Just when you stopped regularly hearing about COVID-19 in the news, another infectious disease began to dominate headlines this spring: bird flu.
The good news is—for now, at least—public health experts don’t see this latest bout of avian influenza evolving into the likes of the coronavirus pandemic. But given the disease has spread to poultry in 48 states, dairy cows in nine, and two people in Texas and Colorado, you may be worried about its potential impact on your health.
Fortune spoke with a trio of epidemiologists, who below answer critical public health questions about bird flu.
What is bird flu?
The type of bird flu that’s currently circulating is a highly pathogenic avian influenza (HPAI)—a disease that can seriously sicken wild and domestic birds, posing a major threat to the poultry industry and thereby the global economy, according to the U.S. Department of Agriculture (USDA). The other kind, low pathogenicity avian influenza, causes mild infection in wild fowl but can turn into HPAI in poultry.
Influenza viruses are divided into four types, A, B, C, and D, explains Edwin Michael, PhD, an epidemiologist at the University of South Florida College of Public Health. What we think of as the seasonal flu in humans is caused by types A and B. Bird flu falls under the umbrella of influenza A viruses. The strain currently spreading in the U.S. is H5N1, an HPAI named for proteins on the virus’s surface.
“It can spread very quickly through the bird population—wild birds. That can spread it all across the world,” Michael tells Fortune. “From the bird, the virus is shed in mucus, saliva, feces, and then that can go and get into domestic poultry.”
When HPAI strains reach domestic poultry, they can kill entire flocks within days, the USDA says. Birds with HPAI infection may show a slew of symptoms including diarrhea, lack of energy and appetite, and a drop in egg production. Because HPAI can’t be treated, poultry depopulation is the sole solution.
Such culling can be effective in curtailing the spread in birds crucial to U.S. agriculture and food production, says Dr. William Schaffner, a professor in the Division of Infectious Diseases at Vanderbilt University Medical Center.
“This particular bird flu virus, in its variants, has actually been around for a long time—over a decade,” Schaffner tells Fortune. “It’s been gradually spreading but recently, somehow, for reasons that at least I don’t understand, its spread around the world has become much more prominent and much more widespread.”
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How does bird flu spread to humans and other mammals?
H5N1 is a shapeshifter, so to speak, due to its segmented DNA, Michael explains.
“For example, if a human gets infected with a bird flu and also carries a human influenza A virus, these two viruses can exchange genetic material. This is known as genetic shift,” Michael says. “That can form very new viruses [and] cause epidemics.”
Flu pandemics are rare, though, occurring roughly three times a century, Michael says. Perhaps the most notable is the so-called “Spanish flu” of 1918–1919, when the Centers for Disease Control and Prevention (CDC) estimates “an avian-like H1N1 virus” killed 50 million, and possibly as many as 100 million, people worldwide.
You probably remember the swine flu pandemic of 2009, caused by a previously unknown H1N1 virus that contained swine, human, and avian genes. How did that happen? Because bird flu viruses don’t always effectively spread among humans, they sometimes need help from other species to thrive, Schaffner explains.
“The pig’s respiratory tract is set up in such a way that it can accept bird flu infections and—and—infections with a human virus,” Schaffner says. “You could think of the pig as a test tube into which goes a bird flu and a human flu, and if that happens simultaneously in the same pig, then those two viruses get together and can have the capacity to exchange genetic elements.”
While the latest H5N1 strain isn’t known to spread among humans, it has already spilled over to more than a dozen species of wild mammals, from black bears to a bottlenose dolphin. As recently as May 3, red foxes in New York and Michigan tested positive, according to the USDA’s Animal and Plant Health Inspection Service (APHIS).
“This bird flu virus is not relying on pigs but is mutating on its own to become a more effective infection in a variety of different mammalian species,” Schaffner says. “The one that has been in the news is dairy cattle, which surprised every virologist.”
When did bird flu most recently appear in the US?
Government agencies have been monitoring the latest bird flu outbreaks since Jan. 12, 2022, when a Northern Shoveler duck in Hyde County, North Carolina, tested positive for a Eurasian strain of H5N1.
Among wild birds, the disease had spread throughout more than 1,100 counties as of May 8, 2024, infecting nearly 9,400 fowl.
The situation is worse in U.S. poultry. As of May 7, nearly 91 million birds, including commercial poultry and backyard flocks, had been infected. This encompasses more than 1,100 outbreaks in 522 counties across 48 states.
APHIS recorded the first poultry infection in a commercial turkey flock in Dubois County, Indiana, on Feb. 8, 2022.
Though H5N1 has been circulating nationwide in wild birds, domestic poultry, and mammals since 2022, you may have only been hearing about it recently because of its jump to dairy cattle. The first positive case was documented in Texas on March 25, 2024.
As of May 7, nearly 40 dairy cattle herds had been infected across nine states.
The logistics of modern industrial farming may be fueling the H5N1 spread to cattle, Michael says.
“Look at the density of animals in those farms—those are not natural settings,” Michael tells Fortune. “So as soon as you get [an infection], these things will spread very quickly among farm animals.”
Michael adds, “We have to shed a light on how farming is done. That’s the trade-off, you want cheap meat and all the rest of it, but then you farm animals in this way and you’re opening the door up for other things.”
Is it safe to drink milk and eat chicken?
The Food and Drug Administration (FDA) acknowledges the presence of bird flu in dairy cows is “a novel and evolving situation.” That said, no previous studies have been done on the effects of pasteurization—a sterilization measure that kills harmful bacteria—on HPAI in bovine milk. However, the FDA says existing research has informed its understanding and conclusion that milk consumption is safe.
The FDA and USDA are conducting a national commercial milk sampling study, which includes milk-derived baby formula. As of May 1, no HPAI had been detected in any formula products. While bird flu had shown up in milk, sour cream, and cottage cheese samples, pasteurization inactivated the virus.
“There is no need to be concerned about milk, eggs, [and] chicken as sources of infection,” Schaffner tells Fortune. “I’m out there drinking milk, and we had chicken last night for dinner.”
Both Schaffner and the FDA urge the public to steer clear of raw, unpasteurized milk. The CDC recommends cooking eggs and poultry to an internal temperature of 165 degrees to kill bacteria and viruses, H5N1 included.
How worried do you need to be about H5N1 affecting people?
Simply put, don’t panic, says Michael Osterholm, PhD, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
“There’s no evidence yet at this point that this is an imminent risk to humans,” Osterholm tells Fortune. “We’ve not seen it cross over to humans in a way that would support that it’s going to be the next pandemic virus.
“On the other hand, a reassortant event, or continued mutations, could occur tomorrow.”
Two humans have so far caught H5N1, according to the CDC. A person exposed to dairy cattle in Texas tested positive April 1, 2024, and a person in Colorado involved in culling poultry tested positive April 28, 2022. No human-to-human transmission has been confirmed, and the more recent patient’s only symptom was conjunctivitis, or pink eye.
“We do have two, three receptor sites in our eyes, and so having a case of conjunctivitis would not be considered unusual,” Osterholm says. “There was no evidence of any kind of respiratory infection, which is the key piece for serious illness and then being able to transmit the virus.”
The Texas case marks the first time a human has been infected with H5N1 through contact with a mammal, according to the World Health Organization (WHO). Even so, WHO considers the public health risk to the general population low. For farmworkers and other industry staff likely to be exposed to the virus, the risk is low to moderate.
Going forward, assuring the health and safety of agriculture workers is paramount to keeping H5N1 from spreading to the wider population, Michael says.
“I don’t think this one is going to be a major outbreak (in humans),” Michael says. “[But] the risk is always, always there because of the way we manage nature.”
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