A Deadly European Fungus Threatens the Future of American Salamanders

Researchers are working to find a way to mitigate the damage to local populations of the amphibious creatures if a skin eating fungus makes its way to the U.S. from abroad.
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It all started in 2008 when scientists started noticing fire salamanders in the Netherlands were disappearing. By 2011, nearly all salamanders in affected Dutch populations had been killed off, as were many in neighboring Germany and Belgium.

In 2013, scientists identified the culprit: a new-to-science chytrid fungus they named Batrachochytrium salamandrivorans (Bsal). Its species name means "salamander-eating" because that's literally what it does. As Bsal takes hold, it eats away a salamander's skin, leading to ulcers that get bigger and bigger until pieces of skin start sloughing off.

"Bsal is an acute infection that just turns them into little masses of slime in three to four days," said University of California–Berkeley salamander expert David Wake in 2015.

"It looks like somebody has taken a cigarette butt and burned the animal," said Matt Gray, an ecologist at the University of Tennessee.

"It's death by a thousand holes," said Molly Bletz, a disease ecologist at the University of Massachusetts.

Research indicates Bsal evolved in Asia and spread to Europe via the pet trade. But as bad as it's been for salamanders there, biologists fear this may just be the tip of the iceberg. Because, any day now, they expect it to show up in the United States, home to more salamander species than any other country in the world.

To head off a pandemic, researchers say the best thing would be to ban the import of all amphibians into the U.S. But they're not waiting around for that to happen, and are developing a response plan in the meantime to help deal with an outbreak in North America should one occur.

But a big part of dealing with an outbreak is knowing what impacts it will have if (or more likely when) it begins. To figure this out, researchers around the U.S. have been examining how the 200-plus species of salamanders here react to Bsal. While they still have a way to go until they reach that number, preliminary results from the dozens of species they've tested so far paint a somber picture—most are susceptible to infection. And those that don't die outright may act as carriers, spreading the disease and ushering in a pandemic.

A Formidable Fungus

Batrachochytrium salamandrivorans is a fungus in Chytridiomycota group of fungi. Like other chytridiomycetes, Bsal reproduces and spreads through the release of "zoospores," which, unlike most fungal spores that simply drift around, have little tail-like appendages called flagella that they use to propel themselves through their environment. Bsal also exists as "encysted" spores that don't move and can survive being in drier environments.

Once released, Bsal zoospores swim around until they come into contact with a potential host—say, a young fire salamander. They then burrow into the salamander's skin and set up camp, eating away skin tissue as they rapidly multiply.

Like other amphibians, salamanders rely on their skin for many purposes. They absorb water through it, use it to maintain proper electrolyte balance, and even breathe through it. And so, losing their skin to an infection like Bsal can be a death sentence, killing a salamander in as little as two weeks after infection begins.

But not all salamanders have the same reaction to Bsal. Some live through the infection, and others seem to be able to resist infection completely. Researchers are still trying to figure out why some species are more susceptible than others, but they have a few ideas.

"Some species, such as the aquatic ones, are going to be more in contact with the fungus, so they're going to be more susceptible just based on exposure," said Ana Longo, an amphibian disease ecologist at the University of Florida.

Prior to becoming an assistant professor at the University of Florida this summer, Longo researched chytrid susceptibility at the University of Maryland (UMD). While there, she and her colleagues collected salamanders from sites in Maryland and Florida, brought them back to the UMD lab, and examined their response to Bsal.

First, they monitored the salamanders for a couple weeks to make sure they weren't already compromised by other diseases. Then they exposed them to Bsal they'd cultured in petri dishes over 10 days.

In all, they've tested 13 species, including eastern newts (Notophthalmus viridescens) and those in the Plethodon and Desmognathus genera.

"We're testing every genus we can," Longo said.

So far, all salamanders tested by Longo and her team have been susceptible to Bsal infection.

They also exposed them to a different chytrid fungus, Batrachochytrium dendrobatidis, which primarily affects frogs and has contributed to the extinctions and declines of around 200 species around the world since it was first described by scientists in the 1970s. Bd is already globally present and many, if not most, salamander populations are currently living with it. But Longo and her colleagues thought this frog-killing fungus might actually be advantageous for salamanders because perhaps it could prime their immune systems to fight off Bsal more effectively.

But the opposite proved true.

"What we're seeing is when they have both at the same time, it's the worst combo that they can get," Longo said. "Even if they can fight Bd infection and clear it, Bd suppresses their immune systems, so Bsal more easily takes over and wins the battle."

Super Spreaders

Susceptibility research has revealed more bad news for North American salamanders. Longo said that amphibians that survive infection can act as reservoirs for the fungus and spread it to new areas. One of the species she's most worried about when it comes to this is the eastern newt. Not only is this species very widespread, but it also has a terrestrial "wandering" phase that could shuttle Bsal between water sources. This combination, Longo says, makes the eastern newt a potential "super spreader."

"If Bsal arrives, then the first species it's going to encounter are the newts that are highly susceptible," Longo said. "So it's going to spread really fast."

Other labs around the U.S. have upheld Longo's findings on eastern newt susceptibility and its potential as a carrier. One of these is run out of the University of Tennessee by ecologist Matt Gray and wildlife pathologist Debra Miller. They took a deeper dive into eastern newt susceptibility by comparing the susceptibilities of different newt populations around the country to see if members of the same species could have different reactions to Bsal. They also looked at whether some newt life stages are more susceptible to Bsal than others.

While the UT group hasn't yet published its findings, its preliminary results indicate that, yes, newt susceptibility does seem to differ significantly with population and age. For instance, the researchers found that all newts from a population in Michigan became diseased when exposed to Bsal while just 20 percent of newts from the Blue Ridge region of Tennessee did. They also found newts were more susceptible to Bsal during their terrestrial, juvenile stage than later on as aquatic adults.

The UT lab has finished up testing 30 species and is beginning another round for 18 more. In addition to eastern newts, they've identified several species as being particularly susceptible. Among them is the black-spotted newt, an endangered aquatic species native to southern Texas and northeastern Mexico that looks a bit like a tiny polka-dotted crocodile. The striped newt, a southeastern U.S. species with orange-red racing stripes down its sides was also found to be highly susceptible to Bsal, as were the dramatically colored large-blotched ensatina that lives in the western U.S. and the splay-foot salamander native to northern and eastern Mexico.

The UT researchers are focusing their testing efforts on "species of greatest conservation need" to identify Bsal-susceptible salamanders that are already threatened or have limited ranges. They highlight the striped and black-spotted newts as particularly vulnerable. "Bsal represents a significant conservation risk to N. meridionalis and N. perstriatus, which are species of greatest conservation need in the southern United States," they write.

The UT lab isn't just testing salamanders; they're also seeing if frogs can harbor Bsal. And, according to test results, they can. Cope's grey tree frogs, Chiricahua leopard frogs, eastern spadefoot toads, and American toads can all contract Bsal but do not die from it, meaning that they can potentially act as carriers and spread it to new areas where susceptible, vulnerable salamanders may live.

This isn't the first time frogs have been identified as potential Bsal carriers. Previous research discovered the fungus infecting the skin of small-webbed fire-bellied toads. A closely related species, the oriental fire-bellied toad, is popular as a pet in the U.S. with an estimated 3.5 million of them traded in the country between 2001 and 2009. This discovery prompted biologists to urge the federal government to extend a 2016 ban on the import of salamanders to all amphibians in the hopes that doing so will reduce the chances of a North American Bsal epidemic.

Big Impacts and a Little Hope

The U.S. has the unique distinction of being the world's hotspot of salamander diversity. Scientists think this is because the Appalachian Mountains, which skirt along the eastern side of the country, were ground zero for the evolution of the largest salamander family, Plethodontidae. These little salamanders have diversified to suit a wide variety of ecosystems across the country—from northern leaf litter to southern swamps, even living arboreally in West Coast trees. But they all have one defining characteristic: they don't have any lungs. Instead, they breathe entirely through their skin.

While the UT lab's research has found that a few plethodontid salamanders are resistant to Bsal, the family remains one of the most susceptible to the disease. And since there are so many plethodontid species, researchers worry a Bsal epidemic could lead to more than a few extinctions.

But even if species don't completely blink out, they stand to undergo drastic declines that may have repercussions on the surrounding environment. Many salamanders are hyper-abundant in their habitats and, because of this, have important ecosystem roles.

"The biomass of these animals is really high," Longo said. "Even though they are really small, they compose a big part of the ecosystem."

Take, for instance, the red-backed salamander. Studies of this species found that they are important regulators of insects and other arthropods that eat fungi. These fungi, in turn, break down leaf litter and recycle its nutrients back into the forest soil. Researchers worry that if red-backed salamanders were to suddenly disappear, forests might essentially starve.

Yet more research indicates salamanders may even play an important role in carbon cycling. A 2014 study by the U.S. Forest Service estimates a single woodland salamander keeps around 200 kilograms of carbon per hectare out of the atmosphere because it eats so many of the insects that would otherwise release carbon dioxide and methane by consuming leaf litter.

The potential magnitude of the effect of woodland salamanders on carbon sequestration is staggering," the researchers write in their study, "and poses a provocative new perspective on the contribution of biodiversity in general, and woodland salamanders in particular, to forest and global sustainability."

And so, with a lot to lose, researchers are scrambling to minimize surprises and figure out what to do once Bsal gets here. To this end, a multidisciplinary team that includes Matt Gray has been working on a strategic response plan that they plan to release next month. Gray, Miller, and the rest of the UT team are also rolling out a new research project funded by a $2.5 million grant from other U.S. universities. They'll be studying how Bsal develops as a disease, how it spreads, and how salamanders' immune systems respond to infection.

"With eastern North America as a global hotspot for salamander biodiversity, this research will allow science-based decisions to be made on Bsal response actions most likely to thwart an outbreak in the USA and elsewhere," Gray said.

Hope may also lie within salamanders themselves—specifically, in their slime. A study published in July in PLoS One found Bsal spores were killed when they came in contact with salamander skin slime. Technically and charmingly called a "mucosome," an amphibian's slime is a slippery soup of bacteria, proteins, antimicrobial peptides (little molecules produced by the host's immune system that kill pathogens), and other compounds. The study found the mucosomes of some species were better at killing Bsal than those of others, which the authors attribute to the presence of certain bioactive proteins.

"Our results indicate that mucosomes of salamanders might provide crucial skin protection against Bsal, and could explain why some species are more susceptible than others," they write.

The UT team also hints that mucosomes may have played a role in the Bsal responses of different populations of eastern newts: "Interestingly, mucosome inactivation of Bsal zoospores by Michigan newts was lower than Tennessee newts."

There is still much to figure out. But there's still time to do so. The fact that Bsal hasn't yet been detected in the U.S. gives Longo hope that she and other researchers will be able to learn more and formulate a salamander-saving game plan before it gets here.

"We are in a very good position right now," Longo said. "The fungus has not yet arrived the United States or North America that we know of, so if we can predict which species need [conservation] priority—which species are more susceptible—then we can redirect resources to concentrate our efforts on these species that really need protection.

"It allows us to develop better strategies against the fungus."

This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.

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