Avian flu continues to wreak havoc. Here's the latest.

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Over the past three years, a highly contagious and often deadly form of bird flu has taken a staggering toll on animals around the world.

The virus, known as H5N1, has infected birds in more than 80 countries. It has infiltrated large commercial poultry farms and small chicken houses, affecting 72 million farmed birds in the United States alone, according to the Department of Agriculture. It has affected a wide range of wild bird species, killing thousands of gulls and terns. And it has appeared repeatedly in mammals, including foxes, skunks, bears, cats, sea lions and dolphins. (It has also caused a small number of deaths in people, mainly in those who had close contact with birds. The risk to the general public remains low, experts say.)

The virus is not over yet. It is increasing again in Europe and North America and causing massive animal mortality events in South America. It also appears to be spreading for the first time in the Antarctic region.

“It remains unprecedented,” said Thomas Peacock, a virologist at the Pirbright Institute in England. “By several measures, we are at the worst point in our history, particularly in terms of geographic spread, how widespread it is among birds and how many mammals are being infected.”

However, in Europe, where the virus has been circulating longer, early signs suggest this winter may not be as bad as the last few, Dr. Peacock said. And there is very preliminary evidence that some wild birds may be developing immunity to the virus.

Here's the latest:

The current version of the virus has spread around the world at an astonishing speed. After emerging in 2020, it quickly began causing outbreaks in Europe, Africa and Asia. In late 2021, it appeared in North America and swept through Canada and the United States. In the fall of 2022, the virus appeared in South America and spread to the tip of the continent in just a few months.

This rapid southward spread raised concerns that the virus would soon reach Antarctica, which provides critical breeding habitat for more than 100 million birds. And in October 2023, the virus was found for the first time in the Antarctic region, detected in brown skuas on Bird Island, South Georgia. Since then, scientists have identified additional confirmed or suspected cases in gulls and petrels, as well as elephant seals and other animals in the region, according to the Antarctic Wildlife Health Network.

Although the virus has not yet been reported in continental Antarctica, scientists said they expected news to come any day now. “It's probably already in Antarctica, but it hasn't been detected,” Dr. Peacock said.

Many of the region's birds and marine mammals are already struggling to survive in the face of climate change and other threats. And because Antarctica has never before been hit by a highly pathogenic bird flu virus, its wild animals could be especially vulnerable to it, scientists say.

In the United States, summer provided a respite from what had already become the worst bird flu outbreak in the country's history. Between May and September, the country recorded only several small outbreaks in poultry and cases in wild birds decreased.

“We breathed a sigh of relief for several months when things really calmed down,” said Rebecca Poulson, an avian influenza expert at the University of Georgia. “But she's back. Or maybe it never left.”

Since early October, the virus has affected more than 1,000 poultry flocks in 47 states; 12 million farm birds have been affected, according to the USDA

Europe has documented a similar pattern, with virus detections rising sharply in late October, according to a recent surveillance report from the European Center for Disease Prevention and Control.

Although the virus is still relatively new, these seasonal cycles could be here to stay. “My gut would tell me it could be part of the new normal,” Dr. Poulson said.

Hot, humid weather traditionally does not favor the spread of flu viruses, and many birds remain stationary in the summer and spend those months in their breeding areas. In the fall, many birds begin to migrate and bird populations increase with young birds that have little exposure to the flu. All of these factors can fuel autumn surges. (The virus can also break out in spring, when birds migrating in the other direction congregate in high densities.)

Now that the virus has been circulating for several years, critical questions have arisen regarding immunity: Do birds that survive a brush with the virus gain some immunity against it? Could that lessen the ferocity of these outbreaks?

There is little data so far, but in a recent study, scientists found potential signs of immunity in boreal gannets, a seabird species that suffered heavy losses in H5N1 outbreaks in 2022. “This is encouraging, especially for species with threatened populations,” said Diann Prosser, a wildlife research ecologist at the U.S. Geological Survey's Eastern Ecological Sciences Center.

More anecdotally, in Europe, some of the bird species that were hit hard in previous years do not appear to be dying at the same rate, Dr. Peacock said.

Scientists said they expected birds that survived the infection to develop some degree of immunity to the virus. But what that means for the future of panzootics (the animal version of a pandemic) will depend on a variety of factors that are harder to pin down, such as how strong that immune protection is, how long it lasts, and how well it is maintained. in the face of a virus that has been evolving rapidly.

“I would expect the development of immunity within wild bird populations to affect the trajectory of panzootics, although the specific pathway is difficult to predict,” Dr. Prosser said.

Although the virus is primarily a threat to birds, it has appeared with unusual frequency in mammals, especially wild scavengers such as foxes. Many of these cases have likely been dead-end infections, in which mammals contracted the virus after eating infected birds and then died without transmitting the virus.

But some larger outbreaks have caused concern. In the fall of 2022, the virus affected a mink farm in Spain and in recent months it has been detected in numerous fur farms in Finland, which house mink, foxes and raccoon dogs. In Peru, H5N1 has been linked to mass die-offs of South American sea lions.

Viral samples taken from some of these animals contain mutations that are known to make the virus better adapted to mammals. Although it is not unusual to see such mutations appear when mammals are infected, these findings, combined with the size and speed of the outbreaks, have been concerning. “It looks like there was probably mammal-to-mammal transmission in at least a couple of cases,” Dr. Peacock said.

Although human infections remain rare, a version of H5N1 that spreads more easily among mink or sea lions could also spread more easily among humans, potentially triggering another pandemic, scientists fear.

Several curious outbreaks in cats have also been reported this year. One, at a cat shelter in South Korea, was linked to contaminated food, which has also been suggested as a potential cause of infections in cats in Poland. Although it is unclear whether the virus was transmitted from cat to cat, the viral samples showed signs of mammalian adaptation. And each infection of a mammal provides more opportunities for the virus to mutate and evolve, posing risks not only to humans but also to other wild creatures.

“We are concerned that these viruses will jump to mammals and then perhaps more specifically to humans,” Dr. Poulson said. “I always like to point out that wildlife is important in its own right. “And this has proven to be a really devastating virus for mammalian and bird species.”



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