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In late 2021, Tufts University virologist Wendy Puryear began to worry. The avian flu virus was behaving oddly. It usually passes through wild birds with little harm. But in Europe, a highly pathogenic strain known as H5N1 was killing a range of avian species, such as Mute Swans in France and a White-tailed Eagle in Scotland, and infecting mammals like otters and foxes.

Then it hopped across the Atlantic. Carried long distances by migrating waterfowl, gulls, and shorebirds, within months the virus found almost every corner of North America—and eventually South America—until the situation reached “whole new levels of crazy,” Puryear says.

By now, USDA’s National Wildlife Disease Program has in nearly 150 avian species, such as Canada Geese, Brown Pelicans, Red-tailed Hawks, and Snowy Owls, and recorded thousands of dead birds, says program lead Julianna Lenoch. Meanwhile, H5N1 hit North America’s farms. More than 55 million poultry have died—some from flu and others culled to contain it—at a cost of billions of dollars. At grocery stores, egg prices skyrocketed.

Despite surging and cresting in the past year, the now-global strain is showing no signs of burning out. As a result, birds are facing their own pandemic, with unknown long-term consequences for avian health. Public health agencies are for human infections. While rare, people can contract bird flu through close contact with infected birds or contaminated surfaces. What’s more, there’s a low but real risk the strain could mutate to enable people to pass the virus to others, a scenario that’s caused  human pandemics over the last century. 

Although it snowballed recently, today’s situation has been brewing since 1996 when a garden-variety avian influenza acquired mutations on a farm that made it lethal to poultry. Over time, the virus appeared in Asia, Europe, and Africa, with an outbreak hitting North America in 2014 before fizzling. Experts think further mutations may be behind its recent ability to cause more severe disease in wild birds and infect more birds and some mammals.

To keep tabs on H5N1’s prevalance and distribution, USDA and other agencies are collecting samples from tens of thousands of wild birds—mostly dabbling ducks, which transmit the virus when they congregate (usually without getting ill). As birds return from their migration journeys this spring, scientists will be looking at whether the virus has changed genetically over the winter in ways that make it more or less dangerous, and whether it affects bird behavior during the breeding season.   

While determining the flu’s long-term effects will take more time and surveys, researchers are especially worried about its impact on vulnerable avian populations, including some colonial waterbirds, long-lived seabirds, and birds of prey. In June, for example, almost 1,500 Caspian Terns died on Lake Michigan islands—about 60 percent of Wisconsin’s population, including many breeding adults. Such losses, says Timm Harder, a veterinary virologist at Germany’s National Reference Laboratory for Avian Influenza, could leave “a deep scar” on future generations.

Bald Eagles, which can get sick after consuming infected waterfowl, also face challenges. In the United States, more than 300 Bald Eagles tested positive in 2022. A recent study reported an unusually poor eagle nesting season in avian flu hotspots in Georgia and Florida. The authors the disease may pose an “impending threat” to a species that conservationists spent decades bringing back from near extinction.

It’s a disheartening situation for bird lovers. While songbirds are mostly unaffected so far, some authorities have urged the precaution of taking bird feeders down during local outbreaks, so birds don’t gather and risk getting the flu. (People who keep backyard chickens or other domestic birds should especially avoid attracting wild avian visitors). Լ����Ƶ generally advises removing feeders if at least one local wildlife agency calls for it and regularly cleaning any bird feeders or baths in use. Also important: Don’t touch live or dead birds.

Broader efforts to rein in the virus also face uncertainty. Wild birds pass it to poultry and vice versa, and even the best measures to avoid such intermingling haven’t been foolproof. Rodents, farm equipment, or even the wind may move the virus around. Given this outbreak’s staying power, and experts are debating the use of vaccines. But giving doses to billions of farmed birds might not be feasible, Harder says, and vaccinating wild birds through bait they consume is even harder. While inoculations could target high-risk animals such as free-range chickens that interact with wild birds, incomplete measures also risk speeding the rise of new variants, says University of Missouri virologist Henry Wan.

These considerations may feel all too familiar to a world still reeling from the COVID-19 pandemic. Over the last few weeks, H5N1 has begun appearing more widely in mammals, including wild bears and sea lions and among farmed mink. Amid these troubling developments, many experts ramped up calls for more urgent measures, such as better biosecurity on farms and improved global cooperation, to avoid the risk that the virus could adapt to infect people more easily. The goal: ensure that bird flu won’t trigger another major public health crisis.

This story originally ran in the Spring 2023 issue as “Bird Flu Blazes On.” To receive our print magazine, become a member by .