A deadly disease that is wiping out the world’s frogs and toads probably originated in the Korean peninsula. The disease is being spread by the pet trade, so banning the trade in amphibians from south-east Asia could help slow it down.
The disease is the chytrid fungus, also known as Batrachochytrium dendrobatidis or Bd. The amphibians it infects tend to die of heart attacks, as the fungus stops their skin from regulating the movement of water and electrolytes. The fungus has already wiped out 200 of the world’s 7800 amphibian species and infected at least 700.
Matthew Fisher of Imperial College London, UK and his colleagues have spent 10 years figuring out where chytrid came from by studying its genetics. The team examined 234 samples of the fungus from amphibians all over the world.
They found that Oriental fire-bellied toads from South Korea carried a hitherto-undocumented variant called BdASIA-1. Fisher believes this is the “mother strain” that started the current epidemic.
Mother of the plague
“It’s four to five times more genetically diverse than the other Bd strains we sequenced,” says Fisher. BdASIA-1 carries unique genetic sequences that could only have evolved during a long period of isolation.
All the other strains seem to be younger than BdASIA-1 and probably originated from it. They all had chunks of DNA that match it, but none had all the unique regions found in the mother strain.
The team also used their genetic data to estimate when the lethal strain emerged. They knew when some of the samples were collected, and combined this with estimates of mutation rates. It seems the lethal chytrid strains emerged in the early to mid-20th century.
This was an ideal time for the fungus to escape its Korean homeland. International trade was expanding, and there were world wars to boot. “It’s one genome that got into one individual amphibian exported to Haiti, and then you have the chain of infection that goes global,” says Fisher.
Spread by pets
Today, the amphibian pet trade is the major conduit for the fungus. The team found that all the major lineages are being carried around in the pet trade.
“It proves the pet trade is moving Bd diversity around the planet, and this is going on in real time,” says Fisher.
There could well be other strains lurking undiscovered in south-east Asia, which are as harmful or worse than BdASIA-1, Fisher warns. The region is a hotspot of chytrid biodiversity.
That means we should clamp down on the trade in south-east Asian animals, says Fisher.
In 2016, the US banned imports of salamanders from the far east to prevent the introduction of Bsal, a chytrid fungus that infects salamanders. Europe introduced a similar ban in February. “I’d like to see it extended to frogs and toads,” says Fisher.
Journal reference: Science, DOI: 10.1126/science.aar1965
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