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A pelican that likely died from avian influenza on a beach in Lima, Peru, on December 1, 2022.
Ernesto Benavides/AFP via Getty Images

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A frightening virus is killing a massive number of wild birds

Scientists have never seen anything like it.

Benji Jones is a senior environmental reporter at Vox, covering biodiversity loss and climate change. Before joining Vox, he was a senior energy reporter at Insider. Benji previously worked as a wildlife researcher.

In the past two years, a viral disease has swept across much of the planet — not Covid but a type of avian flu. It’s devastated the poultry industry in the US, Europe, and elsewhere, sickening millions of farmed birds, which either die from infection or are killed by farmers seeking to stem the spread.

The poultry outbreak has become an animal welfare crisis. It’s also one reason eggs have become so expensive; there are simply fewer hens to lay them.

But the virus is causing another major crisis that’s drawn far less attention: the death of wild birds.

The ongoing outbreak of avian flu has killed hundreds of thousands — if not millions — of wild birds, including endangered species like the California condor. It’s one of the worst wildlife disease outbreaks in history. Having now spread across five continents and hundreds of wildlife species, scientists call the current outbreak a panzootic, meaning a pandemic among animals.

“What we’re seeing right now is uncharted territory,” said Andrew Ramey, a wildlife geneticist at the US Geological Survey, one of the federal agencies involved in testing wild birds for the virus.

A flock of snow geese in flight at Bosque del Apache wildlife refuge in New Mexico. Thousands of snow geese have been affected by avian flu.
Vicki Jauron/Getty Images

The number of dead birds in itself is historic, but so is the virus’s biology. Typically, avian influenza viruses only cause severe disease and death in domestic birds like chickens and farmed ducks; they sweep through populations, killing upward of 90 percent of the flock.

This virus, however, is different. It’s hammering wild birds and other wildlife, including mammals.

“It’s causing a high amount of mortality in a huge breadth of wild birds, which is not something that has been seen before,” said Wendy Puryear, a molecular virologist at Tufts University who studies viral evolution.

This is especially concerning because birds are already at risk across the world. North America alone has lost an astonishing 3 billion breeding birds in the last half-century, due to threats like climate change, predation by feral and pet cats, and the loss of grasslands and other habitats. This panzootic is only making an ongoing extinction crisis worse.

The virus could also pose a threat to us. While it doesn’t readily infect and spread among people today, the avian virus could evolve traits that make it more dangerous to humans as it circulates among wild animals. That’s another reason scientists are taking the outbreak among wild birds so seriously.

An unusual avian flu

Viruses that cause avian flu are actually pretty common. They’ve been circulating for eons among wild birds — and especially waterfowl, such as ducks and geese — without causing them much harm. These mild forms of infection are called low-pathogenic avian influenza, or LPAI, which means they’re typically not deadly.

On occasion, a low-pathogenic virus can jump from wild birds to poultry farms. As the virus replicates in densely packed warehouses of farmed birds, it can quickly evolve and pick up adaptations that make it highly deadly to poultry. At that point, it gets dubbed a highly pathogenic avian influenza virus, or HPAI virus. Historically, however, most of these HPAI viruses haven’t killed large numbers of wild birds, even if they did spill out of the farm and back into wild populations.

Then came an avian flu outbreak on a goose farm in China.

A view from above of a farmer feeding more than 6,000 geese at a farm in Jiangsu province, China. Dense farms of waterfowl are especially susceptible to avian flu.
Zhang Lianhua/VCG via Getty Images

In the spring of 1996, influenza caused by a virus known as H5N1 (named for the kinds of proteins found on its surface) spread among the geese. It was highly pathogenic and killed more than 40 percent of the farm birds it infected.

Descendants of this virus have since triggered a new era for bird flu. They’re not only adapted to spread disease among poultry, but — and this is key — some varieties are also capable of spreading and causing severe disease among wild birds. That’s an important trait that separates this virus from past versions of avian flu.

The US first experienced one of these goose farm viruses in 2014. After spreading to North America for the first time, the virus killed or affected tens of millions of poultry and an unknown number of wild birds, across at least 13 US states. At the time, officials were able to control the outbreak by slaughtering a huge number of farm birds.

The situation today is more dire — and much harder to control.

How bad is the current outbreak?

The world now faces a more frightening version of this goose farm virus that appears better equipped to infect wild birds. First detected on North American soil in the winter of 2021, the virus, which is also a form of H5N1, has since spread throughout the US, into Mexico, and down through Central and South America. It’s infecting birds on every continent now other than Antarctica and Australia (where it almost certainly will arrive soon).

The current outbreak has killed — or forced farmers to cull — more than half a billion poultry worldwide, a simply mind-boggling number of birds.

It’s much harder to estimate the toll the virus has taken on wild birds.

Two dead northern gannets in eastern Canada, on July 8, 2022. They likely died from avian flu.
Lewnanny Richardson/iNaturalist

In the US, suspected or confirmed cases of H5N1 in wild birds are in the tens of thousands, according to a study published in April. Reporting by the Guardian revealed that the flu has killed more than 50,000 birds in the UK. In Eastern Canada alone, roughly 40,000 birds have been reported as sick or dead, likely linked to the flu, according to Stephanie Avery-Gomm, a research scientist with the Canadian government, who cited her preliminary, unpublished research.

Yet most of these numbers are almost certainly underestimates. Government agencies don’t have the resources to test every dead bird. Plus, many individuals die out at sea, or in rural areas that lack any kind of surveillance.

Testing in the US tends to focus only on a small number of avian species, or on birds that die en masse, according to Johanna Harvey, a postdoctoral researcher at the University of Maryland who studies avian influenza. That leaves out smaller-scale outbreaks. Plus, global reporting of H5N1 cases in wild birds is often inconsistent or incomplete, according to a preprint paper published earlier this month.

This suggests that “only a fraction of outbreaks in wildlife have been detected and appropriately reported,” scientists Marcel Klaassen and Michelle Wille wrote in the paper, which has yet to go through the formal peer review process.

A map showing where the H5N1 virus, a highly pathogenic type of avian influenza, has been detected in poultry and wild birds, between December 2021 and March 2023.
Johanna A. Harvey, et al./Biological Conservation

So how big is the number really? It’s likely in the millions globally, according to the paper by Klaassen and Wille. Scientists may never have an exact toll, but the spotty numbers they do have are ringing alarm bells.

“We haven’t seen these kinds of numbers with an influenza outbreak in wild birds previously, ever,” said Puryear, of Tufts, who was not involved in that study.

What avian flu means for biodiversity

The virus that’s killing birds today is highly “promiscuous,” Puryear said — meaning, it’s infecting and causing disease in all kinds of species. Scientists have found it in everything from vultures and bald eagles to American white pelicans and snowy owls.

Birds that nest in colonies have been hit especially hard. These include things like snow geese, terns, and double-crested cormorants. According to Avery-Gomm, more than 20,000 of the potential cases of influenza in eastern Canada came from northern gannets — giant, colonial seabirds that spend most of their lives on the ocean.

Scientists fear these die-offs could make a big dent in some avian populations, especially in ones that are already small.

A pair of endangered California condors soar overhead in California.
US Fish and Wildlife Service

Since March, more than 20 California condors in the Southwest have died, and most of them tested positive for avian flu. The largest bird in North America, with a wingspan that can reach more than 9 feet, the California condor almost went extinct in the 1980s. A successful captive breeding program revived the population, which now stands at roughly 500 worldwide, though they’re still listed by the US government as endangered.

Last summer, meanwhile, bird flu knocked out more than half of Lake Michigan’s population of Caspian terns, another threatened species.

“Large die-offs can impact populations of these species for decades and may contribute to species collapse and further ecosystem damage, particularly given the critical declines seen in North American bird biodiversity over the last half century,” scientists wrote in the April study, which tallied reported and suspected cases of H5N1 in North American birds.

Indeed, prior to the current outbreak, birds were already declining in nearly all habitats in the US, according to Cornell University’s State of the Birds report. Roughly half of all bird species are known (or likely) to be in decline globally.

More troubling still is that avian influenza is also killing many mammals, including foxes, coyotes, mink, and seals. Earlier this year, officials reported that bird flu killed almost 3,500 seal lions in Peru. That’s worrying on a whole different level — because humans are mammals. Could this avian flu become a pandemic?

The frightening link between infected wild birds and human health

No, in its current form, avian influenza is not at all likely to cause a pandemic. While hundreds of humans have contracted H5N1 over the years — and many of them have died — those cases usually involve extremely high exposure to infected poultry. Biologically speaking, the virus isn’t well equipped to overtake our immune systems and spread quickly among human populations.

The problem for us is that viruses, and especially influenza viruses, evolve quickly. Not only do they mutate, but they can also swap entire portions of their genomes with other viruses if they infect the same hosts. Under the right circumstances, this evolution could give them the tools to replicate more easily in mammals, which would make them more threatening to humans. (My colleague Keren Landman and I go into this in detail here.)

The risk of the virus morphing into a human threat remain slim, yet the outbreak in wild birds may push it in that direction.

During most past outbreaks, only poultry farms were badly infected, so countries could kill giant flocks of infected farm birds and exercise other biosecurity measures to stem the spread. That’s what happened during the outbreak in 2014 and 2015. In this case, however, wild birds are also a reservoir for highly pathogenic influenza. So no matter how much culling farmers do, wild birds could still pass H5N1 over to domestic populations.

This is happening already: Most recent outbreaks on farms were started by wild birds, not farm-to-farm spread, Yuko Sato, a poultry veterinarian at Iowa State University, said in a briefing for reporters last month. Infected wild birds can spread the virus through their feces or breath when they flock to reservoirs near farms, or stop over while migrating. One reason wild birds are likely to enter farms in the first place is that so much of their natural habitat has been destroyed.

“The challenge is that you can do all this work to make farms more secure, but that won’t matter if you have lots of infected wild birds,” said Nichola Hill, an infectious disease ecologist at the University of Massachusetts Boston.

Once wild birds contaminate farms, the risks skyrocket. Viruses evolve much more quickly within animals that are packed tightly together, simply because they have more bodies to grow in. They’re also known to pick up adaptations that make them better equipped to replicate among mammals, Puryear said, though it’s not clear why.

The other problem is that when flu viruses are widespread among wild birds, they have more opportunities to spill over directly into mammals. The virus has infected animals like foxes and seals that interact with birds. And as these microbes replicate within their cells, the flu viruses can pick up traits that make them more harmful to humans.

This is not just theoretical. In a recent study, Puryear found that some H5N1 viruses that infected seals in New England had genetic changes that have been shown to make them more efficient at replicating within the cells of mammals. Researchers have found similar adaptations in H5N1 viruses found in foxes and mink. All of these animals likely got sick from wild birds.

A harbor seal basking on the beach in Massachusetts’s Salisbury beach state park.
Ken Canning/Getty Images
A red fox with part of a chicken poking out of its mouth.
Getty Images/iStockphoto

There is some reassuring news, however: Not only is the virus biologically ill-equipped to cause a pandemic, but the US government has also stockpiled vaccines, including those specifically for H5N1. (There are also vaccines for birds, which my colleague Kenny Torrella writes about here.)

“It’s not Covid,” Puryear said, referencing how there were no coronavirus vaccines stockpiled when the pandemic hit. “In theory, we should be able to respond quickly if this becomes an issue.”

What to do for the birds

In time, wild birds will likely develop some immunity to the current H5N1 virus, causing the panzootic to wane. It’s not clear how long that will take, Puryear said, because “this scale of HPAI in wild birds hasn’t occurred before.”

But there are ways to help wild birds short of waiting for them to gain natural immunity. Countries including the US, for example, could ramp up surveillance, so they can better understand how avian flu is spreading, scientists say. That information could help give farmers a heads-up if infected species are moving their way.

In the US, the Department of Agriculture (USDA) and USGS lead surveillance, together with state agencies. Those agencies do most of the field-based sampling, Hill said. According to Puryear, however, government officials simply don’t have the resources or funding to keep up with the outbreak. “They are slammed,” she said.

Part of the problem, I’m told, is that USDA is primarily focused on protecting the poultry industry. Controlling outbreaks on farms draws a lot of the agency’s resources that could otherwise go toward wild birds.

“Wild bird surveillance is a critical part of USDA’s response to highly pathogenic avian influenza,” USDA spokesperson Lyndsay Cole told Vox. The agency and its partners tested more than 30,000 wild birds last year, Cole said. The agency also emphasized that studying the virus among wild birds is important because it can spread between wild and domestic populations. “While there is always room to expand efforts, the surveillance that has been done has been invaluable in identifying how the virus is spreading and where introductions to domestic birds occur,” Cole said.

Academic institutions like Tufts and the University of Georgia, which both do testing, help fill in some of the gaps in surveillance, but those projects cost money, Puryear said. On that note, one solution is to free up more federal funding for groups that are equipped to surveil and test wild birds for H5N1, she said. Surveillance would also benefit from government and state agencies talking to each other more, many scientists told me. (Cole of USDA said the agency is able to “coordinate closely” with its partners to carry out surveillance.)

The public can help, too, Hill said.

Birders are “the eyes and ears of this panzootic,” she said, “and I don’t think they’re being harnessed enough.” People are used to photographing beautiful wildlife on their phones and uploading it to platforms like iNaturalist. But right now, it’s more important that they document dead birds, she said. (iNaturalist actually has a webpage exactly for this purpose. If you find a dead wild bird and are not sure what to do, check out this one-pager from the USDA.)

Ultimately, protecting wild birds and reducing the risk of a pandemic will require that we make much bigger changes, such as to our food system. The normal way many companies raise birds for slaughter — in warehouses, packed tightly with chickens or turkeys — is a recipe for highly pathogenic viruses, Hill said.

“It’s useful to remember that wild birds are the victims here,” Hill said. “They spread HPAI but are not the original source. My motto has become: Bird flu sucks, blame chicken nuggets.”

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