In late January, U.S. health officials announced that a new strain of avian flu, H5N9, was discovered on a duck farm in Merced County, California. They quarantined the farm, and destroyed nearly 120,000 birds.
Disclosure of this new strain comes amid rising concern about the earlier strain, H5N1, which first appeared in the U.S. early last year. That outbreak has infected dairy cows and poultry farms around the country, causing thousands of animals to be killed. It continues to spread.
The good news is that there have been fewer than 100 cases in humans around the world, with one death. The cases have primarily occurred among workers exposed to infected animals. The Centers for Disease Control and Prevention says no cases of bird flu have yet been passed from person to person in the U.S.
But that could change, and quickly, if the virus mutates into a strain that is more easily spread among humans.
UC Berkeley School of Public Health’s John Swartzberg, Health Sciences Clinical Professor Emeritus, Infectious Diseases and Vaccinology, opens up on the risks to humans and livestock.
What is bird flu?
Bird flu, or avian flu, as it’s also known, is an influenza that we call a zoonotic infection. That is, it’s a virus that primarily infects animals and especially birds. Birds are a major reservoir on this planet for influenza.
Unfortunately, influenza can change itself genetically very easily, and so it spins off, often, lots of different variations of itself. And sometimes those variations wind up being able to infect not only birds, but other animals as well. A couple years ago, there was a devastating outbreak of bird flu in seals that devastated the seal population, especially in South America.
What are you seeing now?
Bird flu is changing genetically to infect other animals. We’re seeing a lot of influenza in birds, and it’s been decimating the bird population here in North America. A lot of the birds we get eggs from contract it—that’s why egg prices have gone up. But it’s also spilled into other animals here in North America, especially cows in dairy herds. Once it’s in the cows, it is able to spread to other animals, including humans.
Does it get into milk from dairy cows?
Well, what probably happened was in a particular dairy herd in Texas, around a year ago, the dairy cattle got infected from wild birds. Dairy cattle are often shipped to different dairy herds in the same state or to different parts of the country. So, that was a spillover event, from wild birds in Texas to a dairy herd. From there, as it went from cow to cow, as they shipped cows to different parts of the state or the different parts of the country.
From there, it spread to the rest of the country. It was a spillover event from a wild bird in Texas to a dairy herd.
Was that surprising?
Yes, we didn’t think cows were a very typical host for it to spill over to. The second thing that surprised us was that it infected the udders of cows. We never heard of that before. It turned out that there are sites on the cells in the udders of cows that the virus can attach to reproduce. We didn’t know that.
So, it reproduces in the udders of cows, which is brilliant from the virus’ perspective, because that means that the virus spills into the milk that the cows are producing, and then the milk becomes a vehicle to transmit it to other animals, or more cows.
How did it get into cats?
It went from the dairy cows to the raw milk, to the cats. And that leads to one of the concerns that we have about spillover to humans, if we consume raw milk.
Is pasteurized milk safe?
Yes, pasteurization kills the virus. We’ve done lots of studies now and the good news is that if we consume pasteurized milk, there’s no worry about getting influenza from it.
Dairy workers are exposed to the milk from the cattle they’re working with, so they’ve been exposed to the virus that way, and they’ve gotten avian flu. Fortunately, none of these people have gotten very sick, and some of them didn’t get sick at all. We just know they’ve been infected because we checked their blood, which showed evidence of the infection. So, we know it can spill over from dairy cattle to dairy workers. But to date, there’s been no transmission from a dairy worker to another human being. But that is something that could happen.
Is the strain in cattle the same as in birds?
A different type of avian flu is also infecting these large commercial breeding grounds for birds. That’s where we’ve lost tens of millions of birds in the United States. That is a different strain of influenza than the strain that is infecting cattle.
There are two humans that have contracted that strain, the one that the birds got. One was a 13-year-old girl in British Columbia, who nearly died. I don’t know if she’s out of the hospital yet. It’s been well over a month and she was on a ventilator for a good while. She is alive and getting better. She should recover.
What about the second human case?
This was in a man over age 65 in Louisiana. He died after about a three-week illness. This is the only death to date in the United States from this strain of influenza. For both the man and the girl up in British Columbia, there was no evidence that they transmitted it to another human being.
What misinformation are you reading, or hearing, that you would like to correct?
One thing is about raw milk. There are some people like RFK, Jr., who are “believing” without scientific foundation that drinking raw milk is more healthful. We’ve known forever that drinking raw milk is hazardous because of all the microbes you can consume. It’s led to lots of people getting sick and some people dying.
And that’s before bird flu. We know that there can be an enormous amount of bird flu virus in raw milk, and other mammals that drink raw milk have gotten sick and died from it, for example, cats.
It seems very imprudent that anybody should be drinking raw milk. So, I think one of the myths is that it’s safe to drink raw milk. It’s definitely not safe to drink raw milk, not only because of the possibility of bird flu, but because of all the other pathogens you can get.
Any other myths you’d like to correct?
We have not seen any transmission from eating the flesh of cows. But it would seem prudent that if you’re going to eat beef, it should be cooked sufficiently well to kill any virus. And we know that the standard recommendations that have held for the years that heating meat to 165 degrees is sufficient to kill the virus. So, there’s no danger in eating meat, but it should be cooked.
Is it safe to eat cheese?
If it’s made from pasteurized milk, there’s no concern. If it’s made from unpasteurized milk, there’s a theoretical concern that even with aging, it may not kill all the virus. But we’ve not seen any evidence of cheese that is made from non-pasteurized milk spreading the virus. So, there’s still a question mark attached to that.
Are we heading toward another pandemic?
I think the biggest myth is that bird flu is going to lead to another pandemic. That’s not necessarily a myth, but the honest answer is, no one knows.
It’s important to recognize that we’ve known about avian influenza strains for close to 30 years. We’ve been following them very carefully since then. They’ve killed maybe 900 human beings, and none of those cases has led to a massive outbreak.
It may be that there are reasons that we don’t fully understand, why the virus is not able to make the leap to being effectively transmitted human to human. We just don’t know.
The only honest answer is that we are concerned about what’s going on in cattle and birds. We are certainly concerned about the possibility that it could spread to humans and become humanized, and lead to a pandemic, although I certainly don’t think anybody should be panicking about it.
We should be diligently studying this much better than we are, and trying to control it much better than we are.
What do you mean by “humanized”?
We’ve been talking about strains of virus in birds, but we know there are strains of influenza that have become humanized over centuries, which have led to major outbreaks. These are strains of influenza that at one point came from birds to humans, either directly or intermediately, through another host, like a pig—and then to humans.
Once they infected humans they remained almost exclusively a human virus and they kept circulating constantly around the planet. We’ve had big outbreaks. In the 20th century we had three major outbreaks. The biggest, of course, was in 1918-1919. No one knows how many humans were killed, but over two years, probably the best estimate is that it killed 50 million people.
For COVID, terrible as it was, the official number is a little over seven million humans, but most people think it’s probably at least three times that—probably around 20 to 25 million people killed. But this was over five years.
You mentioned we should be studying avian flu more. What should be done and who should be doing it? Is this a USDA thing or CDC or both?
Well, that’s part of the problem. The USDA is charged with two responsibilities that sometimes work in concert and sometimes are diametrically opposed. One of the responsibilities they have is to assure a healthy agricultural industry for the United States. The second responsibility is to assure safety of the human beings who consume agricultural products in the United States.
With this bird flu problem that spilled over into dairy cattle, there was a lot of pushback from the dairy owners against doing a lot of the epidemiologic work that needed to be done on the dairy farms. USDA was very slow in moving things forward, very slow.
And I think what we’ve seen with this bird flu problem is that the USDA is tilted in favor of protecting the industry, as opposed to protecting the health of humans. CDC is also involved, but the CDC has no authority to go into states and tell them what to do. It has to be done state by state.
So the CDC can tell California what they think we should be doing, but the California Department of Public Health is the entity that has to make the decision. We’re fortunate here in California to have a very proactive public health oriented CDPH, and it’s doing a very good job here in the state, but it’s still hampered somewhat by the problems we were previously talking about. And that was one of the reasons that the governor called it a state of emergency regarding bird flu here on December 18, 2024.
There are other states like Colorado, Wisconsin, and Minnesota, that have also done a very good job. There are other states that have done a lousy job, like Texas.
Should pet owners be worried about this at all?
People should not feed their pets raw milk or any raw meat.
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