Frog ‘saunas’ could help species to battle deadly disease

Bricks could be an unlikely weapon in the fight against one of the world’s most devastating wildlife diseases.

In the past 50 years, chytridiomycosis spread rapidly from its origins in Asia into populations around the world. The fungal disease, caused by Batrachochytrium dendrobatidis (Bd), has been implicated in the extinction of at least 90 amphibian species and declines in over 400 more.

While a few treatments have been developed, including the use of fungicides and probiotic bacteria, they don’t always work. One of the most promising techniques involves raising an infected amphibian’s body temperature, which the fungus cannot cope with.

Rolling this out on a large scale has proven difficult, until now. New research, published in the journal Nature, shows that a simple setup of bricks and plastic sheeting is enough to help at-risk Australian frogs fight off infection and build up immunity to the disease.

Dr Anthony Waddle, lead author of the research, describes the shelters as a “spa” for frogs.

“Lowering mortality rates and boosting immunity to chytrid is the key to protecting amphibians from this disease, which is now endemic around the world,” Anthony says.

“Our results are the first to provide a simple, inexpensive and widely applicable strategy to buffer frogs against this disease in the 25 years since it was identified as a major cause of the global collapse of amphibian populations.”

The battle against chytridiomycosis hots up

Bd thrives in cool and moist environments, making it well-suited to invade the skin of amphibians. As many species depend on their skin to absorb water and oxygen, these infections are often fatal.

But when conditions warm up, Bd struggles. Laboratory experiments have shown that raising the temperature to 30⁰C or more is often fatal to the fungus, while the disease developed more slowly in frogs exposed to a burst of heat every day.

These seem to support observations in the wild, which found that while frogs that bask on rocks warmed by sunlight were able to resist chytridiomycosis, their relatives living in the shadow of forests nearby could not.

Though this heat treatment isn’t suitable for all frogs, especially those which prefer cooler climates, it has offered a new front in the fight against Bd. To investigate how it might be rolled out across infection hotspots, the team designed a cheap and easy-to-make thermal shelter.

It’s made up of 10-hole masonry bricks, painted black and placed in the kind of plastic greenhouse that’s often used to grow a small plot of vegetables. This is then weighed down and left on moist ground, with small gaps left in the covering to allow frogs to freely come and go.

As the bricks absorb sunlight, they raise the temperature in the greenhouse by around 5⁰C. In autumn and winter, when cooler temperatures help to boost the spread of Bd, this could be the difference between life and death for amphibians.

How effective are the thermal shelters?

To test the shelters, the team placed them into semi-controlled outdoor environments known as mesocosms. Half of the shelters were covered with cloths, while the rest were left open to the sunlight.

These areas were populated with 10 green and golden bell frogs, an Australian species at risk of extinction by Bd. Over the next 15 weeks, the frogs were periodically recaptured by the researchers to see how they were faring.

The team found that when the frogs could bask in the shelters, their body temperatures were around 3⁰C warmer than usual, helping them to fight the infection.

“In my experience, most research doesn’t go as planned, so I was surprised just how well the hot shelters allowed frogs to rapidly reduce and clear chytrid infections,” Anthony says.

“Frogs that had cleared their infection using heat were subsequently resistant to disease regardless of their body temperature. This allowed me to think of the shelters as these little factories where sick frogs would go, get better, and come out not only healthy but better able to fight chytrid.”

In the short term, the researchers plan to roll out these shelters across Sydney to support vulnerable populations of green and golden bell frogs – but hope they will one day be used on a much larger scale.

“Eventually, I would like to place the shelters at all of the major priority areas for the species,” Anthony says. “Since the shelters are inexpensive and made from readily available materials, I am also keen to see the general public get involved with their deployment.”

“I have drafted a how-to guide for constructing a shelter and my hope is that people living on Australia’s central coast may take an interest in the frogs and place these in their backyards.”