Why is bird flu so bad right now?

Bird flu is on the rise. In the past few days, both France and the UK have announced new biosecurity measures aimed at containing the rapidly spreading disease. Dozens of infected penguins in South Africa have recently died, and on Wednesday South Korea reported its first case in six months. In the United States, the disease is driving up turkey prices a month before Thanksgiving, when the bird plays a central role. The prevalence of the disease is at its highest on record in Europe, and the number of domestic birds that have died in the past year is nearing a record in the United States. Why is bird flu so bad right now?

The bird flu currently rampant in Europe and North America is mainly caused by a strain called H5N1 – one of several classified as a highly pathogenic avian influenza virus (HPAI) because of its high fatality rate in poultry.

Europe, Asia and Africa have had many outbreaks of HPAI virus since the late 19th century. For about a century, outbreaks were mostly limited to poultry, and culling affected flocks usually prevented the disease from spreading widely among wild birds.

Viruses in a different way

But since the early 2000s, researchers have noted a continued spread of avian influenza among wild birds. Over the past year, this transmission has increased dramatically. The disease also appears to be spreading to mammals more frequently. These unprecedented transmission patterns mean that “something is very different about this virus than it is occurring now,” says Rebecca Poulson, a wildlife disease researcher at the University of Georgia at Athens.

The situation is particularly unusual for North America. An HPAI strain has only been detected once in wild birds there, between 2014 and 2016, after wild birds spread the disease from Eurasia to Alaska. This outbreak killed more than 50 million pet birds in the United States alone and cost $3 billion. But then the virus “seemed to sort of go away,” says Andy Ramey, a wildlife geneticist at the US Geological Survey Alaska Science Center in Anchorage.

Flu on the farm

In December 2021, the highly pathogenic H5N1 strain reappeared in North America, this time in the East. Researchers expect viruses to make the short journey across the Bering Strait to western North America, but “we didn’t really expect this virus to sneak in through the back door,” says Poulson. Since then, the disease has circulated unchecked in wild birds, rather than staying contained mainly in poultry farms, where crowded conditions can encourage virus spread. In both Europe and the United States, the high numbers of infected wild birds could make it easier for the virus to spread to domestic flocks, she notes.

Poulson says it’s inevitable that someday wild birds would again carry an HPAI strain to North America. “It should happen,” she says. “And it just happened now.”

Mutations are important

No one knows why this outbreak hasn’t fizzled out, but virologist Louise Moncla of the University of Pennsylvania in Philadelphia says there are some leading theories. One is that genetic mutations have increased the virus’ ability to replicate, allowing it to spread more efficiently than previous strains. Another reason is that mutations have allowed the virus to infect a wider range of bird species than previous strains were able to. Researchers are testing these ideas, but so far there are “more questions than answers,” says Moncla.

This strain of HPAI also appears to have evolved a propensity to pounce on mammals such as seals, lynx, and skunks, although there is no evidence that it can spread from a single mammal to another. Human cases are rare, even in Europe, where frequent outbreaks in poultry have created opportunities for humans to become infected. This gives Poulson hope that the virus isn’t evolving to infect people more easily, but “the elephant in the room: We don’t know,” she says.

Maybe here to stay

When, if ever, will this eruption subside? Many cases are likely to emerge in the coming weeks, Ramey says, as the birds gather to migrate together. Outside the train season, the number of infections could drop, but “I don’t know whether the general conditions will really improve,” he says.

Poulson thinks it’s likely the virus has passed the point where it could have disappeared from North America again. “There is no evidence that this virus is being suppressed or contained at all,” she says.