Some frogs may be bouncing back after killer chytrid fungus

It’s tough to be a frog once a killer skin fungus moves in. But, in Panama, the amphibians might be fighting back, researchers propose.

More than a decade ago, an amphibian-killing chytrid fungus nicknamed Bd swept through the country. Now some frog species that had nearly vanished from three regions are growing easier to spot again. But tests of the pathogen find no signs that it is weakening, says disease ecologist Jamie Voyles at the University of Nevada, Reno. With the fungus as dangerous as ever, frogs becoming resistant to the pathogen might be enabling the recovery, Voyles and her colleagues report in the March 30 Science.

Despite any glimmer of hope, it’s too early to celebrate frog recovery, protests ecologist Karen Lips at the University of Maryland in College Park. She doesn’t doubt that researchers have found frogs in the devastated regions, but wants more rigorous monitoring before talking population trends.

The three areas in the study have special resonance. They’re where Lips and other scientists set up a disaster watch as they realized that Bd, short for Batrachochytrium dendrobatidis, was sweeping through Central America, killing many amphibians in its path. As the deadly wave approached, the researchers collected data and living animals in the hope they would help in before-and-after studies. The fungus attacked Lips’ site, El Copé, in 2004 and the other two sites in 2006 and 2007.

Voyles was working at El Copé as a graduate student when Bd arrived. She remembers the abundance of sick and dying animals, and the task of collecting the dead. “That was shocking, really — there’s just no other word for it.”

Using archived, frozen samples of the fungus from those sites, Voyles and colleagues present the first comparison of Bd as it was in 2004 versus in 2012 to 2013, when amphibian communities showed signs of recovering. At first, Voyles suspected that the pathogen was growing less dangerous. Bd attacks a wide range of hosts but evolutionarily speaking, pathogens don’t persist if they drive too many of their hosts extinct.

A waning fungal menace could have explained how nine frog species that had almost disappeared in the area were now easier to spot. But the pathogen appears as lethal as ever, she and her colleagues report. Compared with the older versions, the more recent fungus samples grew and reproduced at about the same speeds, and inhibited immune cells at about the same rates. All fungi killed the test frogs of two species.

“That’s an important result,” says evolutionary ecologist James Collins of Arizona State University in Tempe. Biologists who study the interplay of pathogen and host often ask how the two change in relation to one another over time. But having real data on the relationship is much rarer. Also important, he says, is recognizing that “many species at the Panama sites are still missing, so wholesale recovery isn’t occurring.”

To explain the encouraging cases, the researchers propose that in some species, frog skin secretions may be getting better at repelling the fungus. For wild animals of six amphibian species, secretions from pre-disease days were worse at inhibiting fungal growth in a lab test than secretions from the same species at sites that the fungus had hit. In another test, secretions from wild variable harlequin frogs (Atelopus varius) were about eight times as effective at inhibiting the fungus as were those from zoo animals descended from frogs collected before Bd reached their site.

Of the many amphibian populations that Bd savaged around the world (SN: 3/5/16, p. 14), a few have bounced back in numbers. Sierra Nevada yellow-legged frogs now abound again in the California mountains, ecologist Vance Vredenburg of San Francisco State University points out. Yet these animals are descendants of a worryingly tiny percentage of the original, diverse population. Despite rebounds, attacks by deadly fungi are still “a really big deal,” he says.