EDITOR’S NOTE: Call to Earth is a CNN editorial series committed to reporting on the environmental challenges facing our planet, together with the solutions. Rolex’s Perpetual Planet Initiative has partnered with CNN to drive awareness and education around key sustainability issues and to inspire positive action.
Sydney, Australia
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On a bright afternoon under the hot Australian sun, small, dewy faces peer out from the holes in masonry bricks. Surrounded by a simple greenhouse, the bricks form frog “saunas” that are proving to be a warm refuge, protecting these endangered frogs from a catastrophic disease.
They’re part of an experiment led by Anthony Waddle, a researcher at Macquarie University, in Sydney, who created this novel approach to protect them from a deadly amphibian disease.
Waddle, an American-born biologist, wasn’t drawn to Australia for its stunning coastlines or unique marsupials — instead he relocated for a tiny, slimy, sun-loving creature found in freshwater habitats throughout the southeast — the green and golden bell frog.
With bulging eyes, bulbous digits and vibrantly colored skin to match their name, these native frogs are one of the country’s most striking amphibians, but also, one of its most threatened species.
Over the past three decades, green and golden bell frog populations in New South Wales —once their stronghold — have declined to just 10% of their historical levels.
Although habitat loss and climate change have contributed to their collapse, a major driver has been chytrid — an ancient fungus that has morphed into a modern-day killer.
The form responsible for the green and golden bell frog decline, and for mass amphibian die-offs over the last half-century, is known scientifically as Batrachochytrium dendrobatidis. This waterborne pathogen triggers a highly contagious and often fatal disease called chytridiomycosis, affecting frogs, toads and salamanders.
“The chytrid fungus is attracted to their skin, it attaches to it and it dissolves the skin’s barriers,” Waddle said. Put simply, “chytrid is eating their skin.”
For amphibians, skin is more than a protective layer — it is vital for breathing, absorbing water and other regulatory processes. When these functions are disrupted, it places stress on the heart and can lead to cardiac arrest.
“This is the worst disease ever to impact biodiversity, and most people don’t even know it exists,” Waddle told CNN.
Waddle has made it his life’s work to help save species threatened by the fungus. In 2016, the conservation biologist began his work in the lab — developing a vaccine to combat the disease in leopard frogs in the Western US.
He created a weakened form of chytrid that the frogs could easily fend off and discovered it could spread from frog to frog without causing illness — instead functioning as a transmissible vaccine boosting their resistance and improving survival when later exposed.
He then turned his attention to how temperature shapes the disease’s dynamics. Across continents, chytrid outbreaks follow a seasonal pattern — devastating frog populations in cold winters but subsiding in warmer months, indicating the fungus thrives in cooler weather.
This left Waddle wondering: “If the frogs had access to high temperatures, could they get better?”
In 2020, Waddle started to construct small frog saunas using stacked masonry bricks covered with simple greenhouse structures that heat up in the sun. The shelters quickly became popular with the green and golden bell frogs, which are naturally drawn to warmth.
He found that when frogs’ body temperatures reached about 30 degrees Celsius (86 Fahrenheit), a threshold chytrid cannot tolerate, many were able to get rid of their infections.
As a control measure, Waddle also placed frogs in shaded, cooler structures. Frogs with access to the hot shelters warmed up and rapidly cleared the fungus, while those in the cooler enclosures did not, his study, published in the journal Nature, concluded.
Waddle said the sauna method also helps build immunity. “We found that frogs that had an infection cured with heat were 23 times more likely to survive a reinfection,” he told CNN, adding “they do get reinfected, but at really low rates.”
To have a meaningful impact, these saunas would need to be deployed widely across a species’ range. A relatively inexpensive solution, Waddle has published a simple guide to help people build their own.
But these saunas won’t work in all climates or on all species, Waddle noted. For instance, raising the body temperature of Australia’s endangered corroboree frog — an alpine-inhabiting species — would kill it.
Frogs have endured four mass extinctions, evolving repeatedly to survive a changing planet. But now, facing a fast-moving fungal pandemic that has become the most devastating infectious disease in vertebrate history — according to a 2019 retrospective study — researchers like Waddle say they need our help.
The first major amphibian die-offs that scientists could link to chytrid likely began in the 1970s, but it took more than two decades for research to identify the fungus as the culprit.
Untangling chytrid’s role from other environmental pressures is difficult, but research estimates that globally, the fungus has driven around 90 species to extinction and pushed more than 500 others into decline.
Once it enters, chytrid becomes part of the ecosystem and cannot be eradicated. It has been found on every amphibian-inhabiting continent, said Bree Rosenblum, an evolutionary biologist and professor at University of California, Berkeley.
This disease is striking a group already in crisis: “Amphibians are a very endangered group of creatures on our planet; more than a third of amphibians are considered threatened with extinction,” Rosenblum told CNN.
Because they occupy a critical middle rung of the food chain, she said, their disappearance could ripple outwards, setting off chain reactions that affect countless other species.
They also serve as natural pest control, like feeding on mosquitoes that carry diseases such as malaria and West Nile virus.
“Once you start taking species out of ecosystems, it has cascading, catastrophic effects,” she said.
More than two decades ago, Rosenblum started investigating where chytrid came from and how it spread, using genetic tools to reconstruct the fungus’s evolutionary family tree. Her work revealed that chytrid was not a single invader but a collection of lineages that evolved in different parts of the world and later spread globally — likely through wildlife trade and human movement.
“We can reconstruct it the best we can with our genetic data, but something changed in the last decades to hundreds of years that made it much more deadly,” she said. “The big question is, is it just that something changed in the chytrid, or did something also change in frogs to made them less able to fight it off because of other environmental stressors.”
She believes that slowing the disease’s spread through the wildlife trade, and reducing pressures like habitat destruction and pollution, are critical steps in curbing the amphibian extinction crisis.
But “probably the most important thing is buying amphibians time,” she said. “The long-term solution is evolution, so they can adapt and build natural immunity.”
In Northern California’s Cascade Mountains, froglets lie with their legs outstretched in shallow baths — a group of unlikely patients in a fight for survival.
These are Cascades frogs. Endemic to the Pacific Northwest they live in high mountain lakes and meadows from Northern California up to British Columbia, Canada.
They have been disappearing from this landscape, particularly from the southern end of their range, said Dr. Jonah Piovia-Scott, an associate professor of biological sciences at Washington State University.
“I had initially started off studying the impacts of introduced trout on these frogs, and then we realized that something else was also happening,” Piovia-Scott told CNN — “and it turned out that that was chytrid.”
Having studied chytrid for two and a half decades, he knew that antifungal treatments could help eliminate the infection — at least in controlled, laboratory settings.
“We were really interested in seeing whether we can actually have an impact on survival in an at-risk, wild population,” he said.
In 2012, Piovia-Scott’s team tested baths with diluted itraconazole, a common antifungal agent, and found they reduced chytrid infections and improved survival in wild populations of metamorphosed tadpoles (froglets) — the life stage most vulnerable to disease-related mortality.
The team expanded into larger-scale field experiments at meadow sites in Northern California, immersing hundreds of froglets in medicated baths for five minutes a day over six consecutive days.
In a study published in 2022, they found that treated frogs were four times more likely than those untreated to survive their first winter, which is a critical hurdle for young frogs; those that survive it are far more likely to reach adulthood and reproduce.
While antifungal baths are a valuable and scalable tool, Piovia-Scott stressed they are a short-term “Band-Aid.”
“We’re not trying to set up a situation where the only way we can keep these animals alive is treating them with an antifungal chemical every year,” he said. “I don’t think that’s a good long-term solution.”
Instead, he said the treatment works best alongside other strategies, such as translocations — moving frogs back into their former habitats to help reestablish populations. His team is already putting that approach into practice by relocating frogs from a nearby healthy population into protected sites in Lassen Volcanic National Park, a former Cascades frog stronghold, after first treating them.
Ultimately, he echoes Rosenblum: the goal is to maintain populations long enough for natural resistance to develop — which is already taking place. Piovia-Scott explained that from California to Central America, amphibians that suffered severe declines from chytrid are beginning to rebound.
This recovery isn’t because the fungus has weakened, he said — the frogs have adapted. “But you can’t develop resistance if the population doesn’t exist,” Piovia-Scott stressed.
Rosenblum praises the ingenuity behind solutions like frog saunas and antifungal baths, seeing them as proof of what human concern and ingenuity can achieve — while also underscoring the challenges involved in scaling such hands-on interventions across thousands of species and habitats.
“These strategies are worth testing, but global implementation is really hard,” she said, adding that we need to carefully weigh both the benefits and the potential unanticipated effects of intervening in natural habitats.
For Rosenblum, Waddle and Piovia-Scott, the long-term objective is to reach a point where frogs can survive on their own — without constant human intervention.
For Waddle, that kind of futureproofing may lie in synthetic biology — making frogs genetically resistant to chytrid fungus.
Scientists still don’t fully understand how some frogs naturally fight off chytrid, but Waddle said certain amphibians produce antibodies in their skin that can kill the fungus.
Waddle is now testing whether vulnerable frog species can be given those same natural defenses using genetic editing. He stresses the work is in its early days and must be approached with extreme caution.
“We need to make sure it works and doesn’t have negative effects on the frogs and we also have to think through every possible unintended consequence,” he said.
Last October, the International Union for Conservation of Nature (IUCN) adopted its first global policy on synthetic biology, acknowledging that it could play a role in conservation, while also emphasizing the need for careful, case-by-case evaluation of risks and benefits.
For some species, less radical tools may be enough, Waddle acknowledged, but for amphibians that don’t respond to classic conservation measures and are on the brink of extinction, he sees little alternative.
Protecting biodiversity isn’t just about preserving ecosystems, he added. “Frogs have thousands of compounds in their skin with powerful antibacterial properties, and we’re just starting to touch on their impacts,” said Waddle.
At a time when antibiotic resistance is becoming a global crisis, these compounds could be important not just for frogs, but for human medicine, he noted.
Scientists have already seen hints of that potential: a protein found in South Indian frogs has been shown to protect mice from influenza, preventing infection before it takes hold.
For Rosenblum, hope lies not in one tool or technology, but in a global effort to protect amphibians.
“What blankets the world are those collaborative networks of humans who care enough to be inventive on behalf of other species on our planet,” she said. That web of care, she believes, can start anywhere: “If each of us cared about one thing besides ourselves, we would be able to blanket the tree of life over and over again.”