If you give a frog a sauna, it can fight a deadly fungus

For decades, a deadly fungal disease has haunted the world’s amphibians, wiping out frogs, toads and salamanders from the alpine lakes of the United States to the rainforests of Australia. The disease, known as chytridiomycosis, or chytrid, has driven at least 90 species of amphibians to extinction by one estimate and contributed to the decline of hundreds more.

“Chytrid is an unprecedented wildlife pandemic,” said Anthony Waddle, a conservation biologist at Macquarie University in Sydney, Australia. “We are seeing species and populations becoming extinct.”

But like many other formidable enemies, Chytride has an Achilles heel. The fungus that is the main culprit – known as Batrachochytrium dendrobatidis or Bd – thrives in cold weather and cannot withstand heat.

Now, a new study provides evidence that conservationists can keep the fungus at bay by giving frogs a warm place to survive the winter. A simple pile of sun-warmed rocks, the researchers found, attracts the green and gold bell frog, a vulnerable Australian species. These thermal shelters increase the frogs’ body temperature, which helps them fight fungal infections and possibly prepares them for long survival.

“If we give frogs the ability to cure their infections with heat, they will do so,” said Dr. Waddle, the first author of the new paper, published Wednesday in Nature. “And they will likely be resistant in the future.”

The green and gold bell frog, once common in south-eastern Australia, has disappeared from much of the landscape and is now listed as endangered in the state of New South Wales.

In Sydney, where some of the remaining bell frogs reside, chytrid often flares up in winter and early spring, when daytime temperatures can reach 60 degrees. In the first of several experiments documented in the new paper, Dr. Waddle and his colleagues found that the frogs preferred milder climates when they were available. When placed in habitats with a temperature gradient, the frogs gravitated toward areas that averaged 84 degrees Fahrenheit, warmer than ideal for Bd.

In a second experiment, the researchers placed frogs infected with fungi in different climates. Some frogs spent weeks in the relative cold, in habitats set at 66 degrees. Those frogs harbored high levels of mold for weeks. In the months that followed, more than half of them died, said Dr. Waddle.

But frogs that lived in warmer environments or were exposed to a wide range of temperatures recovered quickly from their infections, the researchers found.

Frogs that recovered from chytrid, using this kind of “heat treatment,” were also less susceptible to the disease in the future. When they were re-exposed to Bd six weeks later — without the benefit of a warm habitat — 86 percent of them survived, compared with 22 percent of frogs that had not been previously infected.

Finally, the researchers put these findings to the test in large outdoor enclosures that more closely resembled real-world conditions. The scientists stacked a number of holes-riddled bricks in each enclosure and covered each stack with a small greenhouse. Half of the greenhouses were in the sun and the rest in the shade.

They then released an assortment of frogs into each enclosure. Some of the frogs had never been exposed to Bd before, while others were actively infected with the fungus or had previously survived an infection.

Both the shaded and unshaded shelters attracted frogs, which made themselves at home in the holes in the rocks. But the frogs that had access to the sun-warmed rocks maintained body temperatures about six degrees higher than frogs that were given shaded shelters, the scientists found. That temperature increase was enough to reduce the amount of mold the frogs harbored. “Just a few degrees of difference can tip the balance of the frogs,” Dr. Waddle said.

Frogs that had survived previous exposure to chytrid also showed relatively mild infections, the researchers found, even when they didn’t have access to the sun-warmed shelters.

The results suggest that thermal refuges could act as a kind of “crude immunization,” said Dr. Waddle, which helps frogs survive their first bout with Bd and makes them less susceptible in the future. “Then you seed the population with resistant frogs that would lower the population level of chytrid.”

The strategy won’t work for every endangered amphibian — they’re not all heat-seeking, for example — but it could be a low-cost intervention that benefits many, said Dr. Waddle, who hopes to test the approach with other amphibian and frog species.

In the meantime, he has installed the shelters in Sydney Olympic Park, which is home to a wild population of frogs. He is also mobilizing the public, encouraging local residents to “build a frog sauna,” he said. “We are trying to get people to put them in their backyards.”