In her three many years of working with elephant seals, Dr. Marcela Uhart had by no means seen something just like the scene on the seashores of Argentina’s Valdés Peninsula final October.

It was peak breeding season; the seaside ought to have been teeming with harems of fertile females and large males battling each other for dominance. As an alternative, it was “simply carcass upon carcass upon carcass,” recalled Dr. Uhart, who directs the Latin American wildlife well being program on the College of California, Davis.

H5N1, one of many many viruses that trigger fowl flu, had already killed at least 24,000 South American sea lions alongside the continent’s coasts in lower than a yr. Now it had come for elephant seals.

Pups of all ages, from newborns to the absolutely weaned, lay lifeless or dying on the high-tide line. Sick pups lay listless, foam oozing from their mouths and noses.

Dr. Uhart referred to as it “a picture from hell.”

Within the weeks that adopted, she and a colleague — protected head to toe with gloves, robes and masks, and periodically dousing themselves with bleach — fastidiously documented the devastation. Group members stood atop the close by cliffs, assessing the toll with drones.

What they discovered was staggering: The virus had killed an estimated 17,400 seal pups, greater than 95 % of the colony’s younger animals.

The disaster was the most recent in a fowl flu epidemic that has whipped around the globe since 2020, prompting authorities on a number of continents to kill poultry and different birds by the thousands and thousands. In america alone, greater than 90 million birds have been culled in a futile try to discourage the virus.

There was no stopping H5N1. Avian flu viruses are usually choosy about their hosts, sometimes sticking to 1 sort of wild fowl. However this one has quickly infiltrated an astonishingly big range of birds and animals, from squirrels and skunks to bottlenose dolphins, polar bears and, most not too long ago, dairy cows.

“In my flu profession, now we have not seen a virus that expands its host vary fairly like this,” stated Troy Sutton, a virologist who research avian and human influenza viruses at Penn State College.

The blow to sea mammals, and to dairy and poultry industries, is worrying sufficient. However an even bigger concern, consultants stated, is what these developments portend: The virus is adapting to mammals, edging nearer to spreading amongst folks.

A human pandemic is under no circumstances inevitable. Up to now at the very least, the changes in the virus don’t sign that H5N1 could cause a pandemic, Dr. Sutton stated.

Nonetheless, he stated, “We actually don’t know interpret this or what it means.”

Marine mortalities

A extremely pathogenic pressure of H5N1 was recognized in 1996 in home waterfowl in China. The subsequent yr, 18 folks in Hong Kong grew to become contaminated with the virus, and 6 died. The virus then went silent, but it surely resurfaced in Hong Kong in 2003. Since then, it has prompted dozens of outbreaks in poultry and affected greater than 800 individuals who have been in shut contact with the birds.

All of the whereas, it continued to evolve.

The model of H5N1 at present racing the world over emerged in Europe in 2020 and unfold shortly to Africa and Asia. It killed scores of farmed birds, however in contrast to its predecessors it additionally unfold extensively amongst wild birds and into many different animals.

Most infections of mammals have been most likely “dead-end” circumstances: a fox, maybe, that ate an contaminated fowl and died with out passing on the virus. However a couple of bigger outbreaks urged that H5N1 was able to extra.

The primary clue got here in the summertime of 2022, when the virus killed a whole lot of seals in New England and Quebec. A couple of months later, it infiltrated a mink farm in Spain.

Within the mink, at the very least, the almost definitely rationalization was that H5N1 had tailored to unfold among the many animals. The dimensions of the outbreaks in sea mammals in South America underscored that chance.

“Even intuitively, I might assume that mammal-to-mammal transmission could be very possible,” stated Malik Peiris, a virologist and knowledgeable in fowl flu on the College 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 Atlantic coast.

Alongside the best way, it killed a whole lot of hundreds of seabirds, and tens of hundreds of sea lions, in Peru, Chile, Argentina, Uruguay and Brazil. The ocean lions behaved erratically, experiencing convulsions and paralysis; pregnant females miscarried their fetuses.

“What occurred when the virus moved to South America we had by no means seen earlier than,” Dr. Uhart stated.

Precisely how and when the virus jumped to marine mammals is unclear, however the sea lions almost definitely got here into close contact with infected birds or contaminated droppings. (Though fish make up the majority of sea lions’ food plan, they do generally eat birds.)

Sooner or later, it’s possible the virus advanced to unfold instantly among the many marine mammals: In Argentina, the ocean lion deaths didn’t coincide with the mass mortality of untamed birds.

“This might counsel that the an infection supply was not the contaminated birds,” stated Dr. Pablo Plaza, a wildlife veterinarian on the Nationwide College of Comahue and Nationwide Scientific and Technical Analysis Council in Argentina.

It’s not onerous to think about how the virus may disperse in these animals: Elephant seals and sea lions each breed in colonies, crowding collectively on seashores the place they battle, mate and bark at each other. Elephant seals sneeze all day, dispersing massive droplets of mucus every time they do.

It’s tough to show precisely how and when the virus moved from one species to a different. However genetic analysis supports the idea the marine mammals acquired their infections from each other, not birds. Samples of virus remoted from sea lions in Peru and Chile and from the elephant seals in Argentina all share about 15 mutations not seen within the birds; the identical mutations have been additionally current in a Chilean man who was contaminated final yr.

There are numerous opportunities for H5N1 to leap from sea mammals into folks. One sick male elephant seal that sat for a day and a half on a public seaside in Argentina turned out to hold huge quantities of virus. In Peru, scientists collected samples from sea lion carcasses that lay alongside households having fun with a seaside day.

Scavenging animals, akin to canine, may additionally choose up the virus from an contaminated carcass after which unfold it extra broadly: “Not one of the wildlife exists of their little silos,” stated Wendy Puryear, a virologist at Tufts College who studied the New England seal outbreaks.

In some South American international locations, other than a couple of carcasses that have been buried, the remainder have remained on the seashores, rotting and scavenged upon.

“How do you even scale as much as take away 17,000 lifeless our bodies out in the midst of nowhere, locations the place you’ll be able to’t even convey down equipment, and humongous cliffs?” Dr. Uhart stated.

A mutating pathogen

Flu viruses are adept at choosing up new mutations; when two kinds of flu virus infect the identical animal, they’ll shuffle their genetic materials and generate new variations.

It’s unclear precisely how, and the way a lot, the H5N1 virus has modified because it first emerged. One research final yr confirmed that after the virus entered america, it shortly mixed with other flu viruses circulating right here and morphed into varied variations — some delicate, others inflicting extreme neurological signs.

“So now after 20 years of reassortment, you might have a virus that really does extraordinary effectively in a complete number of avian and mammal species,” stated Vincent Munster, a virologist on the Nationwide Institute of Allergy and Infectious Ailments who has studied the mutations wanted for H5N1 to adapt to folks.

Each new species that harbors the virus creates alternatives for H5N1 to proceed to evolve, and to leap into folks.

And the virus might stumble throughout mutations that nobody has but thought-about, permitting it to breach the species barrier. That’s what occurred within the 2009 swine flu outbreak.

That virus didn’t have the mutations regarded as wanted to contaminate folks simply. As an alternative, “it had these different mutations that nobody knew about or considered earlier than then,” stated Louise Moncla, an evolutionary biologist who research avian influenza on the College of Pennsylvania.

Nonetheless, even when the virus jumps to folks, “we might not see the extent of mortality that we’re actually involved about,” stated Seema Lakdawala, a virologist at Emory College. “Preexisting immunity to seasonal flu strains will present some safety from extreme illness.”

What occurs subsequent

The U.S. is ready for an influenza pandemic, with some stockpiled vaccines and antivirals, however its efforts at monitoring the virus might not choose it up shortly sufficient to deploy these instruments.

It took a number of weeks earlier than farmers, after which officers, knew that H5N1 was circulating in dairy cows.

The dairy farm outbreak has resulted in just one delicate human an infection, however farms are fertile floor for the virus to leap species — from cat to cow to pig and human, in any order.

Many scientists fear specifically about pigs, that are prone to each human and avian flu strains, offering the proper mixing bowl for viruses to swap genes. Pigs are slaughtered when very younger, and newer generations, with no prior publicity to flu, are significantly weak to infections.

Up to now, H5N1 doesn’t appear adept at infecting pigs, however that might change because it acquires new mutations.

“I by no means let my children go to a state truthful or animal farm, I’m a kind of dad and mom,” Dr. Lakdawala stated. “And it’s principally as a result of I do know that the variety of interactions that we enhance with animals, the extra alternatives there are.”

Ought to H5N1 adapt to folks, federal officers might want to work collectively and with their worldwide counterparts. Nationalism, competitors and paperwork can all decelerate the alternate of knowledge that’s essential in a creating outbreak.

In some methods, the present unfold amongst dairy cows is a chance to apply the drill, stated Rick Vibrant, the chief government of Vibrant International Well being, a consulting firm that focuses on bettering responses to public well being emergencies. However the U.S. Agriculture Division has been too restricted in its strategy to testing cows, and has not been as well timed and clear with its findings because it ought to have been, he stated.

On Wednesday, the division ordered that dairy cows shifting throughout state strains to be examined for influenza.

Dr. Rosemary Sifford, the division’s chief veterinarian, stated the employees there have been working onerous to share info as shortly as they’ll. “That is thought-about an rising illness,” she stated.

Authorities leaders are sometimes cautious, eager to see extra knowledge. However “given the speedy pace at which this will unfold and the devastating sickness that it may possibly trigger if our leaders are hesitant and don’t pull the fitting triggers on the proper time, we will probably be caught flat-footed as soon as once more,” Dr. Vibrant stated.

“If we don’t give it the panic however we give it the respect and due diligence,” he added, alluding to the virus, “I imagine we are able to handle it.”