An outbreak of H5N1 avian influenza that started in 2021 has become the largest bird flu outbreak in history, both in the U.S. and worldwide. In the U.S. the virus has led to the destruction of millions of commercially raised chickens, turkeys, ducks and geese, and has killed thousands of wild birds.

Many virologists are concerned that this virus could spill over to humans and cause a new human pandemic. University of Colorado Boulder virologists Sara Sawyer, Emma Worden-Sapper and Sharon Wu summarize the compelling story of H5N1 and why scientists are closely watching the outbreak.

H5N1 is a specific type of influenza virus, predominantly harbored by birds, that was first detected on a goose farm in China in 1996. Recently it has begun infecting an exploding diversity of bird and mammalian species around the globe.

The virus is highly pathogenic to birds, meaning that infections often cause extreme symptoms, including death. But its impact on humans is complicated. There have been relatively few human infections detected – fewer than 900 documented globally over several decades – but about half of those infected individuals have died.

The good news about H5N1 for humans is that it currently doesn’t spread well between people. Most people who have contracted H5N1 have gotten it directly from interacting with infected poultry – specifically chickens, turkeys, ducks and geese, which often are raised in close quarters on large commercial farms.

There are only a small handful of examples of human-to-human spread. Because H5N1 doesn’t spread well between people, and because direct infection of humans by infected birds is still relatively rare, H5N1 has not yet erupted into a human epidemic or pandemic.

The first reason that so much attention is being paid to bird flu right now is that currently H5N1 is causing the largest “bird pandemic” ever recorded. A certain viral variant that arose in 2020, called H5N1 2.3.4.4b, is driving this outbreak.

In agricultural poultry flocks, if a few birds test positive for H5N1, the whole flock is killed regardless of symptoms or infection status. Higher prices for eggs and poultry meat in the U.S. are one result. The Biden administration is considering vaccinating farmed poultry flocks, but the logistics could be quite complicated.

The second reason for increased attention is that H5N1 is now infecting more bird and mammalian species than ever before. The virus has been detected in a broad array of wild birds and in diverse mammals, including badgers, black bears, bobcats, coyotes, ferrets, fisher cats, foxes, leopards, opossums, pigs, skunks and sea lions.

As H5N1 infects more species, it also increases its geographical range and produces more viral variants that could have new biological properties.

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