Tiny saunas help frogs fight off chytrid fungus

Frogs fighting a deadly fungal infection could get a boost from tiny brick saunas.

By spending time in a sun-warmed shelter, green and golden bell frogs infected with chytrid (Batrachochytrium dendrobatidis) warmed up enough to fight off the fungal pathogen, researchers report June 26 in Nature.

Chytrid has contributed to population declines in at least 500 amphibian species (SN: 3/28/19), including the green and golden bell frog (Litoria aurea), which has disappeared from 90 percent of its original Australian habitat. The fungus attacks an amphibian’s skin, blocking electrolytes and fluids from flowing into the body. The loss of these minerals can eventually cause heart failure.

Chytrid thrives at relatively cool temperatures and isn’t viable above 30° Celsius (86° Fahrenheit). Given a choice, bell frogs prefer temperatures around 29° C (84° F) — unfavorably high for chytrid. In winter, when temperatures drop, infection rates spike.

To test whether warm hideouts could help frogs fight the fungus, conservation biologist Anthony Waddle of Macquarie University in Australia and colleagues set up 12 outdoor habitats. Each oasis included water, artificial plants and shelters made of black masonry bricks inside little greenhouses. Four habitats held only healthy frogs, while the remaining eight contained a mix of healthy frogs and ones infected with chytrid. Half of the shelters were shaded with cloth. The other half were left exposed to test a range of toasty temperatures.

All the frogs readily made themselves at home in the saunas. At around 20° to 25° C (68° to 77° F) outside, the unshaded bricks provided frogs with an additional 15 to 20 degrees of warmth. Shaded shelters, meanwhile, were 4.5 degrees cooler on average than the unshaded ones.

Frogs in the warmer, unshaded shelters maintained higher body temperatures and had milder infections throughout the 15-week study period. Still, access to saunas — regardless of relative toastiness — helped infected frogs fight off disease, boosting their survival rate to be similar to that of healthy frogs. 

After frogs with chytrid cleared their infections, they were less susceptible to future bouts of the disease. Frogs that had previously fought off a chytrid infection were 23 times as likely to survive reinfection as frogs who had never contracted chytrid, the team found.

Bell frogs are adapted to urban environments, so these types of shelters could help them and other hard-hit populations develop resistance to chytrid. Bell frogs “used to live in people’s toilets and letterboxes and everywhere before chytrid,” Waddle says. “Setting up these habitats where they still persist might give them enough of a boost that you could see a population increase.”

The shelters are relatively inexpensive and easy to build, so frog fans could put them in their own gardens, says disease ecologist Erin Sauer of the University of Arkansas in Fayetteville.

But don’t rush out to build a backyard frog spa just yet. Not every species will benefit from a balmy hideout, says amphibian biologist Cori Richards-Zawacki of the University of Pittsburgh. Some chytrid-affected species that prefer cooler environs, like the Panamanian golden frog (Atelopus zeteki), might not prefer the shelters. Unlike the bell frog, they could even become more susceptible to chytrid at higher temperatures.

“Chytrid is a huge, massive problem,” Waddle says. The study isn’t a one-size-fits-all solution, he adds, “but it is a glimmer of hope.”