A global plague of avian flu

Bird flu: The basic deets

Say hello to the H5N1 virus: There are many strains of avian influenza, but the most deadly is the H5N1. H5N1 first surfaced in 1996 at a goose farm in China—and has since showed up in at least 63 species of wild birds. In 2002, the virus resulted in the death of more than 140 million birds—due to disease and culling to prevent the spread. The virus is highly contagious, and infection can wipe out 90% of farm birds within 48 hours. 

The threat to humans: The disease can spread to humans—but requires prolonged contact in a contaminated environment:

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 [virologist] Richard Webby.

A major outbreak in 1997 left six people dead and 18 infected. To date, over 800 cases of human H5N1 infections have been reported—with a horrific fatality rate of 53%. In other words, while the chances of contracting the disease are small—for now—it is far more lethal than other forms of influenza. 

The symptoms & treatment: The list of symptoms is long—vomiting, fever, cough, diarrhoea, sore throat, eye infection, muscle aches, respiratory distress. It is also unhelpful since it can mimic many other diseases. Much like Covid, it requires a nasal or throat swab to confirm an infection. There are human vaccines for bird flu that offer protection against severe disease. But they may prove ineffective if the virus mutates. Other antiflu medicines are mostly ineffective against the virus. 

First: The spread to wild birds

In 2021, we got the first sign that this epidemic would be different from the others. The virus had mutated to spread to wild birds—which carried the disease far and wide—across the globe. It first popped up in North America in December 2021—a sick gull near a farm in Newfoundland. By January, it had moved as far south as North Carolina. By February, it was detected in 47 states—and resulted in the culling of 58 million birds—and was labelled the largest avian influenza outbreak in US history. 

A massacre of wild birds: The flu has since rampaged through bird populations across the globe—many of them endangered. Here’s a mind-numbing list from Washington Post: 

About 5,200 common cranes in Israel. More than 2,200 Dalmatian pelicans in Greece, about 40% of the species in southeastern Europe, and roughly 20,000 Sandwich terns in Europe, 17% of the northwest European breeding population. More than 18,000 dead barnacle geese in Scotland. And tens of thousands of gannets in Canada.

 

Last year, ornithologists found about 12,000 dead black-legged kittiwakes in Norway. By July, more than 500,000 birds died in South America, including about 41% of all Peruvian pelicans.

Next: The spread to mammals

Warned by dead minks: By the summer of 2022, the disease had spread to mammals. But the first alarm bell: A mink farm in Spain—where 50,000 infected minks were culled later that year. A study of the outbreak showed that the virus was spreading from one mammal to another—for the very first time! Until now, mammals (including humans) were only infected after exposure to infected birds. The worrying reason for this shift: The H5N1 virus had gained at least one mutation—T271A—that allowed it to spread between mammals. 

Confirmed by dead seals: When the seals first began to die in the summer of 2022 along the coast of Maine, scientists assumed they’d been infected the usual way—by eating diseased birds. But in early 2023, tens of thousands of sea lions died in Peru and Chile—“the earliest known mass sea lion deaths from the virus”:

The sea lions behaved erratically, experiencing convulsions and paralysis; pregnant females miscarried their fetuses. “What happened when the virus moved to South America we had never seen before,” Dr. Uhart said.

But as the virus spread among marine animals in South America, it evolved to jump straight from one to another: 

It is difficult to prove exactly how and when the virus moved from one species to another. But genetic analysis supports the theory the marine mammals acquired their infections from one another, not birds. Samples of virus isolated from sea lions in Peru and Chile and from the elephant seals in Argentina all share about 15 mutations not seen in the birds.

The direct spread of the disease also resulted in greater devastation: 17,000 elephant seal pups died in Argentina—at least 96% of the juvenile population. 

An unprecedented spread: Scientists have been taken entirely by surprise by the virus’ ability to hop species—which gives it dizzying reach: 

Avian flu viruses tend to be picky about their hosts, typically sticking to one kind of wild bird. But this one has rapidly infiltrated an astonishingly wide array of birds and animals, from squirrels and skunks to bottlenose dolphins, polar bears and, most recently, dairy cows. “In my flu career, we have not seen a virus that expands its host range quite like this,” said [virologist] Troy Sutton.

The farthest corners of Earth: As the virus spreads through wildlife, it threatens the survival of vulnerable species—and fragile ecosystems that may never recover: 

The virus is literally reaching the ends of the Earth, killing brown skuas on islands near Antarctica and, for the first time just this winter, a polar bear in Alaska. Scientists worry it is only a matter of time before it reaches penguins and other vulnerable populations on Antarctica itself. “The Antarctic situation is at a precipice,” said [virus ecologist] Michelle Wille... “Many of the animal species that live there are found nowhere else in the world, and many are already facing substantial pressures due to things like fisheries and climate change.”

Coming soon: spread to humans?

A rapidly mutating virus is cause for great concern. A virus that adapts to infect mammals is one step closer to infecting humans. Right now, while the H5N1 virus is more contagious, it lacks one critical element—which protects humans from infection.

The wrong receptor cells: Viruses infect a cell by binding to receptors on its surface. And they tend to specialise in infecting certain species—and evolve to bind to their receptors. This makes them less effective when they infect other species:

Avian flu viruses are adapted to bind to birds’ receptor cells. Humans and other mammals have some avian-like receptors, but they’re typically buried deep in the lungs. Because of this anatomical quirk, it would take an enormous load of H5N1 for one infected mammal to dredge up enough of the virus to infect another mammal. Unless, of course, the virus evolved to bind to mammalian cells in the upper respiratory tract.

So the virus can’t easily bind to cells in our airways—and are therefore much harder to transmit—and therefore can’t cause a pandemic.

But, but, but: Those dead minks have respiratory tracts that are physiologically similar to ours. 

The peril of reassortment: A virus that is mutating to allow mammal-to-mammal transmission could soon acquire a mutation that enables it to spread among humans. Influenza viruses are highly changeable—and are prone to undergoing big shifts called reassortments:

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.

This is what would happen if pigs—which can contract both the human and avian flu—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.” 

Looping back to those minks: Since minks are also susceptible to avian and mammalian flus, scientists are worried about the outbreak at the Spanish farm:

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.

Also, dairy farms: For the very first time, avian flu has spread to dairy animals—infecting at least 13 herds in six states in the US. This is astonishing and worrying—especially since scientists don’t know if the disease is being spread from one cow to another. But it doesn’t seem to cause serious disease among the cows—they mostly feel a bit lethargic. Only one person has been infected—he works on a farm and his main symptom was conjunctivitis. 

Watch out for the pigs: Scientists will be most worried if the virus spreads to pigs—“which are susceptible to both human and avian flu strains, providing the perfect mixing bowl for viruses to swap genes.” Happily, they have been immune so far. 

The bottomline : Every time the virus jumps a species, it has more opportunity to mutate to infect humans. But here’s the good news about the mutating avian flu virus: 

So far, what we know about this strain of avian flu suggests an outbreak among humans is not imminent. The adaptation that the virus made to replicate in migratory birds—and thus spread across the globe—seems to have inhibited its ability to infect humans.

Reading list

New York Times and the Washington Post have the best overviews of the epidemic. For more on the unprecedented outbreak in dairy farms, read PBS NewsHour. The Washington Post explains how the virus infects humans—and why it kills. Vox has more on the risk of the virus spreading among humans. The Conversation has everything you need to know about the disease.