Can bird flu jump to humans? The threat of avian influenza, explained.

Maybe you’ve noticed: Eggs are really expensive.

By late last year, the average cost of a dozen Grade A large eggs had more than doubled since January. They’ve been so pricey — more than $10 at certain retailers — that some people are now smuggling them into the US from Mexico.

One major culprit? The spread of avian influenza, a.k.a. bird flu.

The viral disease has wiped out tens of millions of wild and farmed birds in the US, including egg-laying hens, many of which were not infected but were culled to stop the flu from spreading. The ongoing surge is now considered the largest avian influenza outbreak in US history.

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There’s some comfort in the name, avian influenza. The virus that’s been tearing through poultry farms, known as H5N1, typically targets birds, not humans.

But a recent outbreak of H5N1 at a mink fur farm in Spain has some scientists worried. Farms with dense populations of minks — which are mammals, like us — are ideal places for this virus to acquire new mutations or other genetic changes that could help it spread more easily between humans. And testing at the fur farm revealed the virus had already acquired at least one such mutation.

Meanwhile, wildlife monitoring has shown some other mammals have recently contracted bird flu, including a few grizzly bears in Montana, skunks, and otters.

This makes us wonder: Is bird flu creeping closer to humans?

The short answer: no. In its current form, H5N1 doesn’t have the machinery to easily infect humans or spread quickly among us. That’s the good news. What is concerning is that avian influenza viruses are known to change quickly — especially when they’re abundant and spreading among certain animal populations. Hence why some scientists are worried now.

“Avian influenza is near the top of the list in terms of viruses that have pandemic potential,” Daniel Olson, an epidemiologist at the University of Colorado, told Vox. “Coronaviruses are up there, too, but avian influenza is just as high — and maybe even higher.”

That doesn’t mean avian influenza is about to become the next pandemic. Yet experts are on alert, and are looking for any signs that the situation might change. Here’s what to know about the current human risk of bird flu.

What it would take for bird flu to become a human pandemic

The H5N1 virus that’s spreading now was first detected in the ’90s, at a goose farm in southern China, making it a relatively new type of avian influenza. (There are other strains of bird flu, but for the purposes of this article, we’ll use “bird flu” to mean H5N1.)

In the decades since, the virus has mostly been a problem for birds, especially domestic poultry. It’s highly contagious, and infection can cause severe damage to birds’ internal organs. Outbreaks of the virus can wipe out 90 percent or more of farm birds within 48 hours.

Jim Watson/AFP via Getty Images

A number of mammals including humans have also become infected over the years. While it can kill us — H5N1 has a frighteningly high mortality rate — this virus has yet to become widespread or approach anything close to a pandemic.

For any pathogen to have the potential to cause a human pandemic, it has to have three important qualities, said Tim Uyeki, a medical epidemiologist and bird flu expert at the Centers for Disease Control and Prevention. It must spread easily among humans, particularly through the air. It must cause human disease. And it must be something that most of our immune systems haven’t encountered before — that is, it must be novel.

Thankfully, H5N1 doesn’t meet all of these criteria.

For one, it doesn’t have the right machinery to efficiently infect our bodies and isn’t easily transmitted between them.

To infiltrate a host, viruses first have to bind to certain receptors on the surface of their cells. The virus that’s currently spreading, H5N1, does this using a specific kind of protein known as hemagglutinin 5, or H5. You can think of H5 as a key and receptors as the locks.

Following this metaphor, H5 can unlock certain receptors found in cells that line the respiratory and digestive tracts of birds. By invading those cells and replicating, the virus can damage these vital systems, making it difficult for the birds to breathe and easy for them to spread the virus among themselves (through breath and feces).

Humans have some similar, avian-type receptors in our respiratory systems, too. But for reasons scientists don’t fully understand, they don’t make us as vulnerable to avian flu as birds are. Critically, we also have a higher number of different, non-avian-type receptor that bird flu viruses don’t like to bind to quite as much. The abundance of those non-avian receptors in our noses seems to protect us from being easily infected by viruses like H5N1.

The upshot: H5N1 doesn’t easily bind to cells in our airways, so it’s harder for the virus to infect us. Humans can still be infected, but likely only if we’re exposed to a large amount of virus or the conditions are just right for transmission (though scientists don’t know what those conditions are, exactly). Most people who have come down with bird flu spent a more-than-casual amount of time around birds, usually while working with or around sick flocks.

“If you look at all the H5 infections over the past two decades or more, the vast majority of those reported exposure to sick or dying poultry prior to the infection,” said Richard Webby, a virologist specializing in animal and bird influenza at St. Jude Children’s Research Hospital.

Flu viruses that can’t cause infection in humans’ airways are much harder to transmit among humans — and therefore, they can’t cause a pandemic. Unless something changes.

Bird flu evolves quickly. That’s what makes it a threat.

If there’s anything concerning about the current bird flu, it’s that it could mutate and evolve. If it’s not a threat to humans today, it could become one.

That’s because influenza viruses are incredibly changeable. Like other viruses, H5N1 picks up small mutations as it replicates within a host; over time, that can give the virus certain benefits (though mutations are often bad for the virus).

But influenza viruses can also undergo much bigger and more consequential shifts through a process called reassortment.

Reassortment is like something out of science fiction: When two influenza viruses infect the same cell in the same host, they can trade entire chunks of their genomes with each other, yielding a variety of Franken-flus.

That’s why it’s a red flag for researchers when avian flu spreads among animals that can also easily get sick with other kinds of influenza. Pigs, for example, have flu receptors in their respiratory systems that both human and bird viruses easily bind to, so they can get infected with both. Should these two viruses meet inside these animals, they might swap parts, producing an avian flu that can more easily infect mammals.

The same is true for mink: They can be infected by both avian and mammalian influenza. (The famous and particularly devastating 1918 pandemic likely originated in birds and may have passed through a mammal before jumping to humans.)

Experts fear that in “mixing vessels” like pigs or mink, H5N1 could exchange a segment of its genome that makes it easily transmissible among birds for one that makes it easily transmissible among mammals — and, eventually, to humans. In theory, that could lead to the creation of a virus with all of H5N1’s other bad personality traits — its ability to cause severe disease, for example — with the added advantage of, say, being able to easily infiltrate cells in our airways.

(There are some signs that the H5N1 virus that spread through the Spanish mink farm picked up a mutation that’s known to help it replicate more easily in mammals. It’s not clear, however, if the virus picked up the mutation before or after spreading to minks.)

Ole Jensen/Getty Images

These major genetic shifts are so worrying because they can produce novel viruses that humans have never been exposed to. Although our immune systems likely would be able to recognize and fight off a common strain of flu that’s mutated slightly over time, it’s much harder to mount a quick response to a brand new strain.

The potential to evolve novelty is what puts bird flu on the pandemic potential radar, according to Seema Lakdawala, a virologist and influenza A transmission specialist at Emory University. “Pandemics emerge with shifts,” she said.

Rest assured, not all genetic shifts produce a pathogen with pandemic potential, said Lakdawala. Plus, even if H5N1 does evolve a way to more easily invade our airways, that doesn’t guarantee it will be able to spread among humans. To be easily transmissible, the virus also needs to replicate efficiently once it’s inside the cell, and survive in the air after it’s expelled in a cough or a sneeze.

There’s little evidence that bird flu has adapted to spread easily between mammals, much less between humans. Emerging evidence suggests that in many H5N1 cases among wild animals — and in the latest mink farm cases — infections among multiple animals likely occurred not because of transmission between animals, but because several animals all ate infected birds loaded with virus.

The real public health risk of bird flu

There’s more good news: Even if bird flu does evolve tools to infiltrate a human host and spread among us, we have tools of our own to detect and combat the virus.

The US government already has a stockpile of human vaccines for bird flu, including those specifically for H5N1, according to the CDC. There are also vaccines available for farm birds (though they’re not widely used, for reasons that Vox’s Kenny Torrella explains here).

Meanwhile, oseltamivir, a drug commonly used to treat more common types of flu infections, has been effective at treating human cases of H5 flu. And surveillance of flu in human and animal populations is a global health priority.

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Bird flu does, however, pose an immediate threat to humans, not because we’re at risk of infection but because it’s squeezing the global food supply, according to Carol Cardona, an avian health expert at the University of Minnesota.

Eggs and other poultry products, as well as some wild birds, have long been relatively cheap and vital sources of protein in much of the world. Should avian influenza continue to rip through large farms, or spread to backyard coops, it could extend the cost-of-living crisis.

“The risk to humans is through food and food supply,” Cardona said. “And the people who are being cut off from food supply are at the lowest economic level.”

The US Department of Agriculture and the CDC monitor flu viruses found in both people and animals for signs of a novel virus with the potential to cause human disease — a critical component of pandemic preparedness, said Uyeki. Although the disruptive virus freshest in our minds might be a coronavirus, most human respiratory pandemics in recent memory — those of 1918, 1957, 1968, and 2009 — have been caused by novel influenza A viruses.

“Constant vigilance and surveillance is needed worldwide to monitor the potential threat of these and other viruses as they evolve,” Ukeyi said.

Changes to the environment like deforestation and a warming climate are also leading to more intermingling of different species and the infectious organisms that call them home — including flu. “We have humans and animals living closer together on a larger scale than we have in the past,” Olson said.

That intermingling could at some point create a flu with human pandemic potential, said Lakdawala. “The more attempts these viruses make right at jumping across these hurdles, the more likely they are that some of them may be successful,” she said. “Nature is so good at doing this.”

Or as Cardona put it: “Never bet against a flu virus.”