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A little over a year ago, Marcela Uhart was walking on the beach in Punta Delgada, Argentina. It was peak breeding session on this peninsula known for its rich marine wildlife. Usually, the salty breeze brought with it the sounds of baby elephant seals calling to their moms in high pitched yells.

"This time it was silent," recalls Uhart. "The beaches were just loaded with carcasses. We saw basically every [elephant seal] pup dead. We estimate about 18,000 dead baby elephant seals."

And it wasn't just elephant seals. There were terns – with their yellow beaks and black heads – stumbling about having seizures on the sand. The scene played out again and again in the weeks that followed, up and down the coastline. "It was like birds falling out of the sky, dead," she says.

 at the Karen C. Drayer Wildlife Health Center at University of California, Davis. She arrived at the Punta Delgada beach suited up in a white hazmat suit, gloves and goggles in October 2023 ready to take samples and document the decimation. She's been tracking bird flu as it goes from one bird species to another — from seabirds to waterfowl to birds of prey — and then to marine mammals from sea lions to dolphins and then, sometimes, jumping back to birds.

"It's just like wildfire. I mean it just killed everything it encountered," she says, speaking to NPR from Argentina.

In the U.S., bird flu headlines have focused on an 

. But wildlife researchers like Uhart say the dramatic scene in Argentina is evidence that something new — and ominous — is going on with the virus that causes bird flu. And that ignoring it puts human health in peril.

A 'relay race' that started in 2020 and 2021

"I've been studying flu viruses closely since 1980 and there are days when I wake up feeling like I know less about flu now than I did 10 years ago," says 

, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. That's especially true, he says, of bird flu.

The particular virus that causes bird flu — 

 — itself is not new. It's a disease that originated in east Asia, first detected in China in 1996. The virus has mostly 

 and led to massive culling. It has occasionally spilled over to humans, causing some 400 deaths over the decades, but rarely spreads human-to-human. And while it has jumped over into wild birds periodically — killing many birds in many places — it never took off globally.

Then came 2020 and 2021, when the version that's driving the current outbreak emerged.

The virus evolved so some wild birds are able to migrate just far enough to reach another bird community or mammal population to pass the virus on before dying.

"What you see here is like a relay race," Uhart says. "We were all skeptical that this could be possible, but then somehow this started happening."

"We've gone from this concept of dead birds don't fly to this new virus that seems to be a bit more like dead bird flying," explains 

, head of the Virology Unit at the Institut Pasteur du Cambodge in Cambodia and the director of the National Influenza Center of Cambodia.

This means that the virus doesn't just stick around on poultry farms and periodically jump over into wildlife, but can sustain itself in wild birds, moving from one flock to the next without burning itself out.

Scientists are still trying to understand exactly how the virus spreads between animals. One theory is that scavenger birds feast on mammal carcasses "that are loaded with virus, then they get infected, obviously, and can spread it easily on their feet or beaks," says Uhart. Another theory, she says: "It could be that [animals] poop in the water and the other animals drink [that water]."

Wild birds are particularly well suited to take this virus global — and fast.

"If the natural reservoir for this virus was any other animal species than birds, we might expect to see it only located on a certain continent because animals don't swim across oceans. They don't climb over mountain ranges. The one animal species that does is birds," says Osterholm.

 and jumped over to North America in late 2021. From there, it went to South America leaving, destruction in its wake.

"In South America, it traveled the 6,000 kilometer spine in about six months," says 

, senior research fellow at the Center for Pathogen Genomics at the University of Melbourne. "So this is a virus that's not assisted by airplanes. This is a virus that's traveling by mass mortality after mass mortality after mass mortality after mass mortality." It is killing not just birds in large numbers but also mammals, like elephant seals and sea lions, as well as porpoises, dolphins and otters to a lesser degree.

This year, South America has been a lot quieter. "Surprisingly, Brazil does not have these recent cases in wild birds," says 

, a veterinary biologist at the University of São Paulo who studies avian flu. Her team has tried to determine whether the birds have acquired immunity. So far, results have been unclear. "It's very difficult to understand," she says.

Meanwhile, there's a new part of the planet at risk. Researchers are realizing the virus is now plaguing Antarctica.

They worry about this trend because Antarctica is a particularly hard place to study — and many of the animals that live there don't live elsewhere in the world. Even if there are researchers on the ground to collect samples, "you have to wait for months and months on months, until those samples actually get to a lab, potentially also in the opposite hemisphere of the world," says Uhart. And, by that time, she speculates, the virus will have already spread much further.

This adds to a broader problem: Globally, nobody knows how many wild animals the virus has killed.

"No one's counting. We have no idea," says Wille. "It is a global catastrophe"

Wildlife with the virus are bad news for humans

Researchers say the rapid spread is catastrophic for the animal species that are being hard hit. For example, in 

, thousands of Peruvian pelicans have died. "In a few weeks, almost one in two individuals of a species that just completely disappeared," says Wille. And, she adds, it's still too early to know how these mass die-offs will impact ecosystems but it's likely to be significant.

On top of that, this matters for human health.

Wille and a team of other wildlife experts estimate

 that millions of birds have been infected and died, and tens of thousands of marine mammals have died. Each animal that is infected — particularly mammals — is another chance for the virus to evolve and become better suited to infecting people.

This rapid evolution was evident earlier this year, when the 

 — from Louisiana — died of bird flu. When scientists sequenced the virus from different organs from the body, they found "the virus was changing within," says Uhart. While this patient

didn't pass the virus on to other people, Uhart says this case indicates how quickly the virus can change.

There's another element in this genetic game. The virus is able to mix and match its genes with local influenza viruses if one animal is infected with multiple flu viruses. As this highly deadly bird flu virus arrives in new places it can mingle with a big pool of less deadly flu viruses and create new "Frankenstein" viruses, explains Wille.

And with each genetic change, there is the chance the virus could become more severe.

At the moment, the virus can infect humans but has not yet evolved to jump readily from one human to another. That could change.

And without keeping up with the virus's journey in wild animals, experts worry, humans will be caught unprepared and ill-equipped to tackle bird flu.

"Ever since H5N1 [bird flu] showed up," says Osterholm, "I tend to sleep with one eye open."

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That's the way one scientist puts it — referring to how infected wild birds survive long enough to spread it to birds and mammals around the world. And that's a serious risk for human health.

It's like 'dead birds flying': How bird flu is spreading in the wild

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