Dr. Marcela Uhart has been working with elephant seals for 30 years, but she had never seen anything like this on the coast of Argentina's Valdez Peninsula last October.
It was the peak of the breeding season. The beach was supposed to be filled with harems of fertile females and giant males fighting for dominance. Instead, it was “just carcass upon carcass,” recalls Dr. Woohart, who directs the Latin American Wildlife Health Program at the University of California, Davis.
H5N1 is one of the many viruses that cause bird flu, which has already killed at least 24,000 South American sea lions along the continent's coast in less than a year. This time I came looking for elephant seals.
Puppies of all ages, from newborns to weanlings, lie dead or dying at the high tide line. A sick puppy is lying listlessly and foaming from its mouth and nose.
Dr. Woohart called it “a picture of hell.”
Over the next few weeks, she and her colleagues carefully documented the devastation, covering themselves from head to toe in gloves, gowns and masks and regularly dousing themselves with bleach. Team members stood on a nearby cliff and used a drone to survey tolls.
What they discovered was surprising. The virus killed an estimated 17,400 seal pups, killing more than 95 percent of the colony's young animals.
The disaster is the latest outbreak of bird flu that has been raging around the world since 2020, prompting authorities on multiple continents to cull millions of poultry and other birds. In the United States alone, more than 90 million birds have been culled in a futile attempt to stop the virus.
H5N1 infection continues unabated. Avian influenza viruses tend to be selective about their hosts and usually attach themselves to one species of wild bird. But the virus is rapidly invading an astonishing range of birds and animals, from squirrels and skunks to bottlenose dolphins, polar bears and, most recently, dairy cows.
“In my entire influenza career, I've never seen a virus that expands its host range like this,” says Troy Sutton, a virologist at Pennsylvania State University who studies avian and human influenza viruses.
The damage to marine mammals and the dairy and poultry industries is worrying enough. But experts say the bigger concern is that these developments portend that the virus is adapting to mammals and inching closer to spreading among people.
Human pandemics are never inevitable. Sutton said that, at least for now, the changes in the virus are not indicative of H5N1, which has the potential to cause a pandemic.
Still, he said, “I don't really know how to interpret this or what it means.”
marine mortality rate
A highly pathogenic H5N1 strain was identified in Chinese waterfowl in 1996. The following year, 18 people in Hong Kong contracted the virus and six died. The virus subsequently subsided, but reemerged in Hong Kong in 2003. Since then, he has caused dozens of outbreaks in poultry, affecting more than 800 people who had close contact with the birds.
During that time, it has continued to evolve.
The version of H5N1 currently racing around the world emerged in Europe in 2020 and quickly spread to Africa and Asia. This virus killed large numbers of domestic birds, but unlike its predecessors, it also spread widely to wild birds and many other animals.
Most mammalian infections were probably “dead-end” cases. Perhaps the fox ate the infected bird and died without transmitting the virus. However, several large outbreaks suggested that H5N1 is capable of much more.
The first clues came in the summer of 2022, when the virus killed hundreds of seals in New England and Quebec. A few months later, it invaded a mink farm in Spain.
The most likely explanation, at least in mink, is that H5N1 has adapted to spread between animals. The scale of marine mammal occurrence in South America supports this possibility.
“Intuitively, I think mammal-to-mammal transmission is very likely,” said Malik Peiris, a virologist and avian influenza expert at the University of Hong Kong.
After the virus was first detected in South America in October 2022 in Colombian birds, it swept from the Pacific coast to Tierra del Fuego, the southernmost tip of the continent, and spread to the Atlantic coast.
In the process, hundreds of thousands of seabirds and tens of thousands of sea lions died in Peru, Chile, Argentina, Uruguay, and Brazil. The sea lion suffered convulsions, became paralyzed and acted erratically. A pregnant woman miscarried her fetus.
“When the virus moved to South America, something happened that we had never seen before,” Dr. Woohart said.
It's unclear exactly when and how the virus spread to marine mammals, but the sea lions likely came into close contact with infected birds or contaminated feces. (The majority of a sea lion's diet is fish, but they may also eat birds.)
At some point, the virus likely evolved to spread directly among marine mammals. In Argentina, the number of sea lion deaths did not match the mass mortality of wild birds.
“This may suggest that the source of infection is not an infected bird,” said Dr. Pablo Plaza, a wildlife veterinarian at the National University of Comahue and Argentina's National Council for Scientific and Technical Research.
It's not difficult to imagine how the virus could spread to these animals. Elephant seals and sea lions both breed in colonies, huddled together on beaches, fighting with each other, mating, and barking. Elephant seals sneeze throughout the day, releasing large droplets of mucus each time they sneeze.
It is difficult to prove exactly when and how the virus moved from one species to another. However, genetic analysis supports the theory that marine mammals, rather than birds, were infected. Virus samples isolated from sea lions in Peru and Chile and elephant seals in Argentina all share about 15 mutations not found in birds. The same mutation was present in a Chilean man who was infected last year.
There are many opportunities for H5N1 to jump from marine mammals to humans. A sick male elephant seal left alone on a public beach in Argentina for a day and a half has been found to be carrying a large amount of the virus. In Peru, scientists collected samples from a sea lion carcass lying next to a family enjoying a day at the beach.
Animals that eat carrion, such as dogs, can also pick up the virus from infected carcasses and spread it more widely. “None of the wild animals live in little silos,” said Wendy Puryear, a virologist at Tufts University who conducted the New England study. Seal outbreak.
In some South American countries, except for a few buried carcasses, the rest are left on beaches to rot or be scavenged.
“How do you remove 17,000 bodies in the middle of nowhere, in a place where you can't even knock down machines, and on a huge cliff?” Dr. Uhart said.
mutating pathogens
Influenza viruses are good at finding new mutations. When two influenza viruses infect the same animal, they can shuffle their genetic material and produce new versions.
It is unclear exactly how and to what extent the H5N1 virus has changed since it first appeared. One study last year showed that after the virus entered the United States, it rapidly mixed with other influenza viruses circulating here and mutated into different versions, ranging from mild to one that causes severe neurological symptoms. It was shown that there has been a change.
“After 20 years of reassortment, we now have a virus that actually works surprisingly well in all types of birds and mammals,” said the National Institute of Allergy and Infectious Diseases, which has been studying the necessary mutations. says virologist Vincent Munster. So that H5N1 can be adapted to humans.
Each time a new virus-carrying species emerges, H5N1 continues to evolve and has an opportunity to jump into people.
And viruses may encounter mutations that no one has thought of yet and break through the species barrier. That's what happened with the 2009 swine flu outbreak.
The virus lacked the mutations thought to be necessary to easily infect people. Instead, “there were other mutations that no one knew about or thought about before,” says Louise Moncla, an evolutionary biologist who studies avian influenza at the University of Pennsylvania.
Still, even if the virus were to infect people, “we might not see the mortality rate that we're really concerned about,” said Seema Lakdawala, a virologist at Emory University. “Pre-existing immunity to seasonal influenza strains may provide some protection from severe disease.”
what happens next
The United States is stockpiling vaccines and antiviral drugs to prepare for an influenza pandemic, but virus surveillance efforts may not be able to catch infections quickly enough to deploy those tools.
It took several weeks for farmers and then authorities to learn that H5N1 was circulating in dairy cows.
Due to the spread of infection at dairy farms, there has only been one mild case of human infection, but farms are a fertile place for the virus to jump from species to species, from cats to cows to pigs to humans, regardless of the order in which they are transmitted. It has become a rich soil.
Many scientists are particularly concerned about pigs. Pigs are susceptible to both human and avian influenza strains, making them the perfect mixing bowl for viruses to exchange genes. Pigs are slaughtered at a young age, and new generations who have never had influenza before are particularly susceptible.
So far, H5N1 does not appear to be adept at infecting pigs, but acquiring new mutations could change that.
“I'm one of those parents who never let my kids go to the state fair or animal farm,” Dr. Lakdawala said. “That's mainly because we know that the more interactions you have with animals, the more opportunities you have.”
If H5N1 becomes available to people, federal authorities will need to work with international authorities. Nationalism, competition, and bureaucracy can all slow the exchange of critical information during an ongoing outbreak.
Rick Bright, CEO of Bright Global Health, a consulting firm focused on improving responses to public health emergencies, said the current dairy cow outbreak is, in some ways, an opportunity to practice. Stated. But the U.S. Department of Agriculture only requires voluntary testing of cattle, and the results aren't as timely or transparent as needed, he said.
Dr. Rosemary Sifford, the department's chief veterinarian, said department staff are working hard to share information as quickly as possible. “This is considered an emerging disease,” she says.
Government leaders are typically cautious and want to see more data. But “given the speed at which the infection is spreading and the devastating disease it can cause if leaders hesitate and do not pull the right trigger at the right time, we are once again in a dire situation.” “You'll see it,” the doctor said. Bright said.
He further hinted at the virus, saying, “I believe that if we don't panic, if we respect it and take proper precautions, we can deal with it.”