Date Published: July 18, 2018
Publisher: Public Library of Science
Author(s): Hannah Keely Smith, Frank Pasmans, Maarten Dhaenens, Dieter Deforce, Dries Bonte, Kris Verheyen, Luc Lens, An Martel, Stefan Lötters.
Recently emerged fungal diseases, Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal) are an increasing threat to amphibians worldwide. In Europe, the threat of Bsal to salamander populations is demonstrated by the rapid decline of fire salamander populations in Germany, the Netherlands and Belgium. Although most European urodelans are susceptible to infection in infection trials, recent evidence suggests marked interspecific differences in the course of infection, with potentially far reaching implications for salamander conservation. As a salamander’s skin is the first line of defense against such pathogens, interspecific differences in innate immune function of the skin may explain differential susceptibility. Here we investigate if compounds present on a salamander’s skin can kill Bsal spores and if there is variation among species. We used a non-invasive assay to compare killing ability of salamander mucosomes of four different species (captive and wild Salamandra salamandra and captive Ichtyosaura alpestris, Cynops pyrrhogaster and Lissotriton helveticus) by exposing Bsal zoospores to salamander mucosomes and determining spore survival. In all samples, zoospores were killed when exposed to mucosomes. Moreover, we saw a significant variation in this Bsal killing ability of mucosomes between different salamander host species. Our results indicate that mucosomes of salamanders might provide crucial skin protection against Bsal, and could explain why some species are more susceptible than others. This study represents a step towards better understanding host species variation in innate immune function and disease susceptibility in amphibians.
Wildlife diseases are an increasing threat to biodiversity [1,2]. Over the last two decades amphibian populations have declined due to recently emerged diseases [3–5]. Among them, fungal pathogens, such as Batrachochytrium dendrobatidis (hereafter Bd) and Batrachochytrium salamandrivorans (hereafter Bsal), etiological agents of chytridiomycosis have been wreaking havoc on amphibian populations worldwide [6–12]. In Europe, Bsal emerged suddenly and severely, causing a population of fire salamanders to nearly collapse in the Netherlands, with less than 0.1% of the population remaining [13,14]. Similar population collapses occurred in Belgium  and have been found in Germany . Probably originating in Asia [17,18] this pathogen may have a quick and devastating effect on most salamander species of Europe. The risk of further spread globally has caused proactive responses in the form of taskforces to emerge and trade restrictions to be implemented in the USA, Canada and Switzerland [19,20].
As the surface of the skin represents the first contact between the amphibian host and its chytrid pathogen, the innate immune function of the skin is a key first step influencing subsequent disease dynamics. We found that components on urodelan skin can kill zoospores of both Bd and Bsal. Moreover, Bsal killing activity of the mucosome of different salamander’s species reflect their known susceptibility.