Hope in the fight against the devastating amphibian epidemic: Some frog species could recover from a chytrid fungus infection in sun-warmed structures and build up resistance to the pathogen. This is the result of experiments with Australian golden tree frogs. The amphibians were happy to seek shelter in structures under mini greenhouses where temperatures were harmful to the fungus. This enabled many to recover and then develop resistance to renewed infections. The provision of such “saunas” could ensure the survival of other species or enable them to be resettled, say the researchers.
The disease has already wiped out 90 species in the wild and led to a population decline of more than 90 percent in another 124 species: Chytridiomycosis has led to devastating losses in recent decades, particularly among the world’s frogs and toads. The disease is caused by the fungus Batrachochytrium dendrobatidis (Bd), which has been spread worldwide by humans. It attacks the skin of the amphibians until important functions fail and they finally die.
Despite extensive research, it has not yet been possible to develop effective remedies or control strategies for the disease. In captivity, infected animals can be treated successfully, but in the wild the pathogen is almost impossible to keep under control. Only in the case of one species of toad on Mallorca has it been possible to rid the habitat of the pathogen, at least for a certain period of time – but only by using a fungicide. Until a potentially fundamental solution to the problem is found, more practical protection strategies are needed for the particularly threatened species.
Fighting infection with heat
The research team led by Anthony Waddle at Macquarie University in Sydney has now investigated whether a known weakness of the pathogen could be exploited: it is sensitive to heat. This is because the fungus prefers relatively cool temperatures – the more the temperatures rise above 17 degrees Celsius, the worse it grows. At around 30 degrees Celsius, symptoms of death even set in. Due to the pathogen’s temperature preferences, amphibian species from regions with cool periods of the year are particularly affected by the infections. There was also evidence that particularly warm refuges in the range of certain frog species could offer the animals better chances of survival during epidemics. This is why Waddle and his colleagues came up with the idea of exploring the extent to which providing artificial hotspots could help amphibians in the fight against the infections.
The model frog species they used was the Australian golden tree frog (Litoria aurea), whose population, like many comparable species, has already plummeted due to the epidemic. For the study, the researchers created small frog enclosures in which they built mini greenhouses made of foil material with bricks with holes underneath. The sunlight then warmed these structures to higher temperatures than the surrounding area, the researchers explain. For their experiments, they then placed frogs infected with the Bd fungus in the enclosures, which could visit the “sauna facilities” installed in them as they pleased.
Promising results
As the team reports, their investigations initially showed that the frogs actually sought out the units on purpose: They liked to crawl into the small hotspots in the bricks and enjoyed the temperatures there, which were sometimes around 30 degrees Celsius. As the investigations showed, this had a significant effect on their fungal infections: many recovered completely from the otherwise usually fatal disease through the heat treatment. The success was also more pronounced in units in full sun than in shaded ones, the evaluations showed. “We were able to document that artificial refuges heated by sunlight attract endangered frogs and enable body temperatures that eliminate infections,” the authors summarize.
Subsequent studies confirmed another crucial effect: after the sauna-assisted healing, the frogs developed a resistance to the pathogen. Even under cool conditions that are optimal for growth, the animals’ immune systems were able to keep the fungus in check. The team therefore sees considerable potential in the approach: “In the 25 years since the disease was identified as the main cause of the global collapse of amphibian populations, our results show for the first time a widely applicable strategy to protect frogs,” says Waddle. Since the refuges can be made from cheap and easily available materials, they can be used without any problems by professional and private animal welfare workers, the researchers emphasize.
It remains to be seen to what extent other amphibian species will also visit the sauna facilities of their own accord. But the researchers are confident, at least for heat-loving species. “The results can probably be transferred to some other frog species threatened by chytridiomycosis,” says co-author Lee Skerratt from the University of Melbourne. Senior author Rick Shine from Macquarie University adds: “The fungus is unlikely to disappear, but our approach could help certain endangered amphibians to coexist with chytridiomycosis in their ecosystems,” says the scientist.
Source: Macquarie University, professional article: Nature, doi: 10.1038/s41586-024-07582-y