First Cows, Now Cats. Is Bird Flu Coming for Humans Next?
With so much H5N1 virus circulating across the U.S., scientists worry we are a few mutations away from a potential human pandemic
Feb. 17, 2025
Bird flu is here to stay. The H5N1 avian influenza is proliferating among U.S. cows and there are now two strains circulating among mammals and birds.
Though there are only 68 confirmed cases in people—largely dairy workers—public-health officials think bird flu is likely more widespread. Last week, a Centers for Disease Control and Prevention report documented three asymptomatic cases in veterinarians who took care of cattle they didn’t know had H5N1.
What is clear is that people working on farms are sporadically contracting the virus. What hasn’t yet happened—and scientists believe has yet to happen in the history of the virus—is sustained human-to-human transmission.
Scientists think we could just be a few mutations away from that.
“If the virus figures out a way to adapt and start transmitting between humans, infections in the general population could go up exponentially overnight,” says Scott Hensley, a professor in the microbiology department at the University of Pennsylvania who is working on developing an H5N1 avian-flu vaccine.
We are also in the throes of one of the worst influenza seasons in recent years, raising the possibility of a new and more transmissible bird-flu strain emerging. If two viruses infect the same person—or cattle—there is the possibility of reassortment or a shuffling of gene segments that could result in a more infectious virus.
A pandemic isn’t inevitable and the current public-health threat remains low. Still, there are gaps and delays in federal surveillance and communication of bird flu. And the emergence of a new strain could give the virus more time to adapt to humans. Given that, here’s what to know about the virus and its spread.
Original strain
For the past year the B3.13 strain of bird flu has been circulating among dairy herds, wild birds and poultry. Most of the people infected were exposed to sick cows, birds or poultry and had mild illness—namely conjunctivitis, or red and swollen eyes.
A second strain of bird flu—known as D1.1—recently spread from wild birds and poultry to cows in Nevada. A dairy worker in Nevada subsequently tested positive for this strain.
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Seema S. Lakdawala, an associate professor in the department of microbiology and immunology at Emory University, says she was surprised by the second spillover of bird flu into cattle. “I think most of us thought it was a very sporadic event. But it clearly means that there’s a continued potential for bird viruses to spill over into cattle,” she says.
A federal program to test raw-milk samples for bird flu started in December and pools milk from multiple farms. If a test is positive, officials have to trace it back to a specific farm. This can take weeks and creates a potential public-health threat, says Lakdawala.
The D1.1 strain was responsible for the January death of a patient in Louisiana. This person was over 65 with underlying medical conditions and contracted the virus from a backyard flock of wild birds. A 13-year-old girl in Canada also became severely ill with the same strain last year and had to be hospitalized.
Neither of those cases resulted in any known transmission to other humans. But both resulted in the virus acquiring changes during the infections that enabled it to more easily infect human cells.
Scientists are worried
“This is a major threat right now,” says Hensley. “We know that it’s only one or two mutations away from leading to very severe disease. And influenza viruses acquire mutations all the time.”
Currently the threat of getting bird flu for the average American remains low unless you drink raw milk, work on a farm or handle dead birds.
Cats are also testing positive for bird flu and becoming severely ill or dying.
Suresh Kuchipudi, a veterinarian, professor and chair of the infectious-diseases and microbiology department at the University of Pittsburgh School of Public Health, was senior author of a December study documenting 10 domestic outdoor cats that died of H5N1 in South Dakota. It wasn’t clear how the cats contracted the virus, but some of the dead bodies had bird feathers around them.
Kuchipudi says experimental studies suggest cats can transmit the virus among themselves. This makes it theoretically possible they could infect a human.
“What makes the current situation really different is the force and extent of infection,” he says. “The amount of virus that is circulating and the number of animals infected at the same time is very high.”
Speeding up
The more hosts the virus infects the more likely it is to mutate, notes Kuchipudi. “With each new host, the chances of it picking up the mutations needed for human-to-human transmission become much higher,” he says.
The federal government has a limited stockpile of vaccines against bird flu that could be used if the virus were to start transmitting between people. But the process of ramping up production of the vaccine could be challenging because the flu vaccine is produced in chicken eggs.
Hensley says his lab has been working on an mRNA vaccine against bird flu. Moderna is also working on an mRNA vaccine. While mRNA vaccines are much quicker to manufacture, they still need to go through clinical trials before becoming available for commercial use.
This can take time. And time is of the essence.
“So if this virus does cause a pandemic, it likely will not be super slow, it will likely be a fast process,” says Hensley.
Write to Sumathi Reddy at Sumathi.Reddy@wsj.com
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Appeared in the February 20, 2025, print edition as 'What to Know About Spread of Bird Flu'.