In her three decades of working with elephant seals, Dr. Marcela Uhart had 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 filled with harems of fertile females and enormous males fighting each other for dominance. Instead, it was “just carcass on carcass on carcass,” recalled Dr. Uhart, who directs the Latin American wildlife health program at the University of California, Davis.
H5N1, one of the many viruses that cause bird flu, had already killed at least 24,000 South American fur seals on the continent’s coasts in less than a year. Now he had come for the elephant seals.
Cubs of all ages, from newborn to fully weaned, lay dead or dying at the high tide line. The sick puppies lay listless, with foam coming from their mouths and noses.
Dr. Uhart called it “a picture of hell.”
In the weeks that followed, she and a colleague—protected from head to toe in gloves, gowns, and masks, and periodically doused with bleach—carefully documented the devastation. Team members were atop nearby cliffs, assessing the number of casualties with drones.
What they found was astonishing: The virus had killed approximately 17,400 seal pups, more than 95 percent of the colony’s young animals.
The catastrophe was the latest in a bird flu epidemic that has swept the world since 2020, prompting authorities on several continents to kill millions of poultry and other birds. In the United States alone, more than 90 million birds have been killed in a futile attempt to deter the virus.
No one has been able to stop H5N1. Avian flu viruses tend to be picky about their hosts and are usually limited to one type of wild bird. But it has quickly infiltrated a surprisingly 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 haven’t seen a virus expand its host range in this way,” said Troy Sutton, a virologist who studies avian and human influenza viruses at Penn State University.
The hit to marine mammals and the dairy and poultry industries is quite worrying. But a bigger concern, experts said, is what these developments portend: The virus is adapting to mammals and is getting closer to spreading among people.
A human pandemic is by no means inevitable. At least so far, changes in the virus do not indicate that H5N1 could cause a pandemic, Dr. Sutton said.
Still, he said, “we don’t really know how to interpret this or what it means.”
Marine mortality
In 1996, a highly pathogenic strain of H5N1 was identified in domestic waterfowl in China. The following year, 18 people in Hong Kong were infected with the virus and six died. The virus then went silent, but re-emerged in Hong Kong in 2003. 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 circulating around the world emerged in Europe in 2020 and quickly spread to Africa and Asia. It killed dozens of farm birds, but unlike its predecessors, it also spread widely among wild birds and many other animals.
Most mammalian infections were probably “dead end” cases: a fox, perhaps, that ate an infected bird and died without transmitting the virus. But some larger outbreaks suggested that 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, he infiltrated a mink farm in Spain.
At least in mink, the most likely explanation was that H5N1 had adapted to spread among animals. The magnitude of the outbreaks in marine mammals in South America underscored that likelihood.
“Even intuitively, you would think that mammal-to-mammal transmission is very likely,” said Malik Peiris, a virologist and bird flu expert at the University of Hong Kong.
After it was first detected in South America, in birds in Colombia in October 2022, the virus spread from the Pacific coast to Tierra del Fuego, the southernmost tip of the continent, and to 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, experiencing seizures and paralysis; pregnant females aborted their fetuses.
“We have never seen before what happened when the virus moved to South America,” Dr. Uhart said.
It’s unclear exactly how and when the virus jumped to marine mammals, but the sea lions most likely came into close contact with infected birds or contaminated droppings. (Although fish make up most of the sea lion’s diet, they sometimes eat birds.)
At some point, the virus likely evolved to spread directly among marine mammals: in Argentina, sea lion deaths did not coincide with mass mortality of wild birds.
“This could suggest that the source of infection was not the infected birds,” said Dr. Pablo Plaza, a wildlife veterinarian at the National University of Comahue and Argentina’s National Scientific and Technical Research Council.
It’s not hard to imagine how the virus could spread in these animals: Elephant seals and sea lions breed in colonies, crowding together on beaches where they fight, mate, and bark at each other. Elephant seals sneeze all day long, dispersing large droplets of mucus each time they do so.
It is difficult to prove exactly how and when the virus jumped from one species to another. But genetic analysis supports the theory that marine mammals acquired their infections from each other, not from birds. Virus samples isolated from sea lions in Peru and Chile and from elephant seals in Argentina share about 15 mutations that are not seen in birds; The same mutations were also present in a Chilean who was infected last year.
There are numerous opportunities for H5N1 to jump from marine mammals to people. A sick male elephant seal that sat for a day and a half on a public beach in Argentina turned out to be carrying huge amounts of the virus. In Peru, scientists collected samples from sea lion carcasses lying next to families enjoying a day at the beach.
Scavenger animals, such as dogs, could also pick up the virus from an infected carcass and then spread it more widely: “None of the wild animals exist in their little silos,” said Wendy Puryear, a virologist at Tufts University who studied the habitat. of New England. seal outbreaks.
In some South American countries, except for some corpses that were buried, the rest have remained on the beaches, rotting and being scavenged.
“How can you scale to remove 17,000 bodies in the middle of nowhere, places where you can’t even take down machinery and huge cliffs?” Dr. Uhart said.
A mutant pathogen
Flu viruses are experts at detecting new mutations; When two types of flu viruses infect the same animal, they can mix 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 showed that after the virus entered the United States, it quickly mixed with other flu viruses circulating here and morphed into several versions, some mild and others that caused severe neurological symptoms.
“Now, after 20 years of recombination, we have a virus that actually works extraordinarily well in a whole range of bird and mammal species,” said Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases who has studied the necessary mutations. . that H5N1 adapts to people.
Each new species that hosts the virus creates opportunities for H5N1 to continue evolving and reach people.
And the virus may stumble upon mutations that no one has considered yet, allowing it to cross the species barrier. That’s what happened in the 2009 swine flu outbreak.
That virus did not have the mutations believed necessary to infect people easily. Instead, “it had these other mutations that no one knew about or thought about before,” said Louise Moncla, an evolutionary biologist who studies avian influenza at the University of Pennsylvania.
Still, even if the virus jumps to people, “we may not see the level of mortality that we’re really worried about,” said Seema Lakdawala, a virologist at Emory University. “Pre-existing immunity to seasonal flu strains will provide some protection against serious illness.”
What happens next?
The United States is prepared for an influenza pandemic, with some vaccines and antivirals in stock, but its efforts to monitor the virus may not detect it quickly enough to deploy those tools.
It took several weeks before farmers, and then officials, learned that H5N1 was circulating in dairy cows.
The outbreak on dairy farms has caused only mild human infection, but farms are fertile ground for the virus to jump species: from cat to cow to pig to human, in any order.
Many scientists are particularly concerned about pigs, which are susceptible to human and bird flu strains, providing the perfect vessel for viruses to exchange genes. Pigs are slaughtered when they are very young, and newer generations, without prior exposure to flu, are particularly vulnerable to infections.
So far, H5N1 does not appear able to infect pigs, but that could change as it acquires new mutations.
“I never let my kids go to a state fair or an animal farm; I am one of those parents,” Dr. Lakdawala said. “And it’s mainly because I know that the more interactions we increase with animals, the more opportunities there will be.”
If H5N1 hits people, federal officials will have to work together and with their international counterparts. Nationalism, competition and bureaucracy can slow down the information sharing that is crucial in a developing outbreak.
In some ways, the current spread among dairy cows is an opportunity to practice drill, said Rick Bright, CEO of Bright Global Health, a consulting firm that focuses 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 were working hard to share information as quickly as possible. “This is considered an emerging disease,” she said.
Government leaders are often cautious and want to see more data. But “given the rapid speed at which 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 once again be caught off guard,” Dr. Brilliant said.
“If we don’t panic but do respect and do due diligence,” he added, alluding to the virus, “I think we can manage it.”