What do the northern water snake, little brown bat, fire salamander, whitebark pine, and Panamanian golden frog have in common? Not much, it would seem. They hail from vastly different taxa; ditto for ecosystems. One slithers. One flies. One crawls. One sends out roots, and another hops. But they share at least one common—and deadly—denominator: Fungal diseases have recently decimated their numbers.
Lethal fungi have always been around, but their impact appears to be growing. Recently, scourges of sinister spores have been going on killing sprees, spreading quickly and wiping out local populations.
Take white-nose syndrome: Since it was first reported in New York State in 2006, the disease has killed more than six million bats across twenty-nine states and five Canadian provinces. In March, the fungus at fault showed up in Washington for the first time; this means its reach somehow managed to jump 1,300 miles. Batrachochytrium dendrobatidis, known as Bd or frog chytrid, is probably the most well-known fatal fungus. This pathogen affects more than 500 amphibian species across six continents, and scientists have called chytridiomycosis, the disease it causes, “the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted, and its propensity to drive them to extinction.”
In response to the unprecedented bat and frog population declines, a cross-disciplinary team of epidemiologists, ecologists, and plant pathologists got together a few years ago to determine whether a quantifiable upward trend in deadly fungal diseases is, in fact, occurring, or if we’re simply paying more attention to fungi as a result of a handful of high-profile cases. Their analysis of disease-monitoring programs and peer-reviewed studies, published in Nature in 2012, concluded that “fungi pose a greater threat to plant and animal biodiversity relative to other taxonomic classes of pathogen and hosts, and that this threat is increasing.” In other words: Move over, viruses and bacteria; the wrath of fungi is on the rise.
The authors note that improved awareness and diagnostic capabilities probably do contribute to the perceived fungal threat to some extent. Still, the evidence points to a legitimate trend. So what's changing?
Scientists have a few ideas. First, the world is more interconnected now than at any point in history—we trade and transport plants, wildlife, pets, and ourselves to an unprecedented degree. The number of live plants imported to the United States, for example, increased 33 percent per decade between 1967 and 2009. All this globetrotting gives fungi more opportunities to find their way into new ecosystems and hosts, and as they do, they come across species with little to no defenses against them.
The white-nose syndrome epidemic in North America may be the product of one such introduction. European bats contract the fungus as well, but with one important difference: They don’t get sick. Scientists suspect that the fungus and bats overseas have been evolving in one another’s company for years, rendering the relationship benign. But for North American bats, which had never been exposed to it until the 2000s, contracting Pseudogymnoascus destructans is usually a death sentence (destructans indeed). Given that bats don’t make trans-Atlantic flights, it’s likely that people brought the fungus stateside. Spores could have been stuck on a caver’s gear, for example, unbeknownst to the spelunker.
Batrachochytrium salamandrivorans (Bsal), a newly described fungus closely related to frog chytrid, seems to have found its way to Western Europe via the global pet trade. Bsal infects—but doesn’t kill—Asian newts, but up to 96 percent of infected fire salamanders in the Netherlands have died. Chytrid, too, is linked to international trade in amphibians. A 2009 study found that between 2000 and 2005, the three ports of Los Angeles, San Francisco, and New York imported 28 million live amphibians, and 62 percent of the imported frogs were carrying the offending fungus.
Daniel Henk, a fungal ecologist at the University of Bath in the United Kingdom, says it was only a matter of time before the effects of our increasingly dispersive lifestyle started to catch up with us. “These things sort of percolate in the background for a while, and then suddenly you realize: Oh, your frogs are extinct. That’s a problem,” he says.
In humans, many fungal infections are opportunistic, sickening people with weakened immune systems, like those living with HIV/AIDS. Similarly, fungal pathogens frequently strike species that are already stressed out by habitat degradation, climate change, or other diseases.
Aside from the external factors, fungi inherently possess a number of characteristics that make them particularly good bad guys. They reproduce rapidly, evolve quickly, linger in the environment for years, and infect a wide range of hosts. Henk says pathogens are typically limited in the amount of damage they do to one host by the need to make it to the next one. Kill your host off before it can help you infect another, and your outbreak comes to a dead end. Not so with fungi. Because they multiply fast and persist in the environment without hosts, whether the hosts live or die does not affect the fungi’s ultimate success. They can kill quickly without consequence.
The combination of these qualities makes fungi all but impossible to stamp out, but that hasn’t kept biologists from trying. Research is underway on an oral vaccine for white-nose syndrome—a strategy that has been effective for controlling rabies in wild carnivores and is currently being field-tested for plague in black-footed ferrets. With chytrid, biologists have had varying degrees of success with probiotic treatments, fungicides, and captive breeding.
“There is no magic bullet,” says David Blehert, branch chief of the Wildlife Disease Diagnostic Laboratories at the National Wildlife Health Center in Madison, Wisconsin. Each approach requires significant resources and time—time that threatened species may not have.
Still, an ounce of prevention is worth a pound of cure, and there are some signs of progress on that front. White-nose syndrome had already been in the United States for about three years before scientists fingered Pseudogymnoascus destructans as the culprit, says Blehert, who helped identify the pathogen in 2008. “By the time we discovered it,” he says, “it was likely too late to enact any sort of management to halt it in its tracks.” Fortunately, that experience was not lost on the scientific community, and the order of operations has been markedly different for dealing with Bsal. “For the first time that I’ve seen in my career,” Blehert says, “we’re actually doing a national surveillance program in the United States for Bsal before the disease is known to exist in North America.”
The continent is home to nearly 50 percent of the world’s salamander species, so Bsal’s introduction—which many wildlife managers fear is inevitable—would have a devastating impact on global salamander biodiversity. A task force of scientists, conservationists, and government representatives has been scrambling to prepare for the eventuality since June 2015. This past January, the U.S. Fish and Wildlife Service banned the import and interstate trade of 201 salamander species in an effort to keep the fungus at bay. The U.S. Geological Survey recently identified the regions most at risk of a Bsal outbreak—the Pacific Coast, the southern Appalachian Mountains, and the mid-Atlantic—so that wildlife managers can focus their efforts on those areas.
Reid Harris, director of international disease mitigation for the Amphibian Survival Alliance, is encouraged by the swift action on Bsal. “There’s a plan in place; there’s diagnostic procedures,” he says. “It’s still a little unclear what containment actions would be taken, but people are at least thinking about what the best options might be.”
In our response to dealing with the emergence of these deadly diseases, our best hope may be to imitate the very pathogens that are wreaking so much havoc: Evolve rapidly in our strategies, and spread the message into as many vulnerable ecosystems as possible.
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