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

You may have noticed: Eggs are expensive.

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

One of the main culprits? The spread of bird flu, aka bird flu.

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

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There is some comfort in the name, poultry influenza. The virus circulating in poultry farms, known as H5N1, usually 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 among humans. And tests on fur farms showed the virus at least got it one such mutation.

Meanwhile, wildlife monitoring has revealed several other mammals that have recently contracted bird flu, including several grizzly bears in Montana, skunks, and otters.

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

Short answer: no. In its current form, H5N1 does not have the machinery to easily infect humans or spread rapidly among us. That’s good news. What’s worrying is that avian flu viruses are known to change quickly – especially when they are abundant and spread among certain animal populations. That’s why some scientists are worried now.

“Avian influenza is near the top of the list of viruses with pandemic potential,” Daniel Olson, an epidemiologist at the University of Colorado, told Vox. “Coronavirus is there too, but avian influenza is just as high – and maybe even higher.”

This does not mean that bird flu will be the next pandemic. But experts are wary, and are looking for signs that the situation could change. Here’s what you need to know about the risk of bird flu in humans today.

What it takes for bird flu to become a human pandemic

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

For decades, the virus has mostly been a problem for birds, especially domestic poultry. It is highly contagious, and the infection can cause severe damage to the internal organs of the bird. A viral outbreak can kill 90 percent or more of farm birds in 48 hours.

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Some mammals including humans have also been infected over the years. While it can kill us – H5N1 has a very scary death rate – this virus has not spread or approached anything close to a pandemic.

For any pathogen to cause a human pandemic, it must have three important qualities, says Tim Uyeki, a medical epidemiologist and avian flu expert at the Centers for Disease Control and Prevention. It should spread easily between humans, especially through the air. It certainly causes human disease. And it must be something that most immune systems have not encountered before – that is, it must be new.

Fortunately, H5N1 does not meet all of these criteria.

For one, it does not have the proper machinery to infect our bodies efficiently and is not easily transmitted between people.

To infect a host, the virus must first bind to specific receptors on the cell surface. The virus currently circulating, H5N1, does this using a specific type of protein known as hemagglutinin 5, or H5. You can think of H5 as the key and the receptor as the key.

Following this metaphor, H5 can unlock specific receptors found on the cells that line the respiratory and digestive tracts of birds. By attacking these cells and replicating, the virus can destroy the vital system, making it difficult for the bird to breathe and easily spread the virus among themselves (through breath and feces).

Humans also have similar avian-type receptors in our respiratory system. But for reasons scientists don’t understand, they don’t make us as susceptible to bird flu as birds. Critically, we also have more non-avian-type receptors that avian flu viruses do not like to bind to. The abundance of non-avian receptors in our noses appears to protect us from being easily infected by viruses like H5N1.

The result: H5N1 doesn’t easily bind to the cells in our airways, so the virus has a harder time infecting us. Humans can still get infected, but only if we’re exposed to a large amount of the virus or the right conditions for transmission (although scientists don’t know exactly what those conditions are). Most people with bird flu spend more time around birds, usually when working with or around sick flocks.

“If you look at all the H5 infections over the last two decades or so, most of the ones reported were infected birds that were sick or died before the infection,” said Richard Webby, a virologist specializing in animal and avian influenza at St. Jude Children’s Research Hospital.

Influenza viruses that cannot cause infection in the human respiratory tract are more difficult to transmit between humans – therefore, less likely to cause a pandemic. Unless something changes.

Bird flu is developing rapidly. That creates a threat.

If there is anything about bird flu today, it can mutate and evolve. If it’s not a threat to humanity now, it might be.

It’s because of the influenza virus extraordinary can be changed. Like other viruses, H5N1 takes on minor mutations as it replicates in the host; over time, which can give the virus certain advantages (although mutations are often bad for the virus).

But influenza viruses can also undergo larger and more consequential changes through a process called reassortment.

Reassortment is like nothing out of science fiction: When two influenza viruses infect the same cells in the same host, they can exchange entire pieces of their genomes with each other, producing multiple Franken-flus.

That’s why it’s a red flag for researchers when bird flu spreads among animals that are also susceptible to other types of influenza. Pigs, for example, have flu receptors in their respiratory systems that readily bind to human and bird viruses, allowing them to infect both. If these two viruses meet inside these animals, they may exchange parts, producing avian flu that can more easily infect mammals.

The same goes for mink: It can be infected with avian and mammalian flu. (The famous and particularly destructive 1918 pandemic likely originated in birds and may have passed through mammals before jumping to humans.)

Experts fear that in “mixed vessels” like pigs or mink, H5N1 could swap parts of the genome that make it easy to transmit between birds for one that is easy to transmit between mammals – and eventually, to humans. In theory, that could lead to the creation of a virus with all the other bad personality traits of H5N1 – the 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 indications that the H5N1 virus that spread through Spanish mink farms took on mutations known to help it replicate more easily in mammals. However, it is not clear if the virus took on the mutation before or after it spread to minks.)

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These major genetic changes are of great concern because they could produce novel viruses that humans have never encountered before. Although our immune system can recognize and fight common flu strains that mutate slightly over time, it has a harder time reacting to new strains.

The new emerging potential 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 change,” he said.

Rest assured, not all genetic changes produce pathogens with pandemic potential, Lakdawala said. In addition, even if H5N1 makes a way to more easily invade our airways, this does not guarantee that it can spread among humans. To be easily transmitted, viruses also need to replicate efficiently once inside the cell, and live in the air after being expelled in a cough or sneeze.

There is no evidence that bird flu has adapted to easily spread among mammals, much less among humans. Emerging evidence suggests that in many cases of H5N1 among wild animals – and in the most recent case of mink farms – infection among multiple animals occurred not because of transmission between animals, but because some animals all ate infected birds that were stuffed virus.

The real public health risk of bird flu

There’s better news: Although bird flu has evolved the tools to infiltrate human hosts and spread among us, we have our own tools to detect and fight the virus.

The US government already has stockpiles of human vaccines for bird flu, including one specifically for H5N1, according to the CDC. There is also a vaccine available for farm birds (although it is not commonly used, for reasons explained by Vox’s Kenny Torrella. here).

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

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Bird flu, however, poses a direct threat to humans, not because of the risk of infection but because of the reduction in global food supplies, 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 a relatively cheap and important source of protein in many parts of the world. If avian influenza continues to spread in large farms, or spreads to backyard pens, it could add to the cost-of-living crisis.

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

The U.S. Department of Agriculture and CDC are monitoring flu viruses found in humans and animals for signs of novel viruses with the potential to cause human disease — a critical component of pandemic preparedness, Uyeki said. Although the most recent disturbing virus on our minds may be the coronavirus, most of the human respiratory pandemics in recent memory — namely 1918, 1957, 1968, and 2009 — were caused by the novel influenza A virus.

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

Changes in the environment like deforestation and a warming climate also lead to a mix of different species and infectious organisms that call it home – including the flu. “We have humans and animals living closer together on a larger scale than ever before,” Olson said.

The mix could cause a flu with human pandemic potential, Lakdawala said. “The more attempts this virus makes to jump that hurdle, the more likely some of them will succeed,” he said. “Nature is very good at doing this.”

Or as Cardona puts it: “Don’t fight the flu virus.”