Bd favors frogs, while Bsal prefers salamanders, but both fungi produce the same, brutal disease called chytridiomycosis. When spores of either species attach to a susceptible amphibian, which number in the hundreds, they harvest its skin cells to produce a so-called mother cell that generates even more spores.

This is a nightmare for these animals, which famously use their skin to take in water and, in some cases, breathe.

“Grossly, you’ll see these ulcers being created on the skin,” says Matt Gray, an ecologist at the University of Tennessee, who has been studying the genus Batrachochytrium for years. “It looks like somebody has taken a cigarette butt and burned the animal. Ultimately, it causes disease and death.”

When Bd was discovered in 1998, it was already spiraling into a pandemic, infecting streams and ponds from South America to Australia and, of course, the animals that call them home. Now scientists fear Bsal is headed down the same path — this time at the expense of what could be hundreds of salamander species.

Bsal is native to Asia, scientists believe, where it lives in relative harmony with local amphibians that likely co-evolved with the pathogen over millennia. But sometime earlier this century, Bsal apparently hitched a ride to the Netherlands through the pet trade. There, it escaped into the wild, where it thrived. In a matter of years, Bsal wiped out nearly 96 percent of the country’s native population of fire salamanders.

See more of Mongabay’s series on salamanders and Bsal here.

Scientists say it’s only a matter of time before the fungus travels the same route to the U.S., home to the world’s largest number of salamander species, more than half of which may be susceptible to Bsal infection, according to Gray’s unpublished research. Not even a restriction recently placed on the pet trade — a ban on the import of 201 species that scientists believe to be susceptible — can stop it, they say.

“It really does seem to be a matter of when and not if,” says Priya Nanjappa, the former national coordinator for the nonprofit Partners in Amphibian and Reptile Conservation (PARC), one of two nonprofit groups addressing the threat of Bsal in the U.S.

Like other amphibian experts, Nanjappa is worried. But she points out an important silver lining: Scientists, possibly for the first time, have advanced notice of the emerging threat. They know Bsal is coming, they know what to look for, and, thanks to decades of research on Bd, they have a good idea of how it will spread. So instead of waiting to be caught off guard by an outbreak, they can proactively search for the fungus — and pounce when they find it.

The path of a pathogen

No matter how much time you have, finding Bsal isn’t easy. Salamanders may be difficult to spot on a hike, but they’re incredibly widespread, hiding under logs, in mud and water in New York, California, and every state in between. Then there’s the pathogen itself. There are dozens of ports along the coast into which pet amphibians are imported and thousands of pet stores. Where to even begin?

For all that we don’t know, there’s a lot we do: the distribution of salamander species, the location of pet stores, the quantity of live amphibians imported into different ports. We even know the temperatures in which Bsal can thrive (15 to 20 degrees Celsius, or 59 to 68 degrees Fahrenheit). With data, you can build models. And with models, you can guess where Bsal is most likely to be introduced.

In 2016, researchers from the U.S. Geological Survey (USGS) did just that. Using geographic data on the pet trade, salamanders and the pathogen, they built a series of heat maps showing where Bsal is most likely to enter the U.S. and where salamanders will likely experience the most dramatic declines.

Deep red pixels were concentrated in Florida, Southern California and New York, epicenters of the pet trade. Their maps also showed dark hues in the Appalachian Mountains and Pacific Northwest, where species diversity is greatest.

When Bsal arrives, that’s where it will most likely show up, they reasoned. Now they just had to go look for it.

The search begins

Two decades of research on Bsal’s cousin, Bd, has its advantages. For one, scientists know how to quickly test amphibian skin for different species of fungi. It’s like identifying the killer at a crime scene: When researchers come across a salamander in their search, they collect material on its skin, run a DNA test, and look for a match in their database of deadly fungi, which now includes Bsal.

“You take a swab and rub it along the skin of the animal,” says Dede Olson, an ecologist at the U.S. Forest Service. “The fungi comes off on your swab, and then you run the genetics on that swab to see what you found.”

It’s with this method that USGS researchers traveled to the darkest reds on the map in 2016 to begin testing salamanders — a whopping 10,000 of them. If Bsal was present in the U.S., surely they’d be able to find it.

Robert Fisher is part of the team in Southern California that was responsible for collecting around a quarter of the samples. “We’re close to LAX, which is where most of the Asian salamanders are coming through,” says Fisher, a biologist at USGS. “Whether Bsal has jumped from the pet animals to wild animals is the crux of this surveillance.”

With tools for genetic analysis, finding Bsal is only as hard as finding its wet-bodied hosts — which can be difficult, even though they’re widely distributed.

You’re often standing “chest-deep” in pond water, fighting back mosquitos and flies, says Daniel Grear, a colleague of Fisher’s who leads the surveillance effort at USGS. “Usually it depends on having a team of people with fine mesh nets walking around a pond to where we think the newts are going to be.”

And searching on land isn’t much easier. “They’re really hard to find because they live in burrows,” Fisher says of L.A.’s slender salamanders. “When we started the surveillance, it was so dry. By the time we got the sample kits [to swab the salamanders] it was really hard to get good numbers of them.”

The USGS team persisted, swabbing salamander after salamander. Meanwhile, another search for Bsal was underway, in tanks and terrariums across the U.S.

Wild amphibians tell only half the story. Or at least that was the thinking behind a project led by Blake Klocke, a scientist at George Mason University, designed to sniff out Bsal in the pet trade. In 2014, Klocke asked salamander owners across the country to swab their cold-blooded pets at home and mail the samples to a lab for diagnosis.

Two years later, he had 639 swabs representing 65 species, most of which were susceptible to Bsal. Now he just needed to run the genetic tests.

Scientists hold their breath

Last December, USGS researchers swabbed their 10,000th salamander. After analyzing the final sample, they sighed in relief and discarded the Q-tip-like swab, as they did with thousands of others.

“We didn’t detect it,” Grear says. “If Bsal was present and widespread in any area we sampled we would have detected it — we think. Overall, our confidence that it’s not here and not widespread has increased.”

Results from Klocke’s study on pet salamanders were equally favorable: Nobody’s pet tested positive for the pathogen (though some of them did register positive for Bd, which the researchers also screened for).

“Bsal has not been detected to date in captive collections or in wild populations in the U.S.,” the authors wrote. “Our results suggest that if Bsal is present in private captive U.S. salamander collections, it occurs at extremely low prevalence.”