May 10, 2018

Deadly frog fungus epidemic may have originated in East Asia

A captive Oriental fire-bellied toad (or Bombina orientalis) imported into Europe from South Korea

An Oriental fire-bellied toad (Bombina orientalis) imported into Europe from South Korea. Photo: Frank Pasmans/Ghent University

Scientists may have finally traced the origin of a deadly fungus that is decimating the global amphibian population. In a study published in Science Thursday, researchers report the fungus originated in East Asia, possibly Korea.

Why it matters: The researchers say this provides strong evidence there should be a ban on trade in amphibians from East Asia before irrevocable damage is done to global amphibian biodiversity.

"It would break down the [older] idea the [fungus] Bd is endemic and climate change triggered it."
— James P. Collins, evolutionary ecologist, Arizona State University

Background: The chytrid fungus (Batrachochytrium dendrobatidis, or Bd) was identified in the 1990s as causing the global decline in amphibians that started twenty years earlier, but the timing of the expansion and the geographic origin of the pathogen was unknown. Bd is highly infectious between animals, attacking their skin and causing catastrophic mortality.

  • Out of around 7,800 species, Bd has already wiped out more than 200 amphibian species, Collins said. It has also precipitated sharp declines in others, from Australia to the Iberian Peninsula.

Different theories of its origin:

  • One, as detailed in this 2006 study, is that the fungus is endemic and events like large-scale climate change can trigger its emergence.
  • Others — as seen in the study out today, one from earlier this week on American bullfrogs, and in earlier research — say the fungus originated somewhere else, was transported, and eventually grew to become a global pandemic.
"Something was moving these strains around," Collins says, describing the study. "They're not just popping out because of climate change."

What this study did: Over a 10-year period, the international team of researchers gathered pathogen samples around the world and sequenced the genomes. They identified 4 main lineages of the fungus, 3 of which are distributed globally and one, which demonstrates the most genetic diversity, found only in Korea.

What they found: Using the genetic data, the team estimated the killer strain of Bd currently plaguing amphibians diverged from its most recent common ancestor not thousands of years ago as previously thought, but 50–120 years ago. This coincides with a huge expansion in global trade and human population size, the scientists say.

Study limitations: Study author Simon O'Hanlon, from the Imperial College of London, says while the genetics from the fungus on Korean amphibians "bear all the hallmarks" of being the source region, there has not yet been enough research into isolates in other parts of East Asia or the Indian sub-continent to conclusively declare Korea as ground zero of the epidemic.

  • Collins also points out more evidence is needed to show a direct relationship between global trade and the epidemic, which for now is an "interesting correlation."

What's next: O'Hanlon said steps need to be taken to regulate trade more carefully, especially in exotic pets. "Any sensible steps that can be taken to put regulations in place to prevent the next global panzootic from some currently unknown pathogen would be a good thing," he said, using the term for an outbreak of infectious disease that affects animals.

  • Of note, the U.S. Fish and Wildlife Service concluded last year that Bd is already so prevalent in the country that trade restrictions would not be effective.
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