Correlates of virulence in a frog-killing fungal pathogen: evidence from a California amphibian decline

Abstract

The fungal pathogen Batrachochytrium dendrobatidis (Bd) has caused declines and extinctions in amphibians worldwide, and there is increasing evidence that some strains of this pathogen are more virulent than others. While a number of putative virulence factors have been identified, few studies link these factors to specific epizootic events. We documented a dramatic decline in juvenile frogs in a Bd-infected population of Cascades frogs (Rana cascadae) in the mountains of northern California and used a laboratory experiment to show that Bd isolated in the midst of this decline induced higher mortality than Bd isolated from a more stable population of the same species of frog. This highly virulent Bd isolate was more toxic to immune cells and attained higher density in liquid culture than comparable isolates. Genomic analyses revealed that this isolate is nested within the global panzootic lineage and exhibited unusual genomic patterns, including increased copy numbers of many chromosomal segments. This study integrates data from multiple sources to suggest specific phenotypic and genomic characteristics of the pathogen that may be linked to disease-related declines.

Antifungal Bacteria on Woodland Salamander Skin Exhibit High Taxonomic Diversity and Geographic Variability

“…Lower disease risk in vertebrates has been associated with different characteristics of the microbiome, such as high bacterial species richness (6)(7)(8), specific microbial community assemblages (2,7,9,10), and the presence of microbes that produce metabolites that inhibit growth of pathogens (1,11,12). For amphibians, inter-and intraspecies-specific variations in the skin microbiome (13)(14)(15) may contribute to variation in responses to infection by the deadly fungal pathogen Batrachochytrium dendrobatidis (16,17). Yet we know little about how microbial diversity differs among amphibian host species and environments and how this relates to B. dendrobatidis-associated disease risk.…”

American Bullfrogs (Lithobates catesbeianus) Resist Infection by Multiple Isolates of Batrachochytrium dendrobatidis, Including One Implicated in Wild Mass Mortality

“…FL Bd was originally isolated from dead, naturally infected American bullfrogs from the mass mortality event at Finley Lake (Clifford et al 2012). CM Bd was cultured from Rana cascadae at Lassen National Forest in the southern Cascades mountains and was included here for comparison to FL Bd because CM Bd appears to have only moderate virulence (Piovia-Scott et al 2014). Although all Bd isolates used in this study were collected in 2011, samples were cryopreserved and revived just prior to our exposure treatments and thus had relatively low passage numbers (10-15).…”

“…Just before the first Bd exposure and approximately weekly thereafter, we swabbed frogs for Bd infection as previously described in Piovia-Scott et al (2014). We monitored animals daily, and any frogs lacking a righting reflex or that had other health issues (e.g., untreatable lesions incidental to husbandry) were humanely euthanized via overdose of MS-222.…”

“…We monitored animals daily, and any frogs lacking a righting reflex or that had other health issues (e.g., untreatable lesions incidental to husbandry) were humanely euthanized via overdose of MS-222. Zoospore loads were calculated from quantitative PCR results as in Piovia-Scott et al (2014) except that samples were run in duplicate. Swab samples were only considered positive when both sample runs indicated Bd presence, and infection load was calculated by averaging results from the two positive wells (all raw qPCR data are available as Supplemental Material).…”

Interactions between amphibian skin sloughing and a cutaneous fungal disease: infection progression, immune defence, and phylogenetic patterns

“…However, optimal growth in the amphibian host is likely dependent on the thermal performance curves , and varying responses to temperature variability , of both the parasite and its host. Furthermore, virulence of Bd can vary with the strain Farrer et al 2011), and has been linked to that strain's growth rate, production of zoospores, and zoosporangia size in culture (Piovia-Scott et al 2014;Berger et al 2016), and strain phenotype, specifically zoosporangia size, can predict Bd prevalence in a population (Lambertini et al 2016).…”