by Over the past three years, a highly contagious, often fatal strain of bird flu has taken a toll on animals around the globe.
The virus, known as H5N1, has infected birds in more than 80 countries. It has infiltrated large commercial poultry farms and tiny backyard hen houses, affecting 72 million farmed birds in the United States alone, according to the Department of Agriculture. It struck a wide variety of wild bird species, killing thousands of seagulls and terns. And it has come up repeatedly in mammals, including foxes, skunks, bears, cats, sea lions and dolphins. (It has also caused a small number of deaths in humans, mainly those in close contact with birds. The risk to the general public remains low, experts say.)
The virus is not done yet. It is on the rise again in Europe and North America and is causing major animal mortality events in South America. It also appears to be spreading in the Antarctic region for the first time.
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“It’s always unprecedented,” said Thomas Peacock, a virus expert at England’s Pirbright Institute. “By some measures, we’re at the worst it’s ever been, especially in terms of geographic spread, how widespread it is in birds and how many mammals are infected.”
In Europe, however, where the virus has been spreading the longest, early signs suggest this winter may not be as bad as the last, Peacock said. And there is very preliminary evidence that some wild birds may be developing immunity to the virus.
Here’s the latest one:
The virus is expanding into new territory.
The current version of the virus has spread around the world with amazing speed. After its emergence in 2020, it quickly started causing outbreaks in Europe, Africa and Asia. In late 2021, it appeared in North America, storming through Canada and the United States. In the autumn of 2022, the virus appeared in South America, spreading down to the top of the continent in recent months.
This rapid southward spread has fueled concerns that the virus could soon reach Antarctica, which provides vital breeding habitat for more than 100 million birds. And in October 2023, the virus was found in the Antarctic region for the first time, detected in a brown skuas on Bird Island, South Georgia. Since then, scientists have identified additional confirmed or suspected cases in seagulls 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 were expecting that news any day now. “It’s probably already in Antarctica, but it hasn’t been picked up,” Peacock said.
Many birds and marine mammals in the region are already struggling to survive in the face of climate change and other threats. And because Antarctica has never been hit by a highly pathogenic bird flu virus before, its wildlife could be particularly vulnerable to this one, scientists say.
Seasonal patterns may be emerging.
In the United States, summer brought relief from what was already the worst bird flu outbreak in the nation’s history. Between May and September, only a few small outbreaks were found in the nation’s poultry, and cases in wild birds declined.
“We breathed a sigh of relief for several months when things became very quiet,” said Rebecca Poulson, an avian flu expert at the University of Georgia. “But he’s back. Or maybe he never left.”
Since the beginning of October, the virus has affected more than 1,000 poultry flocks in 47 states; 12 million farmed birds were affected, according to the USDA.
Europe has documented a similar pattern, with a sharp increase in virus detections 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 here to stay. “My gut would say it could be part of the new norm,” Poulson said.
Traditionally, hot, humid weather would not help the spread of flu viruses, and many birds stop in the summer, spending those months breeding. In the fall, many birds begin to migrate, and bird populations swell with young birds that have had little exposure to the flu. All of these factors can lead to a boom in harvest. (The virus can also flare up in the spring, when birds migrating in the opposite direction congregate in high densities.)
The exemption is still a wild card.
Now that the virus has been circulating for several years, critical questions about immunity have arisen: Do birds that survive a brush with the virus acquire some immunity against it—and could that fuel these outbreaks?
There is little data so far, but in one recent study, scientists found possible signs of immunity in northern gannets, a seabird species that suffered heavy losses in H5N1 outbreaks in 2022. “This is encouraging, that especially for species with threatened populations,” said Diann Prosser, research wildlife ecologist at the US Geological Survey’s Eastern Ecological Science Center.
More tellingly, in Europe, some of the bird species that were hit hard in previous years don’t seem to be dying at the same rate, Peacock said.
The scientists said they hoped birds that survived the infection would develop some immunity to the virus. But what that means for the future of the panzootic – the animal version of a pandemic – will depend on a variety of harder-to-find factors, such as how strong that immune defense is, how long it lasts and to whom as well as it holds. against rapidly evolving viruses.
“We would expect the development of immunity within wild bird populations to affect the panzootic pathway, and the specific pathway is difficult to predict,” Prosser said.
Outbreaks in mammals are a cause for concern.
Although the virus is a threat mainly to birds, it is seen with unusual frequency in mammals, especially in wild scavengers such as foxes. Many of these cases were probably fatal infections, in which mammals contracted the virus after eating infected birds and then died without passing on the virus.
But some larger outbreaks are cause for concern. In the fall of 2022, the virus hit a mink farm in Spain, and over the past several months, it has been detected in numerous fur farms in Finland, where there are mink, foxes and raccoon dogs. In Peru, H5N1 has been linked to the mass die-off of South American sea lions.
Viral samples taken from some of these animals contain mutations known to make the virus better adapted to mammals. Although it is not unusual to see these mutations pop up when mammals are infected, these results, along with the size and speed of the outbreaks, are worrying. “It seems likely that there was mammal-to-mammal transmission in at least a few cases,” Peacock said.
Although human infections remain rare, a version of H5N1 that spreads more easily among mink or sea lions could spread more easily among humans, setting off another pandemic, scientists worry.
Some strange 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 unclear whether the virus was spread from cat to cat, viral samples showed signs of mammalian adaptation. And each mammalian infection provides more opportunities for the virus to mutate and evolve, posing risks not only to humans but also to other wild creatures.
“We’re worried about these viruses jumping into mammals and then maybe specifically into humans,” Poulson said. “I always like to point out that wildlife is important for its own sake. And this proved to be a truly devastating virus for mammalian and avian species.”
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