Alpine salamanders at risk? The current status of an emerging fungal pathogen.

Amphibians globally suffer from emerging infectious diseases like chytridiomycosis caused by the continuously spreading chytrid fungi. One is Batrachochytrium salamandrivorans (Bsal) and its disease ‒ the 'salamander plague' ‒ which is lethal to several caudate taxa. Recently introduced into Western Europe, long distance dispersal of Bsal, likely through human mediation, has been reported. Herein we study if Alpine salamanders (Salamandra atra and S. lanzai) are yet affected by the salamander plague in the wild. Members of the genus Salamandra are highly susceptible to Bsal leading to the lethal disease. Moreover, ecological modelling has shown that the Alps and Dinarides, where Alpine salamanders occur, are generally suitable for Bsal. We analysed skin swabs of 818 individuals of Alpine salamanders and syntopic amphibians at 40 sites between 2017 to 2022. Further, we compiled those with published data from 319 individuals from 13 sites concluding that Bsal infections were not detected. Our results suggest that the salamander plague so far is absent from the geographic ranges of Alpine salamanders. That means that there is still a chance to timely implement surveillance strategies. Among others, we recommend prevention measures, citizen science approaches, and ex situ conservation breeding of endemic salamandrid lineages.

Diet diversity and environment determine the intestinal microbiome and bacterial pathogen load of fire salamanders.

Diverse communities of symbiotic microbes inhabit the digestive systems of vertebrates and play a crucial role in animal health, and host diet plays a major role in shaping the composition and diversity of these communities. Here, we characterized diet and gut microbiome of fire salamander populations from three Belgian forests. We carried out DNA metabarcoding on fecal samples, targeting eukaryotic 18S rRNA of potential dietary prey items, and bacterial 16S rRNA of the concomitant gut microbiome. Our results demonstrated an abundance of soft-bodied prey in the diet of fire salamanders, and a significant difference in the diet composition between males and females. This sex-dependent effect on diet was also reflected in the gut microbiome diversity, which is higher in males than female animals. Proximity to human activities was associated with increased intestinal pathogen loads. Collectively, the data supports a relationship between diet, environment and intestinal microbiome in fire salamanders, with potential health implications.

Asymptomatic infection of the fungal pathogen Batrachochytrium salamandrivorans in captivity.

One of the most important factors driving amphibian declines worldwide is the infectious disease, chytridiomycosis. Two fungi have been associated with this disease, Batrachochytrium dendrobatidis and B. salamandrivorans (Bsal). The latter has recently driven Salamandra salamandra populations to extirpation in parts of the Netherlands, and Belgium, and potentially also in Germany. Bsal has been detected in the pet trade, which has been hypothesized to be the pathway by which it reached Europe, and which may continuously contribute to its spread. In the present study, 918 amphibians belonging to 20 captive collections in Germany and Sweden were sampled to explore the extent of Bsal presence in captivity. The fungus was detected by quantitative Polymerase Chain Reaction (qPCR) in ten collections, nine of which lacked clinical symptoms. 23 positives were confirmed by independent processing of duplicate swabs, which were analysed in a separate laboratory, and/or by sequencing ITS and 28 S gene segments. These asymptomatic positives highlight the possibility of Bsal being widespread in captive collections, and is of high conservation concern. This finding may increase the likelihood of the pathogen being introduced from captivity into the wild, and calls for according biosecurity measures. The detection of Bsal-positive alive specimens of the hyper-susceptible fire salamander could indicate the existence of a less aggressive Bsal variant or the importance of environmental conditions for infection progression.

Amphibian gut microbiota shifts differentially in community structure but converges on habitat-specific predicted functions.

Complex microbial communities inhabit vertebrate digestive systems but thorough understanding of the ecological dynamics and functions of host-associated microbiota within natural habitats is limited. We investigate the role of environmental conditions in shaping gut and skin microbiota under natural conditions by performing a field survey and reciprocal transfer experiments with salamander larvae inhabiting two distinct habitats (ponds and streams). We show that gut and skin microbiota are habitat-specific, demonstrating environmental factors mediate community structure. Reciprocal transfer reveals that gut microbiota, but not skin microbiota, responds differentially to environmental change. Stream-to-pond larvae shift their gut microbiota to that of pond-to-pond larvae, whereas pond-to-stream larvae change to a community structure distinct from both habitat controls. Predicted functions, however, match that of larvae from the destination habitats in both cases. Thus, microbial function can be matched without taxonomic coherence and gut microbiota appears to exhibit metagenomic plasticity.

Batrachochytrium salamandrivorans sp. nov. causes lethal chytridiomycosis in amphibians.

The current biodiversity crisis encompasses a sixth mass extinction event affecting the entire class of amphibians. The infectious disease chytridiomycosis is considered one of the major drivers of global amphibian population decline and extinction and is thought to be caused by a single species of aquatic fungus, Batrachochytrium dendrobatidis. However, several amphibian population declines remain unexplained, among them a steep decrease in fire salamander populations (Salamandra salamandra) that has brought this species to the edge of local extinction. Here we isolated and characterized a unique chytrid fungus, Batrachochytrium salamandrivorans sp. nov., from this salamander population. This chytrid causes erosive skin disease and rapid mortality in experimentally infected fire salamanders and was present in skin lesions of salamanders found dead during the decline event. Together with the closely related B. dendrobatidis, this taxon forms a well-supported chytridiomycete clade, adapted to vertebrate hosts and highly pathogenic to amphibians. However, the lower thermal growth preference of B. salamandrivorans, compared with B. dendrobatidis, and resistance of midwife toads (Alytes obstetricans) to experimental infection with B. salamandrivorans suggest differential niche occupation of the two chytrid fungi.