Many of the world's amphibians are staring down an existential threat: an ancient skin-eating fungus that can wipe out entire forests' worth of frogs in a flash.
This ecological super-villain, the chytrid fungus Batrachochytrium dendrobatidis, has driven more than 200 amphibian species to extinction or near-extinction—radically rewiring ecosystems all over Earth.
“This is the worst pathogen in the history of the world, as far as we can tell, in terms of its impacts on biodiversity,” says Mat Fisher, an Imperial College London mycologist who studies the fungus.
Now, a global team of 58 researchers has uncovered the creature's origin story. A groundbreaking study published in Science on Thursday reveals where and when the fungus most likely emerged: the Korean peninsula, sometime during the 1950s.
From there, scientists theorize that human activities inadvertently spread it far and wide—leading to amphibian die-offs across the Americas, Africa, Europe, and Australia.
“[The pathogen's spread] could have happened from any one event, from the cumulative number of events, or maybe some big anthropogenic events like the Korean War,” says Imperial College London researcher Simon O'Hanlon, the study's lead author.
Now that researchers know where the fungus came from, they can monitor this hotspot of chytrid diversity for new, deadly species. The findings also serve as a stark reminder that if left unsecured, global trade can unwittingly fuel ecological catastrophes.
'Carpets of Dead Frogs'
The fungus—called Bd for short—is so deadly because it targets amphibians' porous skin, which the animals use to breathe and drink water. Bd unspools the skin's proteins and feasts on the resulting spaghetti of amino acids. As it does, infected animals grow lethargic, shedding their skin in a death spiral that ends with heart failure in a matter of weeks. Some amphibians can tolerate or resist Bd, but the fungus can infect at least 695 species to varying degrees.
“This is pretty unusual for a disease, to see it affect so many different species,” says University of Maryland biologist Karen Lips, an amphibian-decline expert who wasn't involved with the new study.
In person, Bd infestations can look like biblical plagues. Each August, adult midwife toads in the French Pyrenees climb out of their birth lakes for the first time. The infected toads barely make it to shore. “They'll do one last hop, and then they'll expire in your hands,” says Fisher, one of the study's coauthors. “You can walk the lakes—it's just carpets of dead frogs.”
Similar die-offs started popping up in the 1970s, but researchers didn't realize these “enigmatic declines” were a global phenomenon until the 1990s. In 1997, researchers first described Bd, and within a decade, they had connected it to the slaughters. Meanwhile, Bd's killing spree continued. From 2004 to 2008, one site in Panama lost 41 percent of its amphibian species to the fungus.
Most of the once-mysterious slaughters are now attributed to the “Global Panzootic Lineage,” a lethal strain nicknamed BdGPL. But where did this killer come from? And when and how did it blaze a trail around the world?
Building the Bd Library
To find out, researchers spent a decade building a global genetic library of Bd, a quest that has taken scientists across six continents. For instance, study coauthor and National Geographic Young Explorer Jennifer Shelton spent part of 2017 exploring the mountains of Taiwan, searching for infected salamanders.
When researchers found a sick amphibian, they first snipped off one of the animal's toes, a non-lethal way to sample its tissue. Scientists then had to isolate the severed toe's Bd, grow it in a petri dish, and then sequence its DNA.
O'Hanlon and Fisher's team sequenced 177 Bd genomes from around the world, combining them with 57 previously published ones. Researchers compared these 234 genomes to map out a fungal family tree, which revealed four distinct Bd lineages.
Samples from the Korean peninsula showed greater genetic diversity than any other Bd site on Earth, confirming earlier hunches that the region was Bd's ground zero. And once researchers figured out Bd's mutation rate, they learned that the ancestor of today's BdGPL emerged out of Asia in the early 20th century. Until its global export around the 1950s, the fungus had coexisted peacefully with local fauna.
Researchers theorize that human activity spread infected amphibians worldwide, either through shipping, the once-booming trade in live-frog pregnancy tests, the amphibian meat and pet industries, or massive events such as the Korean War. At the conflict's height, millions of soldiers and pieces of equipment entered and exited the region—giving amphibians ample opportunity to hitch a ride.
Despite international trade guidelines, it's clear that the global pet trade continues to spread Bd. When team members combed pet stores and markets in Belgium, the U.K., the U.S., and Mexico, they found infected frogs and toads that carried all known lineages of Bd, including the deadly BdGPL.
The Apocalypse's Many Horsemen
Topical fungicides can heal Bd-stricken amphibians, a method that's been tried out successfully in the wild. But at present, wild populations can't be cured on a global scale. For now, researchers say that our best option is to prevent the fungi's further spread. But stopping chytrid is a tall order—especially since there are deadly species other than Bd.
In 2013, researchers identified B. salamandrivorans, a sister species of Bd known as Bsal. Its name translates to “salamander-devouring” for a reason. From 2009 to 2012, the fungus slashed Dutch fire salamander populations by more than 99 percent.
In 2016, U.S. wildlife officials banned the importation of 201 salamander species, expressly to keep Bsal out of the country. However, a 2017 appeals court ruling means that if these salamanders were already in the U.S. before the ban went into effect, their interstate transport remains legal.
The new study also underscores the threat of Bd hybrid strains. Previously, researchers knew that the native Brazilian strain of Bd could hybridize with the killer BdGPL; now, researchers have shown that the African lineage of Bd can do the same. As once-separated fungi mix and mingle on a global scale, who knows what virulent hybrids might arise?
“To me, this is one of the scariest things,” says Lips.
To Save Amphibians, Ban Their Trade?
BdGPL is already in the U.S., and the U.S. Fish and Wildlife Service is actively monitoring its spread. However, the agency isn't blocking the arrival of its sister lineages. In March 2017, it stopped considering a 2009 petition to ban all amphibian imports unless they were Bd-free.
“The Bd fungus is already widely present in the environment in the United States, so regulating importation of amphibians will do little to protect native amphibians ... [and] would be minimally effective at preventing the fungus from further spreading across State lines,” said Dave Miko, the U.S. Fish and Wildlife Service's chief of fisheries and aquatic conservation, in a statement.
“This new paper says that Bd is not just one thing,” counters Lips, who recommends a renewed effort to ban Bd imports. “While we have one form of it here, we want to keep out the other forms because they might be worse.”
At a minimum, Lips says that internationally traded amphibians should be tested for Bd, which doesn't happen consistently today. The U.S. Department of Agriculture, for one, doesn't require health screenings on imported pet amphibians.
For O'Hanlon and Fisher, the most promising solution to Bd and Bsal is the most sweepingly idealistic: a complete ban on the global amphibian pet trade.
“Do we really need to be mining our environments for organisms and selling them around the world for financial profit, just so we can keep them in terrariums in our living rooms and say 'Hey, that's cool?'” asks Fisher. “It seems like a fairly innocent pastime, but we're putting entire ecosystems at risk.”