In the 1970s and 1980s, frogs and other amphibians seemed to be disappearing overnight. By 1999, researchers had determined the culprit was a deadly disease caused by chytrid fungus which infected the animals with tiny, swimming spores.
Today this disease, called Chytridiomycosis, is thought to be one of the deadliest pathogens on the planet. It infects hundreds of species of amphibians and is thought to have wiped out a third of all frog species. These animals are important contributors to biodiversity, insect and disease control and may even be sources of new types of medicine.
For decades, scientists hoping to save these semiaquatic animals from extinction have been trying and failing to pin down the origins of this mysterious killer. They knew it developed from a common ancestor, but could not agree on where or when. Now, an international group of scientists has compared the genomes of 177 samples of the deadly fungus from six continents. They determined that the pathogen most likely arose on the Korean Peninsula 50 to 100 years ago and spread through global trade.
Their research, published on Thursday in the journal Science, reiterates that the pathogen comes in many different strains, some more virulent than others. It suggests that new variations of the fungus can still develop and spread disease without proper protections.
A photo provided by Dirk Schmeller of researchers collecting a sample of the fungus Batrachochytrium dendrobatidis, using a swab taken from an amphibian. Source: Dirk Schmeller via The New York Times
Previously, researchers were limited by the scrappy bits of the fungus’ DNA they could obtain by smearing a cotton swab across the skin of a frog or a salamander.
But by sequencing the full genomes of samples of the fungus, also called Batrachochytrium dendrobatidis or Bd, from all over the world, the team, led by Simon O’Hanlon and Matthew Fisher, infectious disease epidemiologists at Imperial College London, found that samples shared the most genetic information with a group obtained from frogs that live on the Korean Peninsula, suggesting this was where it had originated.
They also discovered that this Korean lineage contained strains that were more genetically diverse than any others — and because it infected animals but did not kill them, it likely had been living with amphibians, who learned to tolerate or defend against it, for some time.
The researchers think the virulent, global strain emerged within the past century coinciding with rapid development in global technology, commerce and trade. During this time, animals stowed away in equipment or produce or they were traded directly as scientific or medical specimens, food and pets. These amphibians likely harboured the fungus, allowing it to become more virulent and spread to others not adapted to it.
And it is still happening, said O’Hanlon.
“I don’t think we really knew the scope of it,” said Karen Lips, an amphibian ecologist at The University of Maryland who wrote an accompanying commentary. “All the different flavours of chytrid are in the trade. And they’re hybridising and being moved about. None of this is good news for amphibians.”
Many regulators have assumed that animals can harbour only one kind of chytrid fungus. But Lips said this paper brings attention to how diverse strains of the fungus could slip through the cracks and cause greater declines if trade is left unregulated.
“They like to say, well the horse has left the barn, and I say, well maybe one horse left the barn and all the other ones are still in there,” she said. “We need to make sure they don’t get out either, and that they don’t mate and have babies that also escape.”
That might require enforcing trade bans of species known to harbour the disease — as they have done for some salamanders that harbour a sister fungus called Bsal — or quarantining and testing amphibians for Bd, especially those arriving from Asia, she said.
But she emphasised that, “this is a much bigger picture than frogs, chytrid and amphibians.” This case merely highlights the importance of preventing the global spread of infectious diseases for all kinds of plants and animals.
