The H5N1 bird flu is a growing threat for farm animals and humans—how serious is it?

A concerning strain of bird flu, a virus called H5N1, is spreading in farm animals across the United States. And early this month, one person in Texas was infected with the virus after being exposed to a sick dairy cow. The rapid spread of H5N1 is sparking public health concerns across the globe—and not just among those who work on farms or near birds. 

That’s because the virus is creeping closer and closer to human populations; over 40 new non-avian mammal species have tested positive with the virus in the last two years, fueled by its vicious spread in populations of wild and domestic birds, including chickens. 

The Centers for Disease Control and Prevention says there are over 90 million cases in ​​wild birds, commercial poultry, and backyard hobbyist flocks in all 50 states. In chickens in particular, the mortality rate is 90 to 100 percent. Recently, in West Texas, over a million chickens were killed to prevent H5N1 from spreading rapidly. The last two years have been particularly bad—with the virus jumping from birds to other mammals, including mink, foxes, sea lions, and, as of 2024, livestock and a number of barn cats. The Food and Drug Administration (FDA) has also found inactive viral remnants of the bird flu in pasteurized milk, suggesting that pasteurization does its job of killing the virus and other bacteria, but that the virus has been spreading among dairy cows for much longer than presumed. The FDA released a statement saying that the viral remnants pose no threat to human health. 

The human case in Texas, which was reported as a mild infection, was not the first time a person has come down with H5N1: globally, there have been about 889 reported human cases and 463 deaths since 2003, according to the World Health Organization. But it was the first time the virus passed between a non-avian species to a person.

Nahid Bhadelia, founding director of Boston University’s Center on Emerging Infectious Diseases (CEID), has kept her eye on the evolution of H5N1 for years, and believes public health experts must launch into action now to stop the virus from adapting any further to humans. Bhadelia, a BU Chobanian & Avedisian School of Medicine associate professor of medicine, sat down with The Brink to share her insights about the ongoing situation, what needs to be done, and how cautious people should be. 

The Brink: This strain of avian flu has been circulating in nature for decades. Why is it suddenly causing a public health scare now?

Bhadelia: The reason it is of concern right now for public health authorities is that we’re seeing many more species, particularly mammalian species, infected with this virus than have been before the last two years. If a virus can adapt to a lot of different hosts—particularly hosts that are closer to human physiology—then it is more likely to adapt to us. There’s also been more animals dying from it, like seals in Peru and New England harbor seals. There are infections popping up in many more countries and continents. That means the virus is getting better at killing off large mammalian species in more places. And every time it infects a new individual mammal, it gets another physiology to learn from, which might have a viral mutation that gives it the benefit to potentially adapt to humans. Now, in the particular incident with the dairy cows, it’s coming physically closer to people—a lot of people interact with dairy cows. So, the idea that the virus has moved closer to domesticated animals sets it up for more potential exposures to humans. Also, new research shows that the virus may have made a jump from birds into cows in December 2023 and has since been transmitting between cows, suggesting this outbreak may be more widespread than we currently think.

It sounds like a lot of people globally have gotten sick or died from H5N1—has the virus already been spreading from birds to humans?

Yes. There was a cluster of H5N1 cases in Cambodia last year, and that was spread from poultry to humans. This happened from wild birds mingling with domestic birds. The driving forces for this can be traced back to farms developing and replacing wetlands. There are a lot more chances for geese and other wild birds to interact with domestic poultry—they get sick, and then humans who are around the sick poultry get the infection. And it’s evolving. The CDC has an article that talks about a couple of the mutations that have occurred that have made it more adaptable to mammalian species. Since the burden of infection in birds is so high, more mammalian species encounter birds that are sick. We now need to know whether there’s been a lot of mammalian-to-mammalian transmission occurring, as it appears might be happening between cows. It’s when the virus becomes so good at transmitting between mammals—or worst-case scenario, between humans—that it can have a chain of transmission like the way we saw with COVID.

How worried should the general public be about H5N1?

I agree with the CDC’s assessment that the general risk to the general population is low. I recently did a tweet thread about why this is concerning—which is that there are signs this is a virus on the runway to getting adapted to humans. For H5N1, the signs are all there. And there is a risk to dairy workers and other people who work on farms close to infected animals, which is why the CDC has released new guidance on how they can protect themselves. I don’t think this is something the general public needs to worry about right now; the public health experts, and those who fund public health programs—Congress, in particular—need to pay attention. Public health folks need to work on controlling and testing right now.

So, what needs to be done for this to be controlled?

In my mind, a true public health success would be to keep H5N1 from adapting to humans any further. And we can. To avoid situations where this virus can infect more people, we need widespread testing to better understand how the virus is presenting in individual animals, and how it’s getting transmitted from bird to bird, bird to mammal, and mammal to human. We also need to be ready to manufacture a greater number of H5N1 vaccines that are in the national stockpile.

For the person who got sick in Texas, which sounds like it was a mild case, did they get sick from being in close contact with an infected cow?

Yes. The CDC technical summary says that it was the first known mammal-to-human transmission of H5N1.

Do we know how it spreads then, like through exposure to air droplets or surfaces?

Generally, influenza viruses spread through contact and respiratory droplets within one to two feet. Seasonal influenza is not good at spreading through longer distances—I [recognize] we said that about COVID at first, and then found out it was traveling in the air. But from everything we currently know about most influenza viruses, it’s most efficient at transmitting through close contact or through contacting bodily fluids on surfaces. The USDA suspects a lot of cow-to-cow transmission may be through contaminated milking equipment. That’s because there’s a lot of virus in the milk and the mammary glands of the cows. But the only way to know this for sure is by testing. There needs to be testing of every cow, even if they’re asymptomatic. And then, a deep genetic analysis can show which animal a disease jumped from. That’s how we really can figure out the spread. More data about dates and locations of infections and related sequences would provide understanding about how this virus is spreading between mammals and birds, between mammals themselves. There are other questions, like how long does the virus stay in the mammary glands of the cows? How long would the cow shed virus? The USDA just released an order that requires farms to test lactating cows before transferring them across state lines. But there isn’t enough testing in humans or in animals being done.

I’m guessing the flu shot won’t protect people from H5N1?

Seasonal flu vaccines are different, yes. However, I believe that the more flu vaccines you get over your lifetime, the more repertoire of antibodies you have and, maybe, if there’s a new flu in the future, your body potentially created some antibodies that are more effective. Having an avian influenza vaccine is something scientists have been investigating for a long time, and we have a certain number of stockpiled avian flu vaccines like we do for other diseases, and testing is currently underway to confirm how good of a match these are to this particular clade [type] of H5N1.
 
There are flu vaccines for poultry, but the problem with vaccinating chickens is that producers can’t sell them on the international market. If you vaccinate chickens, then the antibodies for the avian flu are positive, and the chickens that test positive for the antibodies can’t be sold. The USDA recently said they’re going to work on a vaccine for cows too. It might take a while, but it’s a good idea, just in case it gets to beef cattle. The CDC passed guidance about what people can do in the meantime, which includes staying away from dead birds, and not eating undercooked meat or eggs, and raw milk or cheeses.

Is there worry that this virus can impact the food supply?

There is concern for the virus to impact not only dairy cows, but beef cattle. As far as milk consumption goes, pasteurization kills the virus. Pasteurization is a process that gets rid of all bacterial and viral diseases so that we can consume milk. But there’s a growing human practice of consuming raw milk. Even after guidance went out that said not to drink raw milk, I’ve heard that rates of drinking raw milk have gone up. So, the idea that people could get infected with raw milk that hasn’t been pasteurized, with the virus in it, is another concern. Agencies are keeping an eye on other downstream products, including cheese and beef.

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