A fungal infection some scientists consider one of the worst wildlife diseases of all time is wreaking havoc on the global frog population. Now, scientists say they’ve discovered a way to help frogs fight back: tiny saunas.

Dozens of frogs placed inside hollow black bricks in Australia were doing more than soaking up the sun’s heat in the winter of 2021. Inside the roughly 100-degree boxes built to mimic saunas found in spa resorts, the frogs were fighting off chytridiomycosis, a fungal infection that causes their skin to grow up to 40 times thicker than normal.

The heat healed the infection within a few weeks, and about 70 percent of the infected frogs survived the 15-week experiment, lead researcher Anthony Waddle said. Waddle and a team of biologists released the results last week in the Nature journal, hoping their simple invention will contribute to solving a massive wildlife issue.

Waddle built the shelters using black bricks and greenhouse nets.

“It will be freezing cold outside, but the minute you step inside [the shelter] … I would just be sweating profusely because of the humidity and heat,” Waddle, a postdoctoral researcher at Macquarie University in Macquarie Park, Australia, told The Washington Post.

Chytridiomycosis, which stems from Batrachochytrium dendrobatidis, a waterborne fungus, was believed to first be found in Asia in the 1930s before trade and travel caused it to spread rapidly across the world. The contagious fungus, which has led dozens of amphibian species to the brink of extinction, causes breathing problems until many amphibians’ hearts stop.

Scientists have tried to spare amphibians by removing infected species from their habitats, chemically disinfecting their homes and heating their water sources to fight the fungus. In 2021, Waddle created a vaccine for frogs against Batrachochytrium dendrobatidis. However, he wanted to invent a solution that frogs could use themselves, especially in the winter, when chytridiomycosis cases are highest.

In December 2020, Waddle placed a few green and golden bell frogs, which are endangered in Australia’s New South Wales state, near a metal fence post, which was cold on one side and hot on the other. The frogs gravitated toward the warm side.

Then, researchers split 66 infected frogs between warm and cold areas in their lab. The frogs in the warm area, which was about 86 degrees, fought off the infection, while those in the cold area, which was about 66 degrees, remained infected.

Those results led researchers to believe that frogs would choose to — and benefit from — living in a warm habitat if researchers created one.

Scientists put to use their hardware supplies for the main experiment: clay bricks, black paint, greenhouse nets and cable ties. They painted the bricks black to attract heat from the sun. Then they stacked 10 bricks, which each featured 10 small holes, on top of each other. They covered multiple stacks of bricks with a greenhouse net to retain heat, and the cable ties stabilized the shelters.

“I didn’t think it would work because of its simplicity,” Waddle said.

On Macquarie University’s campus in July 2021, the researchers placed the shelters in tubs with gravel, water, artificial plants and flowerpots to mimic the frogs’ typical habitats. Then, 239 frogs were put into the tubs and could choose between an unshaded shelter or a shelter shaded with a cloth. Most gravitated to the warmth of the bricks in the unshaded shelters.

The unshaded shelters were about eight degrees warmer than the shaded habitats, and it made a difference. About a month after launching the experiment, researchers swabbed the frogs’ skin and found the infection was healing quickest in frogs in the unshaded shelters.

By November 2021 — soon before Australia’s summer began — 167 of the 239 frogs were still alive, Waddle said. Wild frogs typically start dying about three weeks after they’re infected, according to the Ohio Department of Natural Resources.

The researchers also found that frogs that survived chytridiomycosis became more resistant to the disease — a promising sign for the survival of the species, which can live about 15 years in captivity.

Bryan Pijanowski, a forestry and natural resources professor at Purdue University, said in an email to The Post that the shelters Waddle built offer a “bit of optimism” to solving a disease that has wiped out at least 90 amphibian species.

“These are dire numbers that require novel approaches to reverse course,” he said.

Waddle has set up a few shelters in Sydney Olympic Park, Australia, where one of the largest remaining populations of green and golden bell frogs reside. He plans to monitor the population for the next few years.

He said he hopes that parks and homeowners will implement their own “frog saunas.” He created a public guide to building them, estimating that they each cost about $80.

“Conservation research is a lot of loss,” Waddle said. “You just try things, they don’t work. You try things, they don’t work. But we got something, and it’s something we can deliver right away.”

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