Katherine J. Wu
February 15, 2023
Eagles Are Falling, Bears Are Going Blind
It was late fall of 2022 when David Stallknecht heard that bodies were raining from the sky.
Stallknecht, a wildlife biologist at the University of Georgia, was already fearing the worst. For months, wood ducks had been washing up on shorelines; black vultures had been teetering out of tree tops. But now thousands of ghostly white snow-goose carcasses were strewn across agricultural fields in Louisiana, Missouri, and Arkansas. The birds had tried to take flight, only to plunge back to the ground. “People were saying they were literally dropping down dead,” Stallknecht told me. Even before he and his team began testing specimens in the lab, they suspected they knew what they would find: yet another crop of casualties from the deadly strain of avian influenza that had been tearing across North America for roughly a year.
Months later, the bird-flu outbreak continues to rage. An estimated 58.4 million domestic birds have died in the United States alone. Farms with known outbreaks have had to cull their chickens en masse, sending the cost of eggs soaring; zoos have herded their birds indoors to shield them from encounters with infected waterfowl. The virus has been steadily trickling into mammalian populations—foxes, bears, mink, whales, seals—on both land and sea, fueling fears that humans could be next. Scientists maintain that the risk of sustained spread among people is very low, but each additional detection of the virus in something warm-blooded and furry hints that the virus is improving its ability to infiltrate new hosts. “Every time that happens, it’s another chance for that virus to make the changes that it needs,” says Richard Webby, a virologist at St. Jude Children’s Research Hospital. “Right now, this virus is a kid in a candy store.”
A human epidemic, though, remains a gloomy forecast that may not come to pass. In the meantime, the outbreak has already been larger, faster-moving, and more devastating to North America’s wildlife than any other in recorded history, and has not yet shown signs of stopping. “I would use just one word to describe it: unprecedented,” says Shayan Sharif, an avian immunologist at Ontario Veterinary College. “We have never seen anything like this before.” This strain of bird flu is unlikely to be our next pandemic. But a flu pandemic has already begun for countless other creatures—and it could alter North America’s biodiversity for good.
Deadly strains of avian flu have been ferried onto North American shores multiple times before, and rapidly petered out. That was the case in 2014, when a highly virulent version of the virus crossed the Pacific from Asia and invaded U.S. poultry farms, forcing workers to exterminate millions of chickens and turkeys. The brutal interventions worked: “They did all the right things, and nipped it in the bud,” says Nicole Nemeth, a veterinary pathologist at the University of Georgia. Hardly any wild birds were affected; egg prices bumped up briefly, then settled back roughly to baseline. “It just kind of died down,” Nemeth told me. “And everyone breathed a sigh of relief.”
This time, though, the dynamics are different. The epidemic, which first erupted in Europe in the fall of 2021, appears to have crossed the Atlantic into Canada, then zigzagged down into the U.S. around the start of last year. American scientists have detected the virus in more than 150 wild and domestic avian species and at least a dozen different types of mammals. It’s by far the longest and most diverse list of victims the virus has ever claimed on this side of the world.
Some birds are likely to make it through the outbreak just fine. For as long as humans have been aware of this particular strain of avian influenza, scientifically classed as H5N1, the virus has been relatively gentle on certain types of waterfowl, especially mallards, pintails, and other so-called dabbling ducks. “Some populations have a prevalence as high as 40 percent, and they’re as normal as normal could be,” Stallknecht said. Those same species have also been some of avian flu’s best chauffeurs in the past, silently spreading the feces-borne infection across countries and continents during their seasonal migrations.
Others haven’t been so lucky. Some of the same respiratory issues that strike humans who have the flu—sneezing, coughing, pneumonia when the disease gets severe—can hit birds, too. But across a variety of susceptible species, necropsies show more extensive damage, with evidence of virus in various organs, including the liver, gut, and brain. The neurologic problems can be among the worst: Swans might swim in listless circles; geese might waddle shakily onto shores, their necks twisted and turned; eagles might flap defeatedly from their perches, unable to launch themselves into the air. Michelle Hawkins, a veterinarian at UC Davis, told me that several of the red-tailed hawks she’s treated in her clinic have arrived with their eyes shaking so vigorously from side to side that the spasms turn the animals’ head; others appear to gaze off into nothingness, unresponsive even when humans approach.
Death can come swiftly—sometimes within a day or two of the infection’s start. Birds simply keel over as if they’ve been shot, their bodies dropping like rocks. On poultry farms, outbreaks can wipe out entire flocks in just two or three days. In wilder settings, locals have spotted bald eagles plummeting out of their nests, leaving shrieking chicks behind. By the time infected birds reach Hawkins in her clinic, “they’re usually almost dead,” she told me. “And we can’t figure out how to help them except to put them out of their misery.” Hawkins estimated that in the past few months alone, her team’s euthanasia rate has gone up by about 50 percent.
Mammals so far haven’t fared much better. Last spring, the corpse of a dolphin infected with the virus was found wedged into a canal in Florida—around the same time that Wisconsin locals happened upon litters of ailing fox kits, drooling, twitching, and struggling to stand in the hours before they seized and died. In the fall, three young, flu-stricken grizzlies in Montana were euthanized after researchers noticed that the disoriented animals had begun to go blind. Wendy Puryear, a molecular virologist at Tufts University, told me that seals sick with the virus will sometimes convulse so badly that they can barely hold their bodies straight. Every seal she’s seen that tested positive has ended up dead within days. This month, researchers in Peru reported that they were picking up the virus in some of the sea lions that have died by the hundreds along the country’s coast; a similar situation may now be playing out among a number of Scotland’s seals.
It’s hard to say why this outbreak is so much worse than the ones that came before. Microbial evolution may be one culprit: Flu viruses are particularly inclined to tweak their RNA code; when two genetically distinct versions of the pathogens wind up in the same cell, they can also swap bits of their genomes. This iteration of H5N1 may be particularly adept at sparking lethal disease—something Justin Brown, a veterinary pathologist at Penn State, thinks is quite likely, given how many animals have died. It may also be more easily exiting birds’ bodies in feces, or more efficiently entering cells in the airway or gut. “This particular virus seems to be better adapted to wild birds. I think that’s the key thing,” Stallknecht told me. As climate change alters migration schedules, and pushes certain avian species into more frequent contact with one another’s contaminated scat, the risks of intermingling are only growing. The greater the number of infections, the more animals will die. “It becomes a numbers game,” Stallknecht said.
Flu viruses have never had much trouble spreading: They can be breathed out or defecated; they can persist on surfaces for hours, and in cool waters for days. But Webby suspects that the ballooning of this epidemic can be at least partly blamed on the severity of disease. “The easiest birds to catch are the ones that are sick,” he told me. Hawks, eagles, owls, and other predatory birds may be stumbling across dying ducks and eating them, unwittingly infecting themselves. Nemeth thinks that certain species, including black vultures, are now cutting out the middlebird and feasting on the carcasses of their own kin as they continue to die in droves. “They see dead tissue, they’re going to eat it,” she told me. It’s a morbid tragedy of abundance, as the virus climbs the food chain to reach species it hasn’t easily accessed before. “The biggest impact is on these atypical hosts,” Webby told me, which lack the prior exposures to the virus that might have helped protect them.
Predation or scavenging of sick or dead birds is probably how certain mammals—grizzlies, foxes, opossums, and the like—are catching the virus too. The seals and dolphins present a bit more of a puzzle, Puryear told me, though it’s possible to guess at what’s at play. At least some types of seals have been documented consuming birds; other marine mammals might simply be gulping feces-infested water. A recent avian-flu outbreak at a mink farm in Spain suggests a more troubling mode of transmission: mammals repeatedly conveying the virus to one another—a possible first for H5N1. “That is really disconcerting,” Sharif told me. “It tells me the virus is adapting to mammals.”
Animals vulnerable to the virus don’t have many good options for protection. Some avian-flu vaccines have been used on certain poultry farms, mostly abroad. But some of the same issues that plague human-flu vaccines are obstacles in the bird world too, Brown told me: The ingredients of the shots aren’t always good matches for the circulating virus, and the immunizations, which may be pretty good at staving off severe disease, don’t do much to block infection or transmission, making outbreaks tough to contain. Wild birds, which can’t be corralled and immunized en masse, are essentially out of luck. Nemeth told me that some of her colleagues in Florida have been trying to clear the ground of carcasses so that they won’t become sources of infection for yet another unlucky mammal or bird. “But they just can’t keep up with the number of deaths,” she said. Essentially all avian species are thought to be susceptible to infection—and there’s simply no way to reach every bird, says Becky Poulson, an avian-flu researcher at the University of Georgia. After hopscotching across the globe for decades, H5N1 now seems very likely to be in North America for good, “part of the new normal here,” Poulson told me.
Experts told me they’re hopeful that the outbreak will abate before long. But even if that happens, some species may not live to see it. North America’s birds already face a medley of threats—chemical pollution, window collisions, habitat destruction, roving colonies of feral cats—and some of them cannot sustain another blow. “This could be the last nail in the coffin for some species,” says Min Huang, who leads the migratory-bird program at Connecticut’s Department of Energy and Environmental Protection.
The current epidemic “may not be directly affecting us, in that almost none of us are getting sick,” says Kishana Taylor, a virologist at Rutgers University. But the extent of its reach into wildlife means that humans will still notice its many impacts. In a world with fewer birds, other animals—such as coyotes, snakes, and even humans—might go hungrier, while the fish, insects, and rats that birds eat could experience population booms. The treetops and shorelines, once alive with song, could go silent; far fewer seeds might be dispersed. The U.S.’s national bird—one of the country’s few conservation success stories—could once again find itself pushed to the brink. Some locals in the Southeast have already told Nemeth that they’re feeling the absence of vultures, as roadside deer carcasses begin to fester in the sun. The bird pandemic will have its survivors. But they are likely to be living in a world that is quieter, lonelier, and harsher than it was before.