The Origins of the Killer Fungus Driving Frogs to Extinction

Geneticists have finally found the pathogen’s ancestral home.

A green and black frog with a red belly
A Oriental fire-bellied toad imported from South Korea to Europe  (Frank Pasmans)

The arrival of a chytrid fungus can herald nothing short of catastrophe. In the past few decades, one particular type of the fungus, Batrachochytrium dendrobatidis, or Bd, has killed off so many frogs in Australia, Europe, and the Americas that scientists have warned of an impending mass extinction. “We’ll just see lakes surrounded by hundreds if not thousands of dead or dying amphibians,” says the Bd researcher Simon O’Hanlon, of field sites he studied in the Pyrenees.

For the past 10 years, O’Hanlon and his colleague Matthew Fisher, both researchers at Imperial College London, have been hunting for Bd’s origins with DNA. They put a call out for Bd samples all over the world and eventually sequenced 177 genomes. The single place with the most Bd diversity, they found, is the Korean peninsula. This suggests that Bd first flourished there then spread to the rest of the world, likely beginning in the early 20th century.

“There’s a lot more diversity out there than we imagined,” says Karen Lips, an amphibian researcher at the University of Maryland at College Park, who was not involved in the study but wrote a companion perspective piece about it. The concentration of fungal diversity in Asia means the region could be a wellspring for other pathogens. And in fact, another devastating chytrid fungus that infects salamanders, Batrachochytrium salamandrivorans, has also been traced to Asia. If Asia is indeed this fungal-pathogen hot spot, says Lipps, then this information could inform policies limiting their spread—perhaps by targeting and testing their exports.

Massive amphibian die-offs have not been documented in Asia. Perhaps Asian frogs that have long co-existed with Bd have evolved resistance, or perhaps Bd only evolved its deadliness to frogs after it left Asia. (Not all Bd strains are equally deadly.) Lips also notes, however, few amphibian surveys have been done in Asia.

The fungus causes a disease called chytridiomycosis that attacks the skin. You can tell a frog is infected, says Fisher, because “the skin around the joints becomes shredded and sloughs off.” If that isn’t bad enough, frogs also use their skin to breathe and regulate their water and electrolyte balance. In a bad case of chytridiomycosis, the skin simply stops working. The frog loses so many electrolytes that it suffers a heart attack.

Frog die-offs have knock-on effects for the ecosystem. In Panama, Lips has found, streams that once ran clear will turn green without tadpoles to feed on the algae. Once the amphibians that eat mosquitoes and flies are gone, insect-borne diseases could also spread more quickly among humans.

Joyce Longcore, a mycologist at the University of Maine, first identified and named Batrachochytrium dendrobatidis in 1999, after studying blue poison-dart frogs that died at the National Zoo in Washington, D.C. Longcore is a longtime expert on chytrid fungi. “Chytrid fungi have a particular gleam that, after working with, you tend to recognize,” she says, and she saw that gleam in the frog’s skin cells.

Based on how the DNA sequences diverged, O’Hanlon and Fisher found the deadly Bd strain did not emerge 26,000 years ago, as one previous study suggested, but less than 100 years ago. Its spread coincides with the rise of global trade. It is easy for small frogs to hide out in a shipping container. “They’re sticky little characters,” says O’Hanlon. “Bunches of bananas always have frogs in them. I know herpetologists who go to the London docks when they bring in green bananas.”

Global trade has been good for Bd, maybe even good for those invading frogs. But it has been disastrous for native frogs that encountered the deadly fungus for the first time.