Do certain species intensify chytridiomycosis outbreaks? Modeling transmission in the genus Atelopus

Background/Question/Methods

Amphibian communities throughout Panama have declined dramatically due to chytridiomycosis, the disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd). Members of the frog genus Atelopus have been hit particularly hard, with several species listed as critically endangered. Based on field survey data, Atelopus species are often among the first to go extinct in a population, often with other species declining rapidly in the initial outbreak, and then experiencing slower declines once Atelopus has gone locally extinct. We test whether members of the genus Atelopus are facilitating declines in other species through increased transmission of Bd.  We infected individually housed Atelopus zetecki with Bd and measured both infection intensity and zoospore production weekly using qPCR. We then used an individual based modeling approach fit with both laboratory data as well as data collected in the field from Panama.

Results/Conclusions

After 33 days, all animals infected with Bd had died of chytridiomycosis. qPCR data showed zoospore load over time consistent with exponential growth until the death of the animal.  Our laboratory experiments show that animals with higher loads have higher shedding rates, and we fit this to a model where transmission depends on the intensity of infection. Based on our modeling results, we show that if transmission depends on infection intensity, species with higher growth rates and tolerances can cause increased mortality in species that would normally persist with Bd.