In the 1970s and 1980s, frogs and other amphibians seemed to disappear overnight. By 1999, researchers had determined that the culprit was a deadly disease caused by chytrid fungi that infected the animals with tiny, swimming spores.
Today, this disease, called chytridiomycosis, is considered one of the deadliest pathogens on the planet. It infects hundreds of amphibian species and is said to have killed one third of all frog species. These animals make an important contribution to biodiversity, to controlling insects and diseases and can even be sources of new types of medicine.
For decades, scientists have been attempting to prevent these semi-aquatic animals from extinction, and try not to trace their origins to mysterious murderers. They knew it was a common ancestor but could not agree where or when. Now an international group of scientists has compared the genomes of 177 deadly fungus samples from six continents. They found that the pathogen most likely originated on the Korean Peninsula fifty to one hundred years ago and spread through world trade.
Their research, published Thursday in the journal Science, reiterates that the pathogen occurs in many different strains, some more virulent than others. It suggests that new variations of the fungus can still develop and spread disease without adequate protection.
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Previously, researchers were limited by the rough parts of the DNA of the fungus that they could obtain by swabbing a cotton swab onto the skin of an animal frog or salamander
But by sequencing From the entire genome of samples of the fungus, also called Batrachochytrium dendrobatidis or Bd, from around the world, the team led by Simon O & # 39; Hanlon and Matthew Fisher, epidemiologists aboard Imperial College London found that samples are the most genetic Sharing information with a group of frogs living on the Korean peninsula, suggesting that this was the source of the origin.
They also discovered that this Korean lineage contained genetically more diverse strains – and because it infested animals but did not kill them, they probably had lived with amphibians who had learned to tolerate or defend them. for some time.
Researchers believe that the virulent, global burden has occurred within the past century coinciding with the rapid development in global technology, commerce and trade. During this time, animals were stowed in equipment or products, or traded directly as scientific or medical samples, food and pets. These amphibians probably harbored the fungus, which made it more virulent and spread to others unadapted to it.
And it still happens, Dr. O & # 39; Hanlon.
"I do not think we really knew the scope," said Karen Lips, an amphibian ecologist at the University of Maryland, who wrote an accompanying commentary. "All the different flavors of Chytrid are on the market, and they hybridize and move, none of which is good news for amphibians."
Earlier reports on the rescue of frogs and other amphibians
Many regulators have assumed that Animals can harbor only one type of chytrid fungus. But Dr. Lips said that this paper draws attention to how different strains of the fungus might slip through the cracks and cause larger declines if trade is not regulated.
"They like to say, well, the horse left the barn and I say, well, maybe one horse left the barn and everyone else is still in there," she said. "We have to make sure they do not come out and that they do not mate and also have babies who also escape."
That might require the enforcement of trade bans of species known to harbor the disease for some salamanders that harbor a sister fungus called Bsal – or quarantine and testing amphibians for Bd, especially those that come from Asia, she said.
But she emphasized that "this is a much bigger picture than frogs, chytrids and amphibians." This case only highlights the importance of preventing the global spread of infectious diseases to all species of plants and animals.