Confronting inconsistencies in the amphibian‐chytridiomycosis system: implications for disease management

Abstract

Chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), is one of the largest threats to wildlife and is putatively linked to the extirpation of numerous amphibians. Despite over a decade of research on Bd, conflicting results from a number of studies make it difficult to forecast where future epizootics will occur and how to manage this pathogen effectively. Here, we emphasize how resolving these conflicts will advance Bd management and amphibian conservation efforts. We synthesize current knowledge on whether Bd is novel or endemic, whether amphibians exhibit acquired resistance to Bd, the importance of host resistance versus tolerance to Bd, and how biotic (e.g. species richness) and abiotic factors (e.g. climate change) affect Bd abundance. Advances in our knowledge of amphibian-chytrid interactions might inform the management of fungal pathogens in general, which are becoming more common and problematic globally.

Amphibian Symbiotic Bacteria Do Not Show a Universal Ability To Inhibit Growth of the Global Panzootic Lineage of Batrachochytrium dendrobatidis

“…Because of the enormous genetic variability of B. dendrobatidis GPL (10,18,19,24,25), the propensity for B. dendrobatidis to rapidly evolve in situ (10,18,26), and the panglobal, ongoing dissemination of B. dendrobatidis through numerous vectors (11,27), amphibians and their microbiomes can be expected to confront an ever changing and diverse distribution of B. dendrobatidis genotypes. Thus, the pathogen represents a "moving target" for potential interventions (28), and the mitigation of chytridiomycosis in the wild also needs to account for complex interactions between the host, the pathogen, and the environment, as well as multiple pathogen genotypes, in order to be successful (28)(29)(30).…”

“…Studies have repeatedly illustrated the importance in a complex microbiome for resilience of the community in response to a pathogenic infection (33)(34)(35). A bacterial consortium approach that treats microbiomes as a suite of functional traits rather than a substrate for the insertion of candidate bacteria is likely to offer a more comprehensive protection of hosts from B. dendrobatidis and other threatening amphibian pathogens (12,28,36). How the different members of such consortia will be determined is currently unknown, but our results highlight the limitations of a taxonomic approach for understanding what bacterial communities may afford resistance to B. dendrobatidis: both species and genus showed a limited potential to identify potentially inhibitory bacteria in our study.…”

Spatial scale modulates the strength of ecological processes driving disease distributions

“…The third pathogen, the chytrid fungus Batrachochytrium dendrobatidis (Bd), is considered one of the deadliest organisms on the planet because of its association with hundreds of amphibian extinctions in the last half century (21,22). We chose to model the spatial factors affecting these pathogens because (i) spatially explicit datasets of their distributions were available (but were not available for other pathogens or other organisms in general; see Methods); (ii) they span a diversity of taxa (a virus, bacterium, and fungus) and transmission modes (WNV and Lyme are mosquito-and tick-borne, respectively, and Bd is a directly transmitted, water-borne pathogen), and infect various types of hosts (endothermic and ectothermic), increasing the generality of our findings; (iii) they are widespread generalists throughout the United States, providing a spatial extent great enough to conduct largescale analyses; (iv) their abundances or prevalences appear to be partially controlled by a common biotic factor, the richness of potential hosts (19,21,23,24), and by common abiotic factors, including climate and vegetation (20,25,26); and, finally, (v) understanding emerging diseases is of critical importance to biodiversity conservation and human health. Our goal was not to develop and put forth the best possible model to explain the spread of these diseases but rather to test whether spatial scale influences which types of ecological processes are important.…”

“…Because the abundance of all three pathogens has been shown previously to be affected by a common biotic factor, the richness of potential hosts (defined as the richness of all species that receive either successful or failed transmission attempts from a generalist pathogen or vector) (19,21,23,24), we chose to use this factor in our models to represent the subset of biotic interactions that drive the processes causing dilution or amplification effects (5). We used total amphibian richness to predict the spread of Bd, avian richness for WNV, and mammalian richness for Lyme disease (we also initially tested the richness of other taxa for B. burgdorferi; see Methods).…”

Amphibians acquire resistance to live and dead fungus overcoming fungal immunosuppression

“…If wildlife managers could induce resistance in enough individuals of a population, they might be able to drive the basic reproductive ratio ( R 0 ; the average number of infections one infected individual generates in a population of susceptible hosts over the course of its infectious period) of a pathogenic fungus below one, such that ‘herd immunity’ would protect even pathogen-naive members of the population, a concept that is the basis for vaccination campaigns. Consequently, acquired resistance offers a potential tool to rescue animal populations threatened by fungi 14 .…”