Over the past three years, a highly contagious, often fatal 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. According to the Department of Agriculture, the disease has infiltrated large commercial poultry farms and small backyard chicken coops, affecting 72 million farmed birds in the United States alone. It has affected a wide range of wild bird species, killing thousands of gulls and terns. And it has shown up repeatedly in mammals, including foxes, skunks, bears, cats, sea lions and dolphins. (It has also caused a small number of deaths among people, especially among those who had close contact with birds. The risk to the general public remains low, experts say.)
The virus is not finished yet. The disease is again on the rise in Europe and North America and is causing massive animal deaths in South America. It also appears that the disease is spreading in the Antarctic region for the first time.
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“It remains unprecedented,” said Thomas Peacock, a virus expert at the Pirbright Institute in England. “Various measures have put us at the worst level ever, especially in terms of geographic distribution, how widespread it is among birds and how many mammals are becoming infected.”
However, in Europe, where the virus has been circulating the longest, early signs indicate this winter may not be as bad as last, Peacock said. And there is very preliminary evidence that some wild birds may develop immunity to the virus.
Here’s the latest:
The virus is spreading into new territory.
The current version of the virus has spread around the world with astonishing speed. After emerging in 2020, the disease quickly began causing outbreaks in Europe, Africa and Asia. In late 2021, the disease emerged 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 spread south led to concerns that the virus would soon reach Antarctica, which provides crucial breeding habitat for more than 100 million birds. And in October 2023, the virus was first found in the Antarctic region, discovered 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 on the Antarctic mainland, scientists said they expected that news any day. “It’s probably already in Antarctica, but it hasn’t been picked up yet,” 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 wildlife could be particularly vulnerable, scientists say.
Seasonal patterns may emerge.
In the United States, the summer provided a reprieve from what had already become the worst outbreak of bird flu in the country’s history. Between May and September, the country recorded only a few small outbreaks in poultry, and cases in wild birds declined.
“We breathed a sigh of relief for a few months when things really calmed down,” said Rebecca Poulson, an expert on bird flu at the University of Georgia. ‘But it’s back. Or maybe it never went away.”
Since early October, the virus has affected more than 1,000 poultry flocks in 47 states; According to the USDA, 12 million farmed birds are affected.
Europe has documented a similar pattern, with virus detections spiking 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 may be permanent. “My gut would be that this might be part of the new normal,” Poulson said.
Hot, humid weather is not traditionally conducive to the spread of flu viruses, and many birds become dormant during the summer, spending those months in their breeding grounds. In the fall, many birds begin to migrate, and bird populations grow with young birds having little exposure to the flu. All of these factors can cause fall fluctuations. (The virus can also flare up in the spring, when birds migrating in the other direction congregate in high densities.)
Immunity remains a wild card.
Now that the virus has been circulating for several years, critical questions have arisen regarding immunity: Do birds that survive contact with the virus gain some immunity against it – and can that dampen the ferocity of these outbreaks?
There is little data so far, but in a recent study, scientists found potential signs of immunity in gannets, a seabird species that suffered heavy losses in H5N1 outbreaks in 2022. “This is encouraging, especially for endangered species populations,” said Diann Prosser, a research wildlife ecologist at the US Geological Survey’s Eastern Ecological Science Center.
More anecdotally, in Europe, it appears that some of the bird species that were hit hard in recent years are not going extinct at the same rate, Peacock said.
Scientists said they expected birds that survived the infection to develop some level of immunity against the virus. But what that means for the future of the panzootic — the animal version of a pandemic — will depend on a number of factors that are harder to determine, such as how robust that immune protection is, how long it lasts and how well it lasts. against a virus that is developing rapidly.
“I would expect that the development of immunity within wild bird populations would influence the trajectory of the panzootic, while the specific path is difficult to predict,” Prosser said.
Outbreaks in mammals are a cause for concern.
Although the virus mainly poses a threat to birds, it occurs in unusual frequency in mammals, especially wild scavengers such as foxes. Many of these cases are likely to have been fatal infections, in which mammals contracted the virus after eating infected birds and then died without passing on the virus.
But some larger outbreaks have raised concerns. In the fall of 2022, the virus hit a mink farm in Spain, and in recent months it has been found on numerous fur farms in Finland, where minks, foxes and raccoon dogs live. In Peru, H5N1 has been linked to the mass death of South American sea lions.
Viral samples from some of these animals contain mutations known to make the virus more adapted to mammals. While it is not unusual for these mutations to emerge when mammals become infected, these findings, combined with the size and speed of the outbreaks, are concerning. “It appears that mammal-to-mammal transmission has likely occurred in at least a few cases,” Peacock said.
While human infections remain rare, a version of H5N1 that spreads more easily among minks or sea lions could also spread more easily among humans, potentially causing another pandemic, scientists worry.
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 was also suggested as a possible cause of cat infections in Poland. Although it is not clear whether the virus spread from cat to cat, viral samples showed signs of mammalian adaptation. And every infection of a mammal presents more opportunities for the virus to mutate and evolve, posing risks not only to humans but to other wild creatures as well.
“We’re concerned about these viruses getting into mammals and perhaps even more specifically into humans,” Poulson says. “I always want to point out that wild animals are important in their own right. And this has proven to be a truly devastating virus for mammal and bird species.”
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