Over the past three years, a highly contagious and often fatal strain of bird flu has had a staggering impact on animals worldwide.
The virus, known as H5N1, has infected birds in more than 80 countries. It has infiltrated large commercial poultry farms and small backyard coops, affecting 72 million farm birds in the United States alone, according to the Department of Agriculture. The epidemic hit 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 in humans, especially those in close contact with the birds. Experts say the risk to the general public is low.)
The virus is not finished yet. It is on the rise again in Europe and North America, and is causing mass 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 remains an unprecedented situation,” said Thomas Peacock, a virus expert at the Pirbright Institute in England. “In many ways, we are in the worst situation ever, especially in terms of geographic spread, prevalence in birds, and how many mammals are infected.”
But in Europe, where the virus has been circulating longest, early signs suggest this winter may not be as bad as the last few, Peacock said. And there is very preliminary evidence that some wild birds have developed immunity to the virus.
Here is the latest situation:
The virus is spreading to new regions.
The current version of the virus has spread around the world at a dizzying speed. After appearing in 2020, it rapidly began to cause epidemics in Europe, Africa and Asia. It emerged in North America in late 2021 and swept across Canada and the United States. In the fall of 2022, the virus emerged in South America and spread to the tip of the continent within a few months.
This rapid spread southward has led to concerns that the virus will soon reach Antarctica, which provides critical breeding habitat for more than 100 million birds. And in October 2023, the virus was first detected in brown skuas on Bird Island in South Georgia in the Antarctic region. Since then, scientists have identified other 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 expect this news to come any day. “It’s probably already in Antarctica but hasn’t been retrieved 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. Because Antarctica has never before been exposed to a highly pathogenic bird flu virus, wild animals there may be particularly vulnerable to this virus, scientists say.
Seasonal patterns may emerge.
Summer in the United States provided respite from the worst bird flu epidemic in the country’s history. There were only a few small outbreaks in poultry in the country between May and September, and cases in wild birds have decreased.
“We breathed a sigh of relief for a few months when things really calmed down,” said Rebecca Poulson, an avian flu expert at the University of Georgia. “But he came back. Or maybe he never left.
Since the beginning of October, the virus has infected more than 1,000 poultry flocks in 47 states; According to the USDA, 12 million farm birds were affected.
According to a recent surveillance report from the European Center for Disease Prevention and Control, Europe has documented a similar pattern; Detections of the virus increased sharply in late October.
While the virus is still relatively new, these seasonal cycles may be permanent. “Something tells me this could be part of the new normal,” Poulson said.
Warm, humid weather is not traditionally conducive to the spread of flu viruses, and many birds remain inactive during the summer months and spend these months in their breeding grounds. In autumn, many birds begin migrating and the bird population increases with young birds having little exposure to flu. All of these factors could fuel fall volatility. (The virus can also flare up in the spring, when birds migrating in the other direction gather in high density.)
Immunity remains a wild card.
Now that the virus has been circulating for several years, critical questions about immunity have emerged: Do birds exposed to the virus acquire some immunity to the virus, and can this reduce the ferocity of outbreaks?
There is little data so far, but in a recent study scientists found signs of potential immunity in northern gannets, a seabird species that suffered heavy losses in the H5N1 outbreaks of 2022. “This is particularly encouraging for species whose populations are threatened,” he said. Diann Prosser is a research wildlife ecologist at the U.S. Geological Survey’s Eastern Ecological Science Center.
Peacock said some bird species in Europe that were hit hard in previous years did not die at the same rate.
Scientists said they expect birds that recover from infection to develop some degree of immunity to the virus. But what this 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 robust the immune protection is, how long it lasts, and how well it endures. We are faced with a rapidly developing virus.
“While we expect immune development in wild bird populations to influence the course of panzootics, the specific pathway is difficult to predict,” Prosser said.
Outbreaks in mammals cause concern.
Although the virus primarily poses a threat to birds, it has also been seen with unusual frequency in mammals, especially wild scavengers such as foxes. Many of these cases are likely endless infections in which mammals contract the virus after eating infected birds and die before transmitting the virus to others.
But some larger outbreaks have caused concern. In the autumn of 2022, the virus hit a mink farm in Spain, and in the last few months it has been detected in several fur farms with mink, fox and raccoon dogs in Finland. In Peru, H5N1 was associated with mass deaths of South American sea lions.
Viral samples from some of these animals contained mutations known to make the virus better adapted to mammals. While it is not uncommon to see these mutations emerge when mammals are infected, these findings, coupled with the size and speed of the outbreaks, are concerning. “It seems likely that there was a mammal-to-mammal transition in at least a few cases,” Peacock said.
Although infections in humans remain rare, scientists worry that a version of H5N1 that spreads more easily among mink or sea lions could also spread more easily among humans, potentially leading to another pandemic.
Some interesting outbreaks have also been reported in cats this year. One found at a cat shelter in South Korea was linked to contaminated food, which has been suggested as a potential cause of cat infections in Poland. Although it is unclear whether the virus spread from cat to cat, viral samples showed signs of adaptation in mammals. And each infection of a mammal provides more opportunities for the virus to mutate and evolve; This poses a risk not only to humans but also to other wild creatures.
“We are concerned about these viruses spreading to mammals and then especially to humans,” Poulson said. “I always like to point out that wildlife is important for its own sake. “And this has proven to be a truly devastating virus for mammalian and avian species.”
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