Ubiquity of the Pathogenic Chytrid Fungus, Batrachochytrium dendrobatidis, in Anuran Communities in Panamá

Abstract

The pathogenic chytrid fungus, Batrachochytrium dendrobatidis, has been implicated as the main driver of many enigmatic amphibian declines in neotropical sites at high elevation. Batrachochytrium dendrobatidis is thought to be a waterborne pathogen limited by temperature, and the extent to which it persists and causes disease in amphibians at lower elevations in the neotropics is not known. It also is unclear by what mechanism(s) B. dendrobatidis has emerged as a pathogenic organism. To test whether B. dendrobatidis is limited by elevation in Panamá, we sought to determine the prevalence and intensity of B. dendrobatidis in relation to anuran abundance and diversity using quantitative PCR (qPCR) analyses. Sites were situated at varying elevations, from 45 to 1215 m, and were at varying stages of epizootic amphibian decline, including pre-epizootic, mid-epizootic, 2 years post-epizootic, and 10 years post-epizootic. Batrachochytrium dendrobatidis was found in all sites regardless of elevation or stage of epizootic decline. Levels of prevalence and infection intensity were comparable across all sites except at the mid-epizootic site, where both prevalence and intensity were significantly higher than at other sites. Symptoms of chytridiomycosis and corresponding declines in amphibian populations were variably seen at all elevations along a post-epizootic gradient. Because it is inherently difficult to prove a negative proposition, it can neither be proven that B. dendrobatidis is truly not present where it is not detected nor proven that it is only recently arrived where it is detected. Thus, there will always be doubts about whether B. dendrobatidis is enzootic or invasive. In any case, our results, coupled with current knowledge, suggest most clearly that the disease, chytridiomycosis, may be novel and invasive, and that the pathogen, B. dendrobatidis either is, or is becoming, globally ubiquitous.

Reptiles as potential vectors and hosts of the amphibian pathogen Batrachochytrium dendrobatidis in Panama

“…We have shown that, in Panama, lizards and snakes will harbour B. dendrobatidis DNA at non-pathological levels of prevalence and intensity that are comparable to those found among anuran amphibians that are not showing signs of chytriomycosis disease (Kilburn et al 2011). Without histology, we cannot say that we have proven the presence of B. dendrobatidis infection or the presence of infective zoospores on the reptiles, but the assays we used are routinely considered diagnostic for B. dendrobatidis infection in amphibians and, therefore, our findings indicate that infection is highly plausible.…”

“…No individual reptile that tested positive for Batrachochytrium dendrobatidis DNA showed either the symptoms of chytridiomycosis disease or the elevated levels of infection intensity notable in anuran amphibians stricken with chytridiomycosis (Kilburn et al 2011).…”

“…After DNA extraction and purification was complete, samples were sent to the RNomics Platform of Genome Quebec for qPCR analysis, as developed by Boyle et al (2004) and adapted for a singlicate assay as described by Kriger et al (2006). Following Kilburn et al (2011), we were satisfied that triplicate assays provided no greater quality control over singlicate assays with these methods.…”

“…The relatively small sample sizes for reptiles yielded insufficient power to discriminate between seasons or habitats for any analyses. For comparisons with cooccuring amphibians, we used data on B. dendrobatidis DNA prevalence and intensity as presented by Kilburn (2008) and Kilburn et al (2011).…”

“…All DNA extractions and purifications of samples were done as described by Kilburn et al (2011) according to the protocol of Hyatt et al (2007). After DNA extraction and purification was complete, samples were sent to the RNomics Platform of Genome Quebec for qPCR analysis, as developed by Boyle et al (2004) and adapted for a singlicate assay as described by Kriger et al (2006).…”