Emerging diseases pose a growing threat to wildlife worldwide.
Victims of a fungal disease called chytridiomycosis, some of the world’s last southern mountain yellow-legged frogs lie dead in a stream in California’s Kings Canyon National Park. The disease has knocked more than 200 frog species to, or close to, extinction across the Americas. Bearing telltale signs of white-nose syndrome on their faces and wings, little brown bats (below) hibernate in a Vermont mine. First detected in New York in 2006, the disease has spread to 33 U.S. states and seven Canadian provinces, killing millions of bats.
THE SNOW OUTSIDE an old iron mine near New York’s Adirondack State Park was streaked with blood. A few little brown bats flew around the entrance; more crawled on the ground, shivering in the cold, their wing membranes and noses crusted with what looked like white powder. Hawks, ravens and raccoons had turned the scene grisly, tearing into the sick animals as they emerged from the darkness.
“At that time of year, thousands of bats should be inside, in torpor [hibernation], not flying and certainly not ending up as carnage in the snow,” says U.S. Fish and Wildlife Service (FWS) biologist Jeremy Coleman. Instead, multiple caves and abandoned mines he visited during winter 2009 were eerily empty but for tiny wing and finger bones scattered on the floor “like pine needles,” he recalls. “Tiny skulls, too. It was a gruesome sight.”
This is what the disease known as white-nose syndrome (WNS) has done to millions of North American bats. Since first reported in a New York cave in 2006, the causative fungus, Pseudogymnoascus destructans—an invader from Eurasia—has spread the illness to 32 eastern and midwestern states as well as Washington and seven Canadian provinces, infecting at least 11 bat species. Lacking resistance to the nonnative pathogen, diseased bats exit hibernation too soon for still-unexplained reasons.
Bats are far from the only wildlife succumbing to emerging diseases around the world. Across the Americas, a disease called chytridiomycosis (caused by the chytrid fungus Batrachochytrium dendrobatidis, or Bd) has knocked more than 200 frog species to, or close to, extinction while threatening hundreds more. A related fungus, Bsal (for B. salamandrivorans), soon could doom salamanders throughout the United States. In Australia, a contagious cancer has killed 80 percent of Tasmanian devils. Elsewhere, snakes are suffering from a face-disfiguring infection, armadillos are contracting leprosy, birds are embattled by bacteria that cause cholera and botulism, and canine distemper now infects not just domestic and wild dogs but also foxes, bears, raccoons, big cats and even monkeys and seals.
Disease-causing pathogens are, of course, natural components of ecosystems that help keep populations in check—weeding out the weak, selecting for the strong and driving new adaptations. But in recent decades, the wild has gotten sicker, with more new diseases emerging and more old pathogens ransacking populations that previously were out of reach. Though historic data are lacking, “it seems that wildlife is being hit harder by disease than ever before,” says Chris Walzer, executive director of wildlife health for the Wildlife Conservation Society. Indeed, a 2015 U.S. Geological Survey (USGS) report says the emergence of new pathogens and spread of known pathogens since the 1990s have been “unprecedented.”
With many victims also facing habitat loss, climate change and other perils, “emerging diseases are a growing threat to America’s already stressed wildlife populations,” says National Wildlife Federation Chief Scientist Bruce Stein. “These new pathogens are pushing more and more of our native wildlife species toward extinction.”
Historically afflicting mainly plants, “fungi seem to be at the forefront of wildlife disease right now,” says microbiologist David Blehert of the USGS National Wildlife Health Center. While viruses, parasites and bacteria also are important pathogens, the newest and worst diseases of frogs, salamanders, snakes, bats and even trees are fungal. Fungi have a sort of leg up: “It is detrimental for a virus, for instance, to drive its host to extinction,” because it needs that organism to survive, Blehert says. “A fungal pathogen doesn’t have that constraint, as it can live quietly in the environment degrading organic matter” while waiting for a host. The fungus that causes WNS, for example, may lurk in soils and on cave walls for months before infecting a bat.
Context is also important. The fungus now killing North American snakes likely infected the reptiles historically, but only recently have these infections turned deadly. “Complex changing environmental conditions—such as cooler, wetter springs in the U.S. Northeast—may be limiting snakes’ ability to recover from fungal infections that begin during winter hibernation,” Blehert suggests.
Long before snakes turned up sick, amphibians, mainly frogs, were dying in droves. Bd kills these animals by infecting and thickening the skin, thereby reducing cutaneous respiration as well as water and electrolyte absorption. “It is, as far as we know, the most devastating wildlife pathogen that has emerged in the world,” says disease ecologist Matt Gray of the University of Tennessee. Scientists have raced to understand the fungus as it has trounced population after population. Particularly hard hit have been South and Central American harlequin frogs: Since the 1990s, three of 97 known species have gone extinct, with another 82 species declining to critical levels.
That makes Bsal, a salamander-loving strain of chytrid not yet detected in this country, particularly worrisome. Native to Asia, the fungus already has reached Europe via the pet trade. Because China is the primary source of U.S. pet salamanders, Bsal’s arrival in the United States—where Appalachian and southeastern states are global salamander diversity hotspots—may be just a matter of time.
Scientists say Bsal’s impact would reach well beyond the amphibians themselves. “Let’s say Bsal knocked out common woodland salamanders,” says James Madison University ecologist Reid Harris. “Leaf-shredding insects [salamander prey] would shoot up in abundance.” With more leaf litter come more microorganisms working to decompose leaves. “What you get is a big carbon dioxide increase as microbe populations explode.” In addition, “salamanders eat a tremendous number of soil invertebrates ... so they act as an energy store. What happens if that goes away? There could be significant ecosystem effects.”
Some ecosystems already have been upended by disease. Along the Pacific Northwest coast, sea star wasting disease (SSWD) has in some areas killed up to 84 percent of Pisaster ochraceus sea stars, key intertidal predators. The result has been a cascade of ecological changes. “Mussels and barnacles [sea star prey] have increased dramatically, overtaking space from creatures like sea anemones, snails, urchins, kelp and other algae,” says Oregon State University marine ecologist Sarah Gravem.
Elsewhere off the West Coast, SSWD has virtually wiped out sunflower stars, which used to “carpet the sea floor,” Gravem says. These sometimes-hula-hoop-size animals prey heavily on sea urchins, which eat kelp. “Without the stars,” she says, “there’s been an explosion of urchins and decimation of kelp beds,” which provide vital habitat for creatures ranging from seals to economically important abalone and rockfish.
Why is so much wildlife in such poor health? For one thing, “there’s global movement of everything now,” says Priya Nanjappa, former national coordinator for Partners in Amphibian and Reptile Conservation. More travel and more trade mean more microbial hitchhiking—and for the most part these potentially harmful hitchhikers receive little regulatory oversight. In the United States, for example, imported livestock and other agricultural products must meet federal entry requirements to prevent the accidental introduction of pests and pathogens. But unless they’ve been singled out for disease agents that could be transmitted to humans or livestock, wild-animal imports receive no similar scrutiny.
Scientists are particularly worried about the pet trade. “There’s no testing [for pathogens] of animals coming through the aquarium trade, nor of the millions of imported amphibians and reptiles,” says University of Maryland biologist Karen Lips. In response to Bsal’s threat, FWS in 2016 imposed a temporary ban on imports and in-country movement without permits of 201 salamander species. But with hundreds more unregulated species, experts like Lips say the pathogen’s invasion still seems inevitable.
Beyond better opportunities for disease agents to arrive, “a rapidly changing climate is boosting where and when these organisms can establish and spread,” says Stein. In Hawai‘i, for example, warming temperatures have helped invasive mosquitoes that spread avian malaria (caused by an invasive pathogen) thrive at higher elevations, where the insects once were excluded by cool weather. The disease has particularly hurt the islands’ already beleaguered honeycreepers. According to USGS microbiologist Carter Atkinson, who studies mosquito-borne diseases in Hawaiian forest birds, more than half the state’s original honeycreeper species are extinct, and another 11 species are critically endangered. Unless disease can be curtailed, she adds, “we could lose all our remaining threatened and endangered honeycreeper species by midcentury.”
In many regions, the close proximity of domestic and wild animals also gives pathogens easy paths to new hosts. Of grave concern across North America, for instance, is chronic wasting disease (CWD), a deadly neurologic disorder of deer and other cervids—caused by misfolded proteins called prions—that has jumped between captive deer and wild deer, elk and moose. First detected in 1967 in research mule deer herds in Colorado, the disease by 2018 had spread to at least 23 states and two Canadian provinces. Wildlife managers are scrambling to get ahead of CWD in places like Wyoming, whose economy relies heavily on mule deer and elk—favorites of hunters who spend more than $220 million a year in the state.
A handful of fortunate species seem to be evolving resistance to emerging diseases. Some new generations of Pisaster sea stars, for example, are resistant to SSWD. “The genetics have changed just within two years,” Gravem says. Likewise, scientists have observed signs of disease resistance at the genetic level among Tasmanian devils and Central American harlequin frogs. On the island of Hawai‘i, one population of ‘amakihi honeycreepers appears tolerant to the avian malaria that kills its kin.
But wild animals still need help from human hands—and are beginning to get it. To combat deadly plague that has ravaged white-tailed prairie dogs and the endangered black-footed ferrets that eat them, USGS researchers reported in 2017 that they’ve developed an edible vaccine (delivered using peanut-butter-flavored bait) that increases survival rates of plague-exposed animals. The same team now is developing an edible vaccine they hope will be equally effective against WNS in bats. More promising in the short term, other scientists have found that quick doses of UV light can kill the fungus outright. To encourage more such efforts, the National Fish and Wildlife Foundation recently announced $1.1 million in new funding for bat-health research.
In a move to safeguard salamanders, meanwhile, federal wildlife managers have prepared a Bsal rapid-response plan with potential infection scenarios and recommended actions for state and local agencies. Recent research suggests that probiotic bacteria, fungi or proteins in the environment can hinder or kill the fungus. “Understanding these natural protections could lead to new methods of inoculating animals,” says Nanjappa, who worked on the plan. Ideally, “we can treat salamanders ahead of outbreaks and allow them to persist despite a Bsal introduction.”
Boosting such encouraging prospects, Blehert notes that advances in genetic sequencing and analysis now allow scientists to more easily detect and monitor emerging diseases. Thanks to such technologies, it took only months to describe the WNS fungus and, more recently, Bsal, he points out. In contrast, Bd was likely infecting frogs for years before it was identified. “We are entering a new phase,” Blehert adds hopefully, “one that could lead us into a golden age for wildlife disease management.”
Because a quick response to emerging wildlife diseases is key to limiting their spread, the National Wildlife Federation and its partners are advocating for new federal legislation, including the Wildlife Disease Emergency Act, that would provide government the authority to declare and respond to new wildlife diseases as rapidly as it now can to new human or livestock diseases. To learn more, see www.nwf.org/WildlifeDisease.
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