Bird Flu Infects More Mammals. What Does That Mean for Us?

In her thirty years of working with elephant seals, Dr. Marcela Uhart has never seen anything like the scene on the beaches of Argentina's Valdés Peninsula last October.

It was peak breeding season; the beach should have been full of harems of fertile females and enormous males fighting each other for supremacy. Instead, it was "just carcass on carcass on carcass," recalls Uhart, who directs the Latin American wildlife health program at the University of California, Davis.

H5N1, one of several viruses that cause bird flu, had already killed at least 24,000 South American sea lions along the continent's coasts in less than a year. Now the time had come for elephant seals.

Sign up for The Morning newsletter from the New York Times

Puppies of all ages, from newborns to fully weaned children, lay dead or dying at the tide line. Sick puppies lay listless, foam oozing from their mouths and noses.

Uhart called it "a picture from hell."

In the weeks that followed, she and a colleague - protected from head to toe with gloves, gowns and masks and occasionally dousing themselves with bleach - carefully documented the devastation. Team members stood atop nearby cliffs and estimated the toll with drones.

What they discovered was staggering: the virus had killed an estimated 17,400 seal pups, more than 95% of the young animals in the colony.

The catastrophe was the latest in a bird flu epidemic that has spread around the world since 2020, prompting authorities on multiple continents to kill millions of poultry and other birds. In the United States alone, more than 90 million birds have been culled in a vain attempt to deter the virus.

There is no end to H5N1. Bird flu viruses are often picky about their hosts and typically stick to one species of wild bird. But it has quickly infiltrated an astonishingly wide range of birds and animals, from squirrels and skunks to bottlenose dolphins, polar bears and, most recently, dairy cows.

"In my flu career, we have never seen a virus that expands its host range so much," said Troy Sutton, a virologist who studies bird flu viruses and human flu viruses at Penn State University.

The blow to marine mammals and the dairy and poultry industries is worrying enough. But a bigger concern, experts say, is what these developments portend: The virus adapting to mammals and moving closer to spreading among humans.

A human pandemic is by no means inevitable. So far, at least, the changes in the virus do not indicate that H5N1 can cause a pandemic, Sutton said.

Still, he said, "We really don't know how to interpret this or what it means."

Marine deaths

A highly pathogenic strain of H5N1 was identified in waterfowl in China in 1996. The following year, 18 people in Hong Kong became infected with the virus and six died. The virus then disappeared, but in 2003 it reappeared in Hong Kong. Since then, it has caused dozens of outbreaks in poultry and affected more than 800 people who were in close contact with the birds.

Meanwhile, it continued to evolve.

The version of H5N1 currently racing around the world appeared in Europe in 2020 and quickly spread to Africa and Asia. The disease killed dozens of farmed birds, but unlike its predecessors, it also spread widely among wild birds and many other animals.

Most infections in mammals were probably 'death-end' cases: a fox perhaps that ate an infected bird and died without passing on the virus. But a few larger outbreaks suggested H5N1 was capable of more.

The first clue came in the summer of 2022, when the virus killed hundreds of seals in New England and Quebec. A few months later it infiltrated a mink farm in Spain.

In the mink, the most likely explanation was that H5N1 had adapted to spread among the animals. The scale of the outbreaks in marine mammals in South America underscored that likelihood.

"Even intuitively, I would think that transmission from mammals to mammals is very likely," said Malik Peiris, a virologist and bird flu expert at the University of Hong Kong.

After the virus was first discovered in South America, in birds in Colombia in October 2022, the virus moved along the Pacific coast to Tierra del Fuego, the southernmost tip of the continent, and along the Atlantic coast.

Along the way it killed hundreds of thousands of seabirds and tens of thousands of sea lions in Peru, Chile, Argentina, Uruguay and Brazil. The sea lions behaved erratically and suffered from convulsions and paralysis; pregnant females miscarried their fetuses.

"What happened when the virus moved to South America was something we had never seen before," Uhart said.

Exactly how and when the virus spread to marine mammals is unclear, but the sea lions most likely came into close contact with infected birds or contaminated feces. (Although fish make up the majority of sea lions' diet, they sometimes eat birds as well.)

At some point, it is likely that the virus evolved to spread directly among marine mammals: in Argentina, sea lion deaths did not coincide with mass deaths of wild birds.

"This could indicate that the source of the infection was not the infected birds," says Dr. Pablo Plaza, a wildlife veterinarian at the National University of Comahue and the National Scientific and Technical Research Council of Argentina.

It's not hard to imagine how the virus could spread among these animals: Elephant seals and sea lions both breed in colonies and crowd onto beaches where they fight, mate and bark at each other. Elephant seals sneeze all day long, spreading large drops of mucus.

It is difficult to prove exactly how and when the virus moved from one species to another. But genetic analysis supports the theory that the marine mammals acquired their infections from each other, and not from birds. Samples of virus isolated from sea lions in Peru and Chile and from elephant seals in Argentina all share about 15 mutations not found in the birds; the same mutations were also present in a Chilean man infected last year.

There are numerous possibilities for H5N1 to jump from marine mammals to humans. A sick male elephant seal that sat on a public beach in Argentina for a day and a half was found to be carrying huge amounts of the virus. In Peru, scientists collected samples from sea lion carcasses lying next to families enjoying a beach day.

Scavenging animals, such as dogs, could also pick up the virus from an infected carcass and then spread it more widely: "None of the wildlife live in their little silos," says Wendy Puryear, a virologist at Tufts University who is studying the New England University studied. outbreaks of seals.

In some South American countries the rest, apart from a few buried carcasses, is left on the beaches, rotting and swept away.

"How can you even scale up to remove 17,000 dead bodies in the middle of nowhere, in places where you can't even take down machines, and gigantic cliffs?" Uhart said.

A mutating pathogen

Flu viruses are adept at picking up new mutations; When two types of flu viruses infect the same animal, they can mix up their genetic material and generate new versions.

It is unclear exactly how and how much the H5N1 virus has changed since it first emerged. A study last year found that after the virus entered the United States, it quickly mixed with other flu viruses circulating here and morphed into different versions - some mild, others causing severe neurological symptoms.

"So now, after 20 years of reassortment, you have a virus that is doing extremely well in a whole range of bird and mammal species," says Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases, who has studied the necessary mutations . so that H5N1 can adapt to humans.

Each new strain that harbors the virus creates opportunities for H5N1 to continue to evolve and attack humans.

And the virus could encounter mutations that no one has thought of yet, allowing it to break the species barrier. That's what happened with the 2009 swine flu outbreak.

That virus did not have the mutations thought to be necessary to easily infect humans. Instead, "it had these other mutations that no one knew or thought about before then," says Louise Moncla, an evolutionary biologist who studies bird flu at the University of Pennsylvania.

But even if the virus jumps to humans, "we may not see the level of mortality that we're really concerned about," said Seema Lakdawala, a virologist at Emory University. "Pre-existing immunity against seasonal flu variants will provide some protection against severe disease."

What happens now

The U.S. is prepared for a flu pandemic, with some stockpiles of vaccines and antivirals, but surveillance efforts may not pick up the virus quickly enough to deploy these tools.

It took several weeks before farmers, and then officials, knew that H5N1 was circulating among dairy cows.

The outbreak on dairy farms has resulted in only one mild human infection, but farms are fertile ground for the virus to jump across species - from cats to cows to pigs and humans, in no particular order.

Many scientists are particularly concerned about pigs, which are susceptible to both human flu and bird flu, making viruses the perfect mixing bowl for gene-swapping. Pigs are slaughtered at a very young age, and newer generations, who have not previously been exposed to the flu, are particularly vulnerable to infection.

So far, H5N1 doesn't seem adept at infecting pigs, but that could change if it acquires new mutations.

"I never let my kids go to a state fair or animal farm, I'm one of those parents," Lakdawala said. "And that's mainly because I know that the more interactions we have with animals, the more possibilities there are."

Should H5N1 adapt to humans, federal officials will need to work together and with their international counterparts. Nationalism, competition and bureaucracy can all slow down the exchange of information that is crucial in a developing outbreak.

In some ways, the current spread among dairy cows is an opportunity to practice this exercise, says Rick Bright, the CEO of Bright Global Health, a consulting firm focused on improving responses to public health emergencies. But the U.S. Department of Agriculture only requires voluntary testing of cows, and is not as timely and transparent with its findings as it should be, he said.

Dr. Rosemary Sifford, the department's chief veterinarian, said staff there were working hard to share information as quickly as possible. "This is considered an emerging disease," she said.

Government leaders are generally cautious and want to see more data. But "given how quickly this can spread and the devastating disease it can cause if our leaders hesitate and don't pull the right trigger at the right time, we will be caught flat-footed again," Bright said. .

"If we don't panic it, but we do it with the respect and due diligence," he added, referring to the virus, "I believe we can handle it."

c.2024 The New York Times Company