by In her three decades of working with elephant seals, Dr. Marcela Uhart nothing like the view on the beaches of Peninsula Valdés in Argentina last October.
It was peak breeding season; the beach should be overflowing with harems of fertile females and huge males fighting each other for dominance. Instead, it was just “carcass upon carcass upon carcass,” recalled 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, has killed at least 24,000 South American sea lions along the continent’s coasts in less than a year. Now he had come for elephant seals.
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Calves of all ages, from newborns to fully weaned individuals, were dead or dying at the high tide line. Puppies were listlessly sick, foam pouring from their mouths and noses.
Uhart called it “an image from hell”.
In the weeks that followed, she and a colleague – protected head to toe with gloves, gowns and masks, and periodically dousing themselves in bleach – meticulously documented the devastation. Team members stood atop nearby cliffs, assessing the toll with drones.
What they discovered was startling: The virus had killed approximately 17,400 seal pups, more than 95% of the colony’s young animals.
The disaster was the latest in a bird flu epidemic that has hit the world since 2020, prompting authorities on multiple continents to cull poultry and other birds by the millions. In the United States alone, more than 90 million birds have been killed in a desperate attempt to eradicate the virus.
There was no stopping H5N1. Avian influenza viruses tend to be picky about their host, usually sticking to one type of wild bird. But this one has infiltrated a surprisingly wide range of birds and animals, from squirrels and pods of dolphins to bottlenose dolphins, polar bears and, more recently, dairy cows.
“In my flu career, we’ve never seen a virus that expands its host range quite like this,” said Troy Sutton, a virologist who studies avian and human flu viruses at Penn State University.
The blow to marine mammals, and the dairy and poultry industries is a cause for concern. But more worrying, experts said, is what these developments show: The virus is adapting to mammals, edging closer to spreading to humans.
A human pandemic is by no means inevitable. At least so far, the changes in the virus do not indicate that H5N1 can cause a pandemic, Sutton said.
Still, he said, “We don’t really know how to interpret this or what it means.”
Maritime Deaths
A highly pathogenic strain of H5N1 was identified in 1996 in domestic waterfowl in China. The following year, 18 people in Hong Kong became infected with the virus, and six died. The virus then fell silent, but resurfaced in Hong Kong in 2003. Since then, it has led to multiple outbreaks in poultry and infected more than 800 people who had been in close contact with it. the birds.
All the while, it continued to develop.
The version of H5N1 currently racing around the world emerged in Europe in 2020 and quickly spread to Africa and Asia. It killed scores of farmed birds, but unlike its predecessors it also spread widely among wild birds and into many other animals.
Most infections in mammals were probably “dead” cases: perhaps a fox that ate an infected bird and died without passing on the virus. But several 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.
In the mink, at least, the most likely explanation was that H5N1 had adapted to spread among animals. The scale of the outbreak in marine mammals in South America underscored that possibility.
“Even intuitively, I would think it’s very likely that there is mammal-to-mammal transmission,” said Malik Peiris, a virologist and avian 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 swept down the Pacific coast to Tierra del Fuego, the southernmost tip of the continent, and up the coast of the Atlantic.
Along the way, he killed hundreds of thousands of seabirds, and thousands of sea lions, in Peru, Chile, Argentina, Uruguay and Brazil. The sea lions behaved erratically, suffering convulsions and paralysis; pregnant females miscarry their fetuses.
“What happened when the virus moved to South America is something we’ve never seen before,” Uhart said.
It is not clear how and when the virus jumped to marine mammals, but it is likely that the sea lions came into close contact with infected birds or contaminated droppings. (Although the majority of sea lions’ diet is fish, they do occasionally eat birds.)
At some point, the virus probably evolved directly among marine mammals: In Argentina, the sea lion die-off did not coincide with the mass mortality of wild birds.
“This may suggest that the infected birds are not the source of the infection,” said Dr. Pablo Plaza, a wildlife veterinarian at the University of Comahue and the National Council for Scientific and Technical Research in Argentina.
It is not difficult to imagine how the virus could spread to these animals: elephant seals and sea lions breed in colonies, crowded together on beaches where they fight, mate and bark at each other. Elephant seals sneeze all day, spreading large droplets of mucus each time they do so.
It is difficult to prove exactly how and when the virus moved from one species to another. But genetic analysis supports the theory that marine mammals got their infections from each other, not from birds. Virus samples isolated from sea lions in Peru and Chile and from elephant seals in Argentina contain about 15 mutations not seen in birds; the same mutations were also present in a Chilean man who was infected last year.
There are many opportunities for H5N1 to jump from marine mammals into humans. One sick male elephant seal that sat for a day and a half on a public beach in Argentina turned out to be carrying massive amounts of the virus. In Peru, scientists collected samples from sea lion carcasses lying next to families enjoying a day at the beach.
Scavenging animals, such as dogs, could also pick up the virus from an infected carcass and spread it more widely: “None of the wildlife are small silos,” said Wendy Puryear, a virologist at Tufts University who studied on the New England. seal outbreaks.
In some South American countries, instead of burying the few carcasses, the rest have remained on the beaches, rotting and scavenging them.
“How do you even scale to reach 17,000 dead bodies in the middle of nowhere, places where you can’t even bring machinery down, and sheer cliffs?” Uhart said.
A Mutating Pathogen
Influenza viruses are adept at picking up new mutations; when two types of flu virus infect the same animal, they can shuffle their genetic material and generate new versions.
It is not clear exactly how, and how much, the H5N1 virus has changed since it first appeared. One study last year showed that after the virus entered the United States, it quickly mixed with other flu viruses circulating here and mutated into different variants — some mild, others causing severe neurological symptoms.
“So now after 20 years of reassortment, you have a virus that does very well in a whole range of avian and mammalian species,” said Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases who has studied the necessary mutations. for adapting H5N1 to humans.
Each new species that harbors the virus creates opportunities for H5N1 to continue to evolve, and to jump into humans.
And the virus may stumble across mutations that no one has yet considered, allowing it to overcome the species barrier. That’s what happened in the 2009 swine flu outbreak.
That virus did not have the mutations thought to be needed to easily infect humans. Instead, “there were these other mutations that nobody knew about or thought about before,” said Louise Moncla, an evolutionary biologist who studies bird flu 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 disease.”
What Happens Next
The US is ready for a flu pandemic, with several vaccines and antivirals stockpiled, but its efforts to monitor the virus may not catch up fast enough to use those tools.
It took several weeks before farmers, and then officials, knew that H5N1 was circulating in dairy cows.
The dairy farm outbreak 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 any order.
Many scientists are particularly concerned about pigs, which are susceptible to human and bird flu strains, providing the perfect mixing bowl for viruses to exchange genes. Pigs are slaughtered when they are very young, and the newer generations, with no previous exposure to influenza, are particularly vulnerable to infection.
So far, H5N1 does not appear to be adept at infecting pigs, but that may change as it acquires new mutations.
“I never let my kids go to a state fair or an animal farm, I’m one of those parents,” Lakdawala said. “And the main reason is that I know that the more interactions with animals, the more opportunities there are.”
If H5N1 adapts to humans, federal officials will need to work together with their international counterparts. Nationalism, competition and bureaucracy can delay the exchange of information that is critical in an emerging outbreak.
In some ways, the current spread among dairy cows is an opportunity to practice the drill, said Rick Bright, CEO of Bright Global Health, a consulting company that focuses on improving responses to public health emergencies. But the US Department of Agriculture only requires voluntary testing of cows, and its results are not as timely and transparent as they should be, he said.
Dr Rosemary Sifford, the department’s chief vet, said staff there were working hard to share information as quickly as they could. “This appears to be an evolving disease,” she said.
Government leaders tend to be cautious, wanting to see more details. But “considering the speed at which this could spread and the debilitating illness it could cause if our leaders hesitate and don’t pull the right triggers at the right time, we will be caught in equal measure.” Us again,” said Bright. .
“If we don’t give it the panic but we give it the proper respect and diligence,” he said, referring to the virus, “I believe we can manage it.”
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