Science | May 20, 2024

Four Important Questions About Bird Flu, Answered

The virus has killed tens of millions of birds and infected hundreds of species of animals, including dairy cattle in the United States. Here’s what you should know about it

Rangers clear deceased birds from Staple Island in England, where avian flu had a devastating effect on a seabird colony. Owen Humphreys / PA Images via Getty Images

Will Sullivan

Daily Correspondent

Over the past few years, avian influenza has touched almost every corner of the planet. Since 2021, outbreaks have impacted at least 320 species from 21 different orders, most of which are water birds. Tens of millions of poultry in dozens of countries around the world have died either from the disease itself or from preventative culling. The viral infection has also been spreading to mammals—and was detected in dairy cows in the U.S. for the first time in late March.

The Food and Drug Administration (FDA) has determined that milk and other dairy products from cows are safe to consume, since the pasteurization process seems to inactivate the virus. But genomic data suggests that bird flu has been spreading in cows for months, multiple outlets reported at the end of April. The finding suggests that infections are more widespread in the animals than previously known.

Still, the Centers for Disease Control and Prevention (CDC) says that the current health risk to the general public remains low. But with so many headlines about bird flu recently, we reached out to avian influenza experts and scoured the news to provide you with up-to-date answers on four key questions.

What is the history of bird flu?

Researchers have been tracking bird flu since the late 19th century. The first recorded outbreak in U.S. poultry occurred in the 1920s, and outbreaks have periodically cropped up in the decades since. A million birds died in a 1968 pandemic. The H5N1 strain, which has spread widely in animals in recent years and was recently detected in cows, was first identified in 1996 in domestic waterfowl in southern China.

Over the last several years, the virus’ footprint has expanded. “Prior to 2020, this virus was largely restricted to Europe, Asia and Africa,” Michelle Wille, who studies avian influenza in wild birds at the University of Melbourne in Australia, writes via email. Since then, the virus has entered North America at least three different times and spread to South America, the sub-Antarctic and the Antarctic, she says.

Throughout the 21st century, numerous strains of avian influenza have spread among and between poultry and wild birds. The recent spread of the virus is due in part to a genetic tweak that makes it better adapted to waterfowl, Wille says. “Prior, this virus was highly adapted to poultry, and we generally didn’t see sustained transmission in wild birds,” she writes via email.

The last couple of years, the virus has been running rampant in those wild birds. Between October 2022 and November 2023, more than half a million birds across at least 82 species died in South America. Around 40 percent of Peru’s pelicans and 17 percent of Europe’s Sandwich terns died in a matter of months, and researchers have identified the virus in hundreds of wild bird species, Wille says.

Mallard Inspected For Bird Flu — A researcher inspects a dead mallard duck in Scotland after a swan was found in the country with H5N1. Klebher Vasquez / Anadolu Agency via Getty Images

Some researchers estimate that millions of wild birds have died from bird flu. In the U.S., H5N1 has been detected in more than 9,000 wild birds. It killed 36 bald eagles between February and April 2022, as well as 20 endangered California condors in a span of weeks in 2023. On U.S. farms, more than 90 million poultry across 48 states have been killed by the virus or culled in an attempt to stop its spread. Globally, more than 131 million poultry were culled or died in 2022 alone.

How has bird flu impacted mammals?

Over the last couple of years, H5N1 has also been detected in dozens of mammal species. Mammals can get infected when they consume infected birds. A combination of a few mutations that could make the virus better adapted to mammals and a high viral load in the dead bird can lead to these infections, says molecular virologist Daniel R. Perez. He studies the changes in influenza viruses that allow them to cause diseases in humans at the University of Georgia. Infections in other animals can also occur after creatures ingest food or water contaminated by wild bird secretions, or when they come into direct contact with wild birds, Noelia Silva Del Rio, an expert in dairy cattle health at the University of California, Davis, writes in an email.

In one of the earlier instances of bird flu in mammals tied to the recent outbreaks, researchers found red fox cubs in the wild infected with H5N1 in the Netherlands in May 2021. In February 2022, H5N1 was detected in sea lions in Peru. In the span of a few months, more than 5,000 sea lions died. An outbreak on a mink farm in Spain in October 2022 led to the culling of all the minks on the premises—more than 50,000 in total. Across South America, more than 50,000 mammals from at least ten different species died between October 2022 and November 2023. More than 70 percent of southern elephant seal pups died at a beach in Argentina during the peak of the 2023 breeding season, and an estimated 17,400 pups may have died across the entire colony.

Sea Lions in Peru — Dead and dying sea lions were reported in Peru during an increase in avian flu infections. Klebher Vasquez / Anadolu Agency via Getty Images

Perez says researchers can’t definitively say why avian influenza is spreading so widely in animals. “I don’t think we have a good understanding of the molecular reason why these viruses, with this combination of gene segments, with these mutations, now have this inherent ability of infecting many more species than in the past.”

Globally, more than 50 wild mammal species have been affected, Wille says. In the U.S., H5N1 infections were detected in mammals for the first time in 2022. Since then, the virus has infected a range of mammal species across the country, including mountain lions, black bears, polar bears, bottlenose dolphins, harbor seals, coyotes, red foxes, minks, otters, squirrels, raccoons and opossums. The virus also doesn’t appear to only be spreading from birds—evidence suggests it has spread between minks in Spain and between seals in coastal New England.

With bird flu moving into cattle, is it safe to consume milk?

At the end of March, the U.S. Department of Agriculture (USDA) announced that dairy cattle in Kansas and Texas had tested positive for the virus. “Traditionally, flu A viruses don’t replicate in cows, so this is unique,” Perez says.

As of May 13, the virus has been detected in 36 dairy herds across nine states. Testing from the FDA has not found any live, infectious virus in dairy products, showing that the pasteurization process inactivates the virus and that pasteurized products are safe to consume. The FDA recommends that people not consume unpasteurized milk.

“Do I worry about the milk that I will buy on the shelf? Probably not,” Perez says. “But should I be drinking raw milk directly from a cow? Probably that’s a really bad idea.” Some states allow the sale of raw milk. Perez says it wouldn’t be a bad idea for states to rethink that. “Not because I’m saying it’s unsafe—it’s because we don’t know how safe it is,” he clarifies.

Person Shopping for Milk — A person grabs a gallon of milk at a grocery store in California. About a fifth of milk samples recently tested by the FDA had non-infectious viral fragments, which don’t pose a risk to humans. Frederic J. Brown / AFP via Getty Images

The FDA’s initial testing did find that about 20 percent of milk samples from grocery stores contained non-infectious viral fragments, which do not pose a risk to consumers. But the findings did suggest that more cows are infected than previously thought and asymptomatic infected cows are producing milk containing the virus, reports Stat News’ Megan Molteni. At the end of April, the USDA released data on viral genetic sequences from infected cows indicating that the outbreak in cows had started several months earlier than previously thought, the New York Times’ Apoorva Mandavilli wrote last month.

A recent USDA report found that the dairy cattle outbreak started in late 2023, and the USDA has detected the spread of virus between cows in the same herd and from cows to poultry. “Affected animals mainly shed the virus in milk,” Silva Del Rio writes via email. Researchers have theorized that the virus could be spreading between cows through contaminated milking equipment or milkers’ gloves, she says. Or, milk splashes could form aerosols that cows inhale or that contaminate feed, water or other objects.

As part of an effort to slow the spread, the USDA announced at the end of April that dairy cattle would now have to test negative for the virus before being transported across state lines, and labs and veterinarians must now report positive tests to the USDA.

Some researchers have called on the government to share more information on how the virus is spreading and changing. “I think at the level of the U.S. government, everyone is doing everything that they’re being asked to do and more, in terms of generating information as timely as possible,” Perez says.

What threat does H5N1 pose to humans?

Avian influenza viruses are categorized as low pathogenic or highly pathogenic based on how lethal they are to poultry, and H5N1 is considered highly pathogenic. Scientists used to think that highly pathogenic avian influenza didn’t pose a risk to humans, according to Perez. “That changed completely in 1997,” he says. That year, H5N1 was transmitted from animals to humans in Hong Kong, infecting 18 people and killing six. Between 2003 and April 1, 2024, 889 infections and 463 deaths in humans from H5N1 were reported across 23 countries. Bird flu viruses have only spread from one person to a close contact in rare instances and have never resulted in sustained transmission between humans, according to the CDC.

Aside from H5N1, other strains of avian influenza have led to infections in humans. One person in the U.S. tested positive for H7N2 in 2002, and a second tested positive in 2003. H7N3 was detected in two people in Canada in 2004. None of these four cases was fatal. But three people tested positive for and died from H7N9 in China in 2013. Other sporadic cases of avian influenza have occurred in people throughout the 21st century.

Only two people in the U.S. have tested positive for H5N1—one in Colorado in 2022, and one in Texas earlier this year, both of whom only had mild symptoms. The person in Texas works on a dairy farm and reported eye redness as their only symptom. They reported direct and close exposure to sick cows with symptoms similar to those of cows infected with H5N1.

The CDC says that the spread of H5 viruses in animals and in two people in the U.S. has not affected the risk the virus poses to humans, which remains low. “The virus does not show an indication that it’s any better to infect humans than it was ten years ago,” Perez says. Silva Del Rio writes via email that there could be occasional infections among people like dairy workers or workers at milk processing facilities who are exposed to sick animals or contaminated, unpasteurized milk. The virus is not currently spreading between humans, and infections have been limited. If there were multiple, simultaneous reports of H5N1 infections in humans or identified spread between people, that would raise the public health risk, the CDC says.

“The risk is today low, but we should not be complacent, and I don’t think we are being complacent,” Perez says.

Pelican With Bird Flu — A pelican carcass washed up on a beach in Lima, Peru—the bird was likely killed by an outbreak of avian flu. Car / Fotoholica Press / LightRocket via Getty Images

Researchers will continue to monitor the virus for genetic mutations or signs of further spread that might indicate the virus has become more transmissible or poses more of a risk to humans, Silva Del Rio says.

But Perez calls the events of the last couple years “a big tragedy” for wildlife. “We are talking about very fragile ecosystems in which an outbreak like this may wipe out an entire species,” he says. “In that sense, this is a disaster.”