What’s Bird Flu, Why Is It Killing Chickens, and Should We Be Worried?

Experts say that the risk of such a pandemic remains remote, but that governments should take steps to prepare just in case. As for the impact on U.S. chickens and other poultry, much depends on what happens over the next few months, as wild birds that carry the virus migrate across states where chickens are farmed.

Here are answers to the big questions about the avian flu, and what it means for us.

What is an avian flu?

Different influenza viruses can infect all sorts of animals, from dogs to pigs to cows. One type of influenza virus, known as Influenza A, commonly infects both people and birds. Strains of Influenza A found in birds are known as avian flu viruses, or bird flu. Avian flu viruses have crossed over into humans in the past, and wreaked significant havoc: the 1918 influenza pandemic, which killed 50 million people, was caused by an avian flu virus.

Most of the avian flu viruses circulating don’t make the birds they infect very sick. Some strains however, tend to kill farmed poultry very quickly. H5N1, the virus now spreading in wild birds, is one of those more deadly strains, and has been causing sporadic outbreaks since the late 1990s.

How worried should we be about this avian flu causing a human pandemic?

“It’s like asking how likely is it that the San Andreas fault will rupture catastrophically in the next few weeks and devastate Los Angeles,” says John P. Moore, a professor of microbiology and immunology at Weill Cornell Medicine. “At some point, there will probably be a serious earthquake in California. At some point, it is possible that a pandemic bird flu virus may pass into humans. But we’re not there.”

Experts have been concerned about the potential threat from H5N1 for decades, and have been watching it closely. They say that things aren’t dramatically different now, in terms of the risks to humans, than they have been. This “basically seems to be the same thing we have been facing since the early 2000s,” says Stephen Morse, a professor of epidemiology at the Mailman School of Public Health at Columbia University. “And it’s maybe a notch up, but it’s not a big leap.”

That said, the bout of H5N1 currently impacting the U.S. poultry industry has circulated more widely in wild birds than in previous outbreaks, and seems to be infecting some mammals.

“I’m a little more worried now,” says Vincent Racaniello, a professor of microbiology and immunology at Columbia University. “The more the virus reproduces, the more chances a mutation is going to emerge that will allow transmission to people.”

Racaniello said he is a “little worried” that the virus seems to be acquiring some limited abilities to transmit to mammals. “I wouldn’t say this is the next pandemic,” he says. “But it probably merits keeping a close eye on.”

Why do so many wild birds carry the H5N1 virus right now?

The current outbreak in wild birds stems from the emergence, three years ago, of a new family of H5N1 viruses, known as clade 2.3.4.4b. That clade of viruses reached North America in 2021, and is now widespread among wild birds here. Avian flu has been detected in the U.S. in 2022 and 2023 in crows, storks, robins, ravens, owls, egrets, and many other bird species, according to records kept by the United States Department of Agriculture.

According to Dr. Timothy McDermott, an assistant professor and extension educator at the Franklin County, Ohio office of The Ohio State University Extension, it’s largely geese and ducks that are carrying the virus in the U.S. Poultry and wild raptors that have the highest mortality rates once infected.

How many chickens have gotten sick in the U.S.?

Domestic chickens and other poultry get infected when they come into contact with wild birds, or with their droppings. In 2022 and 2023, avian flu has been detected in 321 commercial poultry flocks and 468 backyard flocks across the U.S. When H5N1 is detected, the whole flock is killed as quickly as possible; nearly 59 million birds have been destroyed.

The worst months for the commercial chicken producers were in March and April of last year, when 21 million and 15 million birds were destroyed, respectively. This year, so far, has been milder: Only 41 commercial flocks have had confirmed cases in the last thirty days, and only 330,000 birds were destroyed in February.

The question now is whether the spring migration season will bring more cases to the poultry farms.

“We’re watching to see what level, if any of, detections we will start to get in 2023,” McDermott said. “The thing that we are concerned about would be, is there still a circulating reservoir of the virus that we would see move from the wild bird population into either commercial or backyard flocks?”

What is being done to keep chickens safe?

Commercial farms have implemented biosecurity practices, meant to keep wild birds and their viruses away from the poultry. That includes sanitizing workers’ hands, sanitizing their boots, keeping birds inside when possible, and barring farm workers from owning other birds.

Can you still eat eggs? What about chicken?

Yes. “For those of us who are buying chicken in the grocery, buying eggs in the grocery, this is not a concern,” Morse says.

Proper handling and cooking of poultry products remains important, of course. The CDC says that, while there is “no evidence” of any person ever contracting bird flu from eating poultry products that have been cooked properly, “uncooked poultry and poultry products (like blood) could have been the source of a small number of bird flu virus infections in people in Southeast Asia.”

Does H5N1 make humans sick?

Not often. H5N1 infections in humans remain extremely rare, though when they happen, they’re often deadly. There have been around 900 identified infections globally since 1997, approximately half of which have resulted in death. There have only been a handful of cases since 2020, including one case in the U.S. in 2022. Last month, two infections were detected in Cambodia, setting off public health alarms. One of the patients, an 11-year-old girl, died. Officials say that the cases were not caused by H5N1 viruses of the type currently circulating in birds in the U.S.

The good news is that the virus is very bad at transmitting among humans. It seems to have happened a few times: According to the Centers for Disease Control and Prevention, there was likely very limited human-to-human transmission of H5N1 in 1997 in Hong Kong, and in 2004 in Thailand, among a handful of other instances.

In general, however, H5N1 has not passed easily from person-to-person, a prerequisite for causing a human pandemic.

Isn’t the human death rate 50%?

It is, though a theoretical version of the virus that acquired the ability to transmit efficiently among humans might not be nearly so deadly. “When viruses acquire high transmission potential, they tend to lose virulence,” says Racaniello.

Even if H5N1 were to adapt to infect humans, it wouldn’t necessarily cause a global pandemic overnight, as Covid-19 did. Covid-19’s rapid spread was due to the large number of asymptomatic infections it caused, and the large amount of viral shedding before people grew symptomatic. That allowed seemingly healthy people to spread the virus, which made it particularly challenging to contain.

When SARS, a similar virus to the one that causes Covid-19, spread in the early 2000s, it had a case fatality rate of 10%. That’s far higher than that of Covid-19, but SARS had only limited spread. “When those people started getting sick, they ended up in the hospital,” Racaniello says of the SARS patients. “It was pretty easy to limit the transmission right in the hospital. The real key is how many asymptomatic infections there are, how much pre-symptomatic shedding there is.”

So why did everyone get so worried about H5N1 right now?

Last October, H5N1 seems to have spread through a mink farm in Spain, where minks were being bred for their fur. All 50,000 mink at the farm were destroyed.

The ability of the virus to infect minks was a matter of particular concern because influenza viruses that can infect minks and their relatives can often infect humans. The worry was that the virus had changed in a way that might allow it to infect people more easily, and maybe move among them.

“Our oldest, but also our best animal model for human influenza, the ferret, is a close relative of the mink,” says Morse.

Since then, there have been a number of other instances of large groups of mammals contracting the virus, including 716 sea lions in Peru. The United States Department of Agriculture has been tracking detections of H5N1 in mammals in the U.S.; the department’s website lists more than 140 such detections since last year, in species including red foxes, black bears, bobcats, coyotes, and mountain lions. 

Is there a human vaccine for the bird flu?

The FDA has approved three H5N1 vaccines. One, called Audenz, was developed by a subsidiary of the pharmaceutical conglomerate

CSL

(CSLLY); another was developed by a

GSK

(

GSK

) subsidiary, while a third was developed by

Sanofi

(SNY).

Those vaccines are all older, however, and it’s not entirely clear how effective they would be against the currently-circulating strains of H5N1.

“GSK is closely monitoring current circulating influenza strains and in 2022, GSK was awarded contracts with the US, Canada, the EU and the WHO to supply our pandemic preparedness vaccine in the event of an influenza pandemic declared by the WHO,” a GSK spokesperson said in a statement.

“We stand ready to assist in supporting a global influenza pandemic response should it arise either from H5N1 or any other influenza strain,” a

Sanofi

spokesperson said.

The CDC has said that it has designed and shared with vaccine makers a so-called candidate vaccine virus, which can be used to make an H5N1 vaccine targeted against the current strain. Bloomberg reported earlier this week that

Moderna

(MRNA) is considering testing a mRNA-based bird flu vaccine this year.

What about a treatment?

Experts say that flu antivirals like

Roche Holding
’s
(RHHBY) Tamiflu, also available in generic versions, and Roche’s Xofluza are expected to work in patients infected with the current strain of H5N1, though both need to be given within the first 48 hours after symptoms appear. 

What should governments be doing now to avoid and prepare for the possibility that H5N1 does begin to spread in humans?

“We have done some of the right things,” Morse says of the U.S. response so far. “There has been increased biosecurity on farms.”

Still, there is more to do. Racaniello says the government needs to make sure the stockpiles are full: “We need to make sure we have enough masks for everyone,” he says. “We need to make sure we have enough antivirals stockpiled.”

Racaniello says that governments should also be closely monitoring to make sure any outbreak is caught early. “At sentinel places around the world, we need to be swabbing people and looking to see if this virus is coming out of people,” he says. “Airports, train stations, those sorts of places. We should be at the frontline.”