Prevalence and infection intensity of Batrachochytrium dendrobatidis (Bd) across an altitudinal gradient in Costa Rica

Background/Question/Methods

Amphibian populations are declining globally at a historically unprecedented rate. Numerous examples of these population declines have occurred in seemingly pristine environments where human disturbance is minimal. One factor that may be responsible for many of these declines is the deadly skin disease, chytridiomycosis, which is often caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd). In Central America, many of these declines have been observed at mid and high elevation sites (>800 m elevation) suggesting that Bd prevalence and infection intensity might increase with elevation. We examined prevalence and infection intensity of Bd in amphibian assemblages across an elevational gradient in Costa Rica (from 0-1400 m elevation), in both versants (Caribbean and Pacific). We swabbed 267 frogs from 33 species to detect Bd and quantified the pathogenic load though qPCR approaches in nine localities between 2015 and 2017. We hypothesized that all study locations would exhibit low to intermediate levels of Bd prevalence and intensity of infection suggesting enzootic dynamics. We also hypothesized that disease prevalence and intensity would increase with elevation, because midlands and highlands of Central America exhibit optimal ranges of temperature and humidity similar to those where Bd grows best in lab conditions.

Results/Conclusions

We found a mid to high overall prevalence of Bd (39.7%) and low infection intensity (0.1-94.5 genomic equivalents) across all locations. We also found that 58% of the sampled species tested positive for Bd. Our results coincide with similar studies showing that Bd appears to now be endemic to the region, occurring across all elevations and causing an overall low level of infection. Contrary to our predation, we found no support for our second hypothesis that infection prevalence and intensity varied with elevation. Our findings demonstrate that Bd can be present in multiple species at low elevations, even though most of the documented die-offs associated with Bd have occurred at higher elevations.