Winter ‘sauna’ helps endangered frogs fight off fungal disease

Green and golden bell frogs in an artificial hotspot shelter

Anthony Waddle

One of Australia’s most endangered amphibians can fight off a deadly fungal infection with the help of a naturally heated shelter, which researchers are calling a “frog sauna”.

The disease, chytridiomycosis, has wiped out nearly 100 species of frogs, toads and salamanders around the world.

The green and golden bell frog (Litoria aurea) was once widespread in coastal, south-eastern Australia, but has seen its range shrink by 90 per cent. While other factors, like habitat loss, are contributing, chytridiomycosis is thought to be the biggest threat to the endangered creatures.

It has long been known that high temperatures limit fungal infections. Many frog species, including the bell frogs, are particularly susceptible to the disease in winter when it is difficult to raise their body temperatures – especially when warm spots are hard to find.

To find out more, Anthony Waddle at Macquarie University in Sydney and his colleagues studied two groups of captive frogs that were deliberately infected with chytridiomycosis during winter.

The first group was provided with bricks with holes in them inside an unshaded greenhouse shelter where temperatures reached highs of nearly 40°C (104°F). The second group was provided with the bricks in a shaded greenhouse shelter where temperatures reached 35°C (95°F).

In the frogs offered the warmer shelters, the amount of spores of chytrid fungus present on their skin was 100-fold lower than in the other group.

Chytrid fungus struggles to grow above 28°C (82°F), but the warmer temperature also seems to activate the frogs’ immune system, says Waddle.

“Using the shelters and surviving is like a vaccination for the frogs,” says Waddle. “We have shown that the bell frogs can gain resistance after an infection is cured with heat and this can lead to a 22 times greater chance of surviving a future infection, even under cold conditions.”

While they have only tested the shelters on one species at this stage, the researchers think the technique could work for others facing the threat of chytridiomycosis, providing they naturally seek out warmth when it is cold. Waddle says he can think of at least six Australian species that could benefit.

Importantly, these heat shelters can be readily and inexpensively deployed. “It’s just a small veggie greenhouse from a hardware store and a few bricks and costs only 60 to 70 [Australian] dollars to build,” says Waddle. “I envision people putting them in their backyards to help frogs during winter.”

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