What we know about the deadliest U.S. bird flu outbreak in history

The U.S. is enduring an unprecedented poultry health disaster, with a highly contagious bird flu virus triggering the deaths of some 52.7 million animals.

The culprit is highly pathogenic avian influenza, or HPAI. It has ravaged farm flocks and chicken yards in 46 states since February, when the first cases were reported in commercial flocks.

It's the worst toll on the poultry industry since 2014-2015, when more than 50 million birds died. That earlier outbreak also started in the winter — but while that ordeal was over by the following June, the current outbreak lasted through the summer and has surged anew.

"I'm hopeful that this is not the new normal for us," Richard Webby, director of the World Health Organization's Collaborating Center for Studies on the the Ecology of Influenza in Animals, tells NPR.

Some birds have died from the disease itself, but the vast majority are being culled through flock "depopulation," to try to stop the virus from spreading. That includes millions of chickens and turkeys in barns and backyards that had been raised to provide eggs or meat.

Here's what you need to know about the 2022 outbreak in the U.S.:

52,695,450 million birds have been wiped out

The losses stretch across the U.S., and they're deepest in the country's middle: More than 1 million birds have been killed in each of 11 states that stretch from Utah to the Midwest and on to Delaware, according to the U.S. Department of Agriculture.

In Iowa, two massive egg-laying operations had to cull more than 5 million birds in single incidents earlier this year.

Unlike the 2014-15 outbreak, this one is being driven by wild birds, not by farm-to-farm transmission. For commercial and backyard flocks, many early infections centered along the intersection of the Central and Mississippi flyways of migratory wild birds. As those birds traveled, so did the virus.

"We don't know exactly what it is about it, but it does seem just to be able to grow and transmit better in wild birds," Webby, who is also a member of the infectious diseases department at St. Jude Children's Research Hospital in Memphis, tells NPR.

"Wild birds are the perfect mechanism to spread a virus because they, of course, fly everywhere," he adds.

Influenza viruses are common among wild aquatic birds, which often show no symptoms despite being infected. In January, the dangerous H5N1 flu virus was found in an American wigeon duck in South Carolina — the first U.S. case since 2016, according to the Centers for Disease Control and Prevention. More reports poured in over the following weeks and months, raising alarms as the virus spread to more states.

A highly pathogenic avian influenza virus "can cause disease that affects multiple internal organs with mortality up to 90% to 100% in chickens, often within 48 hours," the CDC notes. "However, ducks can be infected without any signs of illness."

Avian flu poses only a low risk to humans

It's very rare for a human to be infected with the avian virus. The first U.S. case of a person infected with avian influenza A (H5N1) virus was reported in April in Colorado. The patient recovered after experiencing a few days of fatigue.

The virus does not pose a special risk in the nation's food supply, given proper handling. The CDC states that like any poultry or eggs, heating food to an internal temperature of 165˚F kills any bacteria and viruses present, including HPAI viruses.

The virus is like a kid in a candy store

"The bird populations haven't seen viruses like this before," Webby says, "so in terms of their immune response, they're all immunologically naïve to this" influenza virus.

"Right now, it's like a kid in a candy store racing around," infecting bird populations, he says.

U.S. experts had been bracing for an outbreak, watching successful strains of the H5N1 influenza virus proliferate in Europe and elsewhere. Now that the virus is here, it shows no sign of going away.

A key part of the challenge, Webby says, is that like the SARS-CoV-2 virus, the avian flu virus has spun off several variants of concern. And right now, a specific version of the virus — known as clade 2.3.4.4b — is ruling the roost.

"You can think of that like an omicron variant" because of its wide prevalence, Webby says.

Viruses are notoriously quick to mutate. And since its arrival in North America, the avian influenza virus has continued to change.

"When the virus came over into the Americas, it started to interact with the viruses that we have in our wild birds here," picking up different combinations of other genes, Webby says.

Comparing the current U.S. virus to the one in Europe, Webby says, "From the outside looking in, they look very similar. But when you actually go on the inside and take a look, the viruses we have here are quite different now from what was in Europe."

It's possible that wild bird populations will build up an immunity to the virus — but Webby warns that it will take months to understand whether that is happening at a meaningful level.

The outbreak hasn't raised all poultry costs

If you like to eat chicken, you're in luck. This version of the influenza virus doesn't affect "broilers" — chickens raised for meat — as badly it does "layers" — table-egg laying hens — and turkeys.

"For whatever reason, turkeys and layer birds tend to be more susceptible" to the virus, says Amy Hagerman, an assistant professor at Oklahoma State University who specializes in agricultural economics.

"The chicken that most people think of, their chicken tenders, their chicken sandwiches, all of those things haven't tended to have the same kinds of impact," she adds.

Hagerman warns that in a time of inflationary pressure and supply-chain snags, it can be hard to directly link a price hike to the virus. But she notes that U.S. egg prices can be affected if just a few farms have to dispose of their flocks.

"Generally speaking, these complexes are over a million birds, easily," Hagerman says. "It takes fewer egg-laying operations being affected by HPAI to drive up the price of eggs and egg products," she adds, especially since the majority of U.S. production goes to the domestic market.

The virus has hit many turkey farms — but because those operations tend to be smaller and the cases have been spread out over time and space, producers have mostly been able to absorb the losses, building up stocks of frozen turkey ahead of the end-of-year holidays.

"So, yes, we certainly saw an increase in turkey prices in this holiday season," Hagerman says, "but not as much as we might have anticipated given the extent of this outbreak."

What about vaccines?

The presence of the virus in the commercial food chain raises a number of possible trade headaches — but so would using a vaccine to fight it.

"A lot of countries don't use vaccines for this virus in their poultry," Webby says.

"One of the big complications is timing on a vaccine," Hagerman says. "Generally you need two doses of a vaccine and then a length of time to achieve full effectiveness."

"If you have a bird that has a very short feeding window before it's ready for harvest, that can be a lot more challenging because you also need to allow the withdrawal period after the vaccine before the bird is harvested," she adds.

Another central issue is the difficulty of surveillance — of knowing whether a bird is infected with a deadly influenza virus but isn't showing symptoms because they've been vaccinated.

But the calculations might change if the virus is determined to be endemic in wild birds or in a geographic area.

That "certainly seems to be the case in Europe and Africa," Webby says, adding, "my gut feeling is we're headed that way in the Americas as well."

"These are actually discussions that are going on now," Webby says, describing ideas such as what kind of post-vaccination surveillance would be needed to "make sure your trading partners are happy that the virus is not circulating silently."

The last outbreak didn't survive the summer. This one did

Experts say poultry farms should be credited with limiting the virus as much as they have, hailing the success of surveillance and biosecurity programs. But nearly 11 months after the first known wild case in the current outbreak, the deadly influenza virus is still here.

"Generally, when the weather gets hot, influenza goes away for the most part," Hagerman says.

That was the case in the 2014-2015 outbreak — which came when Hagerman worked at the U.S. Department of Agriculture's Animal and Plant Health Inspection Service, or APHIS.

"Bright sunshine and heat kills the virus in the environment," Hagerman says, describing how summer weather helped end the earlier outbreak.

"This time we didn't see that virus circulation going down to zero in our wild bird population" over the summer, Hagerman says. Instead, the virus sort of simmered through the summer months, she adds, "and then we get into the cooler, wetter months of the fall and we see a resurgence."

The long outbreak is discouraging, Hagerman says: "If we look at Europe, we can see that they are on two years of HPAI outbreaks."

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