Editor’s Note (5/22/24): Another case of bird flu has been detected in the U.S. A dairy worker in Michigan, who works on a farm where H5N1 has been found in cows, developed signs of conjunctivitis and tested positive on an eye swab. This is the second case in a person in the U.S. in the current outbreak. The dairy worker has recovered, and the Centers for Disease Control and Prevention says the risk to the public remains low.
A person in Texas has tested positive for the highly pathogenic avian influenza A virus (H5N1), also known as bird flu, the U.S. Centers for Disease Control and Prevention has confirmed. This individual, who was exposed to cattle that were believed to be infected with the virus, reported eye redness—a sign of conjunctivitis—but no other symptoms. The patient is being treated with antiviral medication and is recovering.
Avian flu has been ripping through farmed poultry and wild bird populations around the world in recent years. It has also infected mammalian species ranging from foxes, bears and seals to cats and dogs. And in recent weeks, infections have been found in cattle in six U.S. states: Kansas, Texas, Michigan, New Mexico, Idaho and Ohio.* There is no evidence of human-to-human transmission so far, and the CDC says the risk to the public remains low.
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The new human case is the second known to occur in the U.S. The first was in 2022, when a person in Colorado tested positive for the virus via a nasal swab after having direct contact with infected poultry. That patient reported mild fatigue and later recovered. Previous cases of avian flu in humans were deadly, but they involved a different form of the virus than the one that is currently circulating.
Scientific American talked to Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, about the latest human case and the risk to human health more broadly.
[An edited transcript of the interview follows.]
How worried should we be about the human bird flu case in Texas?
First of all, there’s some clarification here. Neither of these two cases [in the U.S.] has been an actual influenza infection as you think of a respiratory infection. They both have been basically either a nasal swab detection, for the previous one, or in the case of this one, a conjunctivitis—an eye infection. So this isn’t classic influenza at all.
The first one was just someone who was tested routinely. They were working in a barn, depopulating the birds that were dying from flu, and they had some mild symptoms, and they just got tested. They don’t know if the symptoms were related to it, and it could have been that the virus was just picked up in the nose because of just inhaling it in. The second case is a case of conjunctivitis. So that’s not, again, unexpected in that there are receptor sites in the eye for influenza viruses.
That’s reassuring. How likely is the virus to spread more widely among people?
The risk assessment from both [the World Health Organization] and CDC remains that there is low risk for humans with this virus, and I support that. If you look at the cases that have occurred since 1997, when it first emerged in human cases Hong Kong, through to mid-2015 to 2016, those years were by far the highest-risk situation. We’ve seen very few cases in humans since then. And part of that is that the virus has changed—it’s mutated to the point of where it is infecting other mammalian species and birds at an increasing rate. But we’ve had no evidence yet that that’s happened for humans or for pigs. And pigs would be the animal species that, to me, would be the sentinel that I’d be most concerned about.
Why is H5N1 causing milder human infections now?
This virus just is not infecting lower parts of the respiratory tract in humans at this point. Now that can all change in a heartbeat with additional mutations. But at least for now, it’s not an issue.
Previously there have been fatal cases of avian flu in humans in other countries, however. What does that tell us about the risk of human spread?
We’ve been here before. When we saw a major increase in transmission, in human cases, in Indonesia and Thailand and other countries in the 2003–2007 time period, the message was “This is going to be a pandemic virus soon.” Then nothing happened. Then, in 2012, I was at that time on the National Science Advisory Board for Biosecurity, and we had data presented to that group that essentially said, “We’re just one mutation away from the virus becoming a major pandemic virus.” It didn’t happen. Then, if you look at 2015, that was when we saw the major increase in cases in Africa—in Egypt, along the Nile River Valley, in people raising domestic ducks. That time there were [hundreds of] cases in Egypt alone that year in humans. And the thought was that, you know, “it’s just imminent now that we're going to see an influenza pandemic from H5N1.” Nothing happened.
And so I think that we’ve had lessons in the past to be careful about making the leap to “this is going to happen.” At the same time, you know, you have to keep your eye on it. With all the influenza viruses collectively, you have to sleep with one eye open every night if you’re worried about a pandemic.
Why were some of those earlier cases of H5N1 so deadly?
The virus was able to reach receptor sites and cells deeper in the respiratory tract and cause human infection. And it was obviously a severe issue. But even then, we had no evidence these were person-to-person transmitted cases. The exposures were [traced] back to the animals. We haven’t seen any evidence of sustained human-to-human transmission.
Did the current case in the U.S. likely involve a person becoming infected because of contact with a cow?
I can’t speak to which animal it was contact-wise, but that could very well be. There was some question as to whether or not there was also contact with birds on the same farm. So either way, it’s clearly an animal exposure.
How would we know if the virus was spreading more widely in people?
First, you have to have a confirmed case in a human of a respiratory-transmitted virus that then resulted in severe-to-moderate respiratory illness—and then you just start picking up additional human cases in that area.
Are scientists watching H5N1 closely to see how it evolves?
We’re going to be tracking the virus to see is it changing genetically and if the [avian flu group] 2.3.4.4b is the same virus we’re seeing here with the wild birds. In the humans, we have not seen any changes in the virus yet that would support that it’s now more human-adapted. That’s going to be one thing we’ll be looking for. The second thing is just looking at the epidemiology: Are there people who are in contact with potential cases? I say “potential” because we have not had a respiratory transmitted virus case yet in the U.S. So I think that will be important. If we see one, will we see others around it?
Could people become infected by consuming unpasteurized milk or undercooked meat from an infected cow or other animal?
Yeah, well, unpasteurized milk and cheeses I would surely avoid. But I’ve been saying that for 50 years for a whole lot of other reasons, too, public-health-wise. Surely, this is just one more addition to that. I think the second thing is, of course, that, at this point, what data we have supports that pasteurization will kill the virus in the milk.
But could you actually become infected by drinking milk containing the virus?
We don’t know. Will taking the virus down your throat, into your larynx, will that cause the exposure of the virus to cells with the right receptor sites? We don’t know that yet. It’s possible.
How is the fact that H5N1 is infecting cows and goats likely to affect the livestock industry?
I don’t know. I mean, we’ll know in the weeks ahead. This is the time of the year we expect to see migratory birds bringing the virus into many communities that have not seen the virus for months. And each time that happens it's kind of a throw at the genetic roulette table, and we'll see what happens. We don't know. But it's all the more reason why we need to be better prepared for the next pandemic.
At this point, again, the virus, as it is, I don't believe poses a major risk to humans. But that, as I said, could change overnight if there were mutational changes to the virus.
*Editor’s Note (4/11/24): This sentence was edited after posting to correct the description of states where H5N1 has been detected in cattle.