Frogs Fight Back

A variable harlequin frog (Atelopus varius), one of several species that shows signs of recovering from Panama’s initial chytrid fungus outbreak

Cori Zawacki

An infectious skin disease that has devastated amphibian populations worldwide is caused by the chytrid fungus, Batrachochytrium dendrobatidis (Bd). The outbreak has hit frogs especially hard, playing a role in reducing population levels sometimes to the point of extinction. Yet years after chytrid’s arrival in the tropical forest streams of Panama, nine species of affected frogs have rebounded, despite the pathogen’s continued presence. Researchers may have recently determined why some frogs have started to regain a toehold against this virulent disease. 

Biologist Jamie Voyles, of the University of Nevada, Reno, and a large U.S.-Panama research team tracked the trajectory and nature of Bd prevalence over time by collecting more than 2,000 skin swabs from several amphibian species at three Panamanian sites. To determine if the Bd epidemic subsided because the pathogen weakened over time, they compared a range of traits in Bd samples collected during its historic outbreak in 2004 to those in Bd samples collected from 2012 to 2013, after which frogs began exhibiting signs of recovery. The team also exposed two species of frogs susceptible to chytrid to either the historic or more recent Bd samples and noted the frogs’ rates of infection and survival.

To the researchers’ surprise, results showed no evidence that Bd had lessened in intensity, even after the outbreak waned. The historic and recent Bd samples grew at similar rates, exhibited similar fungal spore size and density, and showed no discernible genomic differences. Furthermore, both samples induced similar levels of infection in susceptible frogs. “The pathogen is just as lethal today as it was a decade ago,” Voyles says.

Instead, it seemed that some frogs living with chytrid developed better defenses against it over time. Voyles and her team pitted Bd against the skin secretions from six frog species at locations where the fungus was absent and where the frog species are living alongside chytrid. They also examined how Bd fared against skin secretions of wild frogs from a species that had rebounded from the initial chytrid outbreak, compared to captive frogs of the same species never exposed to chytrid.

While the magnitude of the differences varied among species, wild frogs living alongside the chytrid fungus had skin secretions showing a greater ability to inhibit Bd, compared to frog populations of the same species in the wild or in captivity where chytrid had never been. Precisely how these defenses developed remains for future studies and may offer hope for one day slowing the spread of this critical disease. (Science