understanding the potential response of wildlife populations to emerging infectious diseases.  We found that small differences in how resistance and tolerance change as a function of pathogen intensity can magnify or dilute how perturbations to resistance or tolerance (either through management or natural evolution) affect the survival of infected hosts.  Our models provide a plausible explanation as to why different species, and even different populations of the same species, exposed to an emerging infectious disease may enact different defence strategies when combating the invading pathogen.  

The fungal pathogen Bsal also poses a serious risk to North American salamander diversity.  From a conservation perspective, our work further identifies the extreme susceptibility of North American salamander species to Bsal, with some species having little resistance or tolerance to infection.  While we show that resistance-oriented management targeted at reducing Bsal infection load, such as the application of fungicide or probiotic treatment, can be effective for reducing the negative impacts of Bsal, the extreme susceptibility of salamander species highlights that prevention of Bsal arrival to North America should continue to be a priority.

What would you like to do next?

As a research team, our next goal is to combine this work with other experimental data that our team has collected on the transmission dynamics of salamanders and Bsal. Our goal is to use these data to build population-level models of salamander-Bsal interactions that ask the question: how do changes in host resistance and tolerance affect the magnitude of population declines and population extinction risk following Bsal invasion into North American salamander populations?

About The Author

How did you get involved in ecology?

I’ve always been fascinated by herpetology, but as an undergraduate I had a transformative ecological research experience with Dr. Gina Wimp at Georgetown University looking at the effects of habitat edges on the parasitism rates of a planthopper by a parasitoid wasp. This experience ignited my interested in ecology, particularly in using ecological models to explore the fascinating dynamics of host-parasite systems.   

What is the best thing about being an ecologist?

One of my favourite things about being an ecologist is the feeling you get when you have (or at least think you have!) discovered a potential “truth” about natural world.   I remember the first time I had this feeling was early in my ecological career when I was doing research on macroecological patterns.  I thought I had worked out a simple, yet powerful relationship that explained some of the commonly-observed violations to the often-assumed power law species area relationship. It turned out my “solution” did not actually work. But the feeling of having just peaked under a small corner of Nature’s curtain is something that still inspires my ecological research today. 

Read the article in full here.

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