In her three decades of working with elephant seals, Dr. Marcela Uhart had never seen anything like this on the beaches of Argentina’s Valdés Peninsula last October.
It was peak breeding season; the beach should have been teeming with harems of fertile females and enormous males fighting for dominance. Instead, it was “just carcass after carcass,” recalls Dr. Uhart, who directs the Latin American Wildlife Health Program at the University of California, Davis.
H5N1, one of the many viruses responsible for bird flu, had already killed at least 24,000 South American sea lions along the continent’s coasts in less than a year. Now it was time for the elephant seals.
Puppies of all ages, from newborns to fully weaned children, lay dead or dying on the high tide line. Sick puppies lay listless, foam oozing 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 with gloves, gowns and masks, and periodically dousing each other with bleach — carefully documented the devastation. Team members stood atop nearby cliffs and assessed the toll using drones.
What they discovered was astonishing: the virus had killed approximately 17,400 seal pupsmore than 95 percent of the young animals in the colony.
The disaster is the latest in a bird flu epidemic that has gripped 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 were eliminated in a vain attempt to deter the virus.
Nothing could stop H5N1. Avian flu viruses tend to be picky about their hosts, usually sticking to a single species of wild bird. But it quickly infiltrated a surprisingly wide range of birds and animals, from squirrels and skunks to bottlenose dolphins, polar bears and, more recently, dairy cows.
“In my career in influenza, we haven’t seen a virus expand its host range like this,” said Troy Sutton, a virologist who studies avian and human influenza 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 is adapting to mammals, moving ever closer to spreading among humans.
A human pandemic is by no means inevitable. So far at least, the changes in the virus do not signal that H5N1 can cause a pandemic, Dr. Sutton said.
Still, he said: “We really don’t know how to interpret this or what it means. »
Marine mortalities
A highly pathogenic strain of H5N1 was identified in 1996 in domestic waterfowl in China. The following year, 18 people in Hong Kong were infected with the virus and six died. The virus then became silent, but it resurfaced 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.
All the while, he continued to evolve.
The version of H5N1 currently circulating around the world appeared in Europe in 2020 and quickly spread to Africa and Asia. It killed many farmed birds, but unlike its predecessors it also spread widely among wild birds and many other animals.
Most mammalian infections were probably “no-win” cases: a fox, perhaps, that ate an infected bird and died without transmitting the virus. But a few larger outbreaks suggest H5N1 was capable of more.
The first clue appeared 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 mink, at least, the most likely explanation was that H5N1 had adapted to spread among animals. The scale of epidemics among marine mammals in South America underlines this probability.
“Even intuitively, I think mammal-to-mammal transmission is very likely,” said Malik Peiris, a virologist and avian flu expert at the University of Hong Kong.
After first being detected in South America, in birds in Colombia in October 2022, the virus swept across the Pacific coast to Tierra del Fuego, the southernmost tip of the continent, and up the Atlantic coast .
Along the way, it killed hundreds of thousands of seabirds and tens of thousands of sea lions. Peru, ChileArgentina, Uruguay And Brazil. Sea lions behaved erratically, experiencing convulsions and paralysis; pregnant women have had a miscarriage their fetuses.
“What happened when the virus spread to South America, we’ve never seen before,” Dr. Uhart said.
It is unclear how and when the virus spread to marine mammals, but sea lions most likely entered the area. close contact with infected birds or contaminated feces. (Although fish make up the bulk of sea lions’ diets, they sometimes eat birds.)
At some point, it is likely that the virus evolved to spread directly among marine mammals: in Argentina, sea lion mortality did not coincide with mass mortality…
