Can we stop one of the deadliest organisms in nature?

Vaccinating frogs against a killer fungus that has wiped out 200 species may stop more endangered amphibian species from croaking it. Cathal O’Connell reports.

Batrachochytrium dendrobatidis fungus on the skin of a salamander. This pathogen is responsible for population declines and massive die-offs in amphibians. – Visuals Unlimited/Corbis

If you were trekking through the leafy D’Aguilar mountain range in south-eastern Queensland around 1970 or before you might have noticed, clinging to the underside of a boulder near a fast-flowing stream, a little brown frog with a distinctive ‘H’ on its back. The Mount Glorious Day Frog was common at the time and it came from ancient stock, originating 100 million years ago when Australia was part of Gondwanaland. These frogs had not only survived the catastrophe that killed the dinosaurs, they’d also survived an ice-age, climate shifts and countless predators. Yet, within a brief four-year period in late 1970s every last member of the species was wiped out.

The Mount Glorious Day Frog was one of the first victims of the spread of the deadly fungus Batrachochytrium dendrobatidis (Bd). This indiscriminate amphibian killer, also known as chytrid fungus, has caused the extinction of at least 200 species of frogs around the world in the past four decades. It has also wiped out toad, newt and salamander species, helping make amphibians the most endangered animal group in the world with nearly one third of known species now extinct or nearly extinct.“It could be the second most deadly organism on the planet, behind humans,” says Jason Rohr of the University of South Florida. “I know of no other species implicated in that many extinctions.”

Rohr and his colleagues are testing whether a vaccination program could help to save the remaining amphibians. Their latest results, published this week in Nature, look promising.

Rohr and his team showed that infecting captive frogs with Bd, and then curing them using heat therapy, left the animals better equipped to fight future exposure to the fungus. When these frogs were directly re-exposed to Bd they became less sick than frogs encountering Bd for the first time. What’s more, the frogs’ resistance increased when they were infected and cured several times.

This inoculation strategy may lead to improved conservation measures, says Rohr. So far, the only way to save Bd-ravaged frog species has been through the Amphibian Ark project in which conservationists transfer members of threatened species from the wild to captive breeding zoos. The problem is, when such frogs are re-introduced into the wild they still often die from exposure to Bd. “To improve the chances of success,” says Rohr, “these zoos might be able to induce acquired resistance before these species are released back to the wild.”

The golden frog or Atelopus Zeteki, is one of Panama's national symbols. Its numbers have been declining due to the deadly fungus. – MAURICIO VALENZUELA/Xinhua Press/Corbis

Even more encouragingly, the study also shows that frogs can build resistance through exposure to Bd killed by heat treatment. This means wildlife managers might be able to release dead fungus directly into ponds – that is, immunise amphibians right in the wild. “It’s not like you have to actively give the animal the infection itself,” says Teagan McMahon, co-author of the Nature paper. This finding, she says, provides hope for establishing positive management plans.

Bd researcher Lee Skerratt of James Cook University praises the new experimental approach. He is cautionary, however, about its possible impact on wildlife management. Skerratt notes that although the study shows that inoculating frogs with the dead fungus does provide some protection against the disease, the size of the effect is relatively small. “[Bd] is still killing the majority of frogs after a short period of time,” he says.

On a global scale, Skerratt adds, Bd has been so devastating within a few short decades because it has been able to hitch a ride with humans to every corner of the planet. Consequently, Bd was able to expand at an alarming rate into territories where frogs had no previous exposure and so no immunity. “It’s globalisation,” he says. “The big lesson is that we need to get some sort of biosecurity system in place globally; otherwise, we're just going to have more of these types of events occurring with other species and other pathogens.”

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