Post-epizootic salamander persistence in a disease-free refugium suggests poor dispersal ability of Batrachochytrium salamandrivorans.

Lack of disease spill-over between adjacent populations has been associated with habitat fragmentation and the absence of population connectivity. We here present a case which describes the absence of the spill-over of the chytrid fungus Batrachochytrium salamandrivorans (Bsal) between two connected subpopulations of fire salamanders (Salamandra salamandra). Based on neutrally evolving microsatellite loci, both subpopulations were shown to form a single genetic cluster, suggesting a shared origin and/or recent gene flow. Alpine newts (Ichthyosaura alpestris) and fire salamanders were found in the landscape matrix between the two sites, which are also connected by a stream and separated by no obvious physical barriers. Performing a laboratory trial using alpine newts, we confirmed that Bsal is unable to disperse autonomously. Vector-mediated dispersal may have been impeded by a combination of sub-optimal connectivity, limited dispersal ability of infected hosts and a lack of suitable dispersers following the rapid, Bsal-driven collapse of susceptible hosts at the source site. Although the exact cause remains unclear, the aggregate evidence suggests that Bsal may be a poorer disperser than previously hypothesized. The lack of Bsal dispersal between neighbouring salamander populations opens perspectives for disease management and stresses the necessity of implementing biosecurity measures preventing human-mediated spread.

Environmental determinants of recent endemism of Batrachochytrium dendrobatidis infections in amphibian assemblages in the absence of disease outbreaks.

The inconsistent distribution of large-scale infection mediated die-offs and the subsequent population declines of several animal species, urges us to understand how, when, and why species are affected by disease. It is often unclear when or under what conditions a pathogen constitutes a threat to a host. Often, variation of environmental conditions plays a role. Globally Batrachochytrium dendrobatidis (Bd) causes amphibian declines; however, host responses are inconsistent and this fungus appears equally capable of reaching a state of endemism and subsequent co-existence with native amphibian assemblages. We sought to identify environmental and temporal factors that facilitate host-pathogen coexistence in northern Europe. To do this, we used molecular diagnostics to examine archived and wild amphibians for infection and general linear mixed models to explore relationships between environmental variables and prevalence of infection in 5 well-sampled amphibian species. We first detected infection in archived animals collected in 1999, and infection was ubiquitous, but rare, throughout the study period (2008-2010). Prevalence of infection exhibited significant annual fluctuations. Despite extremely rare cases of lethal chytridiomycosis in A. obstetricans, Bd prevalence was uncorrelated with this species' population growth. Our results suggest context dependent and species-specific host susceptibility. Thus, we believe recent endemism of Bd coincides with environmentally driven Bd prevalence fluctuations that preclude the build-up of Bd infection beyond the critical threshold for large-scale mortality and host population crashes.

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.