More than three years into a worldwide outbreak of bird flu, the virus continues to expand in the U.S., with growing impacts to food production and animals. Over 80 million chickens, thousands of wild birds and dozens of mammal species, including a polar bear, have been infected. 

Now it's running rampant among dairy cows, turning up in 94 herds across 12 states since March. The latest animal to test positive was an alpaca on an Idaho farm.

“It's gigantic, the scope and scale of the presence of the disease,” said Julianna Lenoch, national coordinator for the Department of Agriculture’s wildlife disease program.

This scale ‒ and related concerns ‒ are reflected in the price of eggs, renewed warnings to cook ground beef and eggs thoroughly, and in extraordinary measures dairy and poultry farmers are being asked to take to prevent its spread. 

As the outbreak lingers and expands, it’s prompting growing concerns about the risks to humans and the influence of warmer temperatures and more extreme weather events in making this and future pandemics worse.

How widespread is this bird flu outbreak?

The highly contagious H5N1 virus has spread to six continents since the first detections in Europe and Asia in 2020. It has been reported across North and Central America and most of South America and has been found on every continent except Australia. It also turned up in Antarctica last fall raising alarm about potential consequences for some of the world's most beloved birds: penguins.

Timeline:From chickens to foxes, here's how bird flu is spreading across the US

The U.S. has experienced avian influenza outbreaks in the past, but this one is lasting longer and is more widespread. Domestic poultry flocks, either in commercial operations or backyard flocks and farms, have been infected in every state except Louisiana and Hawaii, including more than 5.9 million birds just since May 1.

Since 2022, infections have been reported in 14 million turkeys and 80 million chickens, including 71 million egg layers. Farmers must kill chickens and turkeys when a poultry flock tests positive, and experts say the slaughter to prevent human infection has helped drive up the cost of eggs.

Infected mammals have been found in 31 states, with the greatest number of infections found in foxes, mice, striped skunks, mountain lions, cats and harbor seals.

Research studies find the prolonged presence and spread of the virus increases the risk of genetic mutations that could allow it to pass more easily from animals to people and among people. 

“The longer we have virus out there, the more possibility there is for changes,” said Lenoch, who oversees the federal program responsible for tracking the virus in wild birds. 

Can humans get bird flu? 

Yes, but the risk in the U.S. is still very low, federal officials reiterated in a Thursday briefing. They say the public should be “alert but not alarmed.” 

Since it arrived during the winter of 2021-2022, four people have tested positive in the U.S. All were exposed to the virus on farms. In the first case, in 2022, the worker was helping to cull infected poultry on a farm. All three patients this year had exposure to dairy cows. Two only reported conjunctivitis, or pink eye, while the third also experienced upper respiratory symptoms. No one in the U.S. has died from the virus, according to federal officials, but deaths have been reported internationally.

So far, for the general public, everything but raw milk is considered safe, federal officials say, as long as you’re cooking ground beef and eggs all the way through. Cooking takes care of any remnants of the virus that could be in egg yolks or in ground beef after once-infected dairy cows are shipped to market to be butchered. 

In milk, pasteurization kills the active virus so it can’t be transmitted, federal research shows. However, harmless traces of the virus remain, and are found in an estimated 20% of the nation’s milk supply. 

The Food and Drug Administration said it does not know if the virus can be transmitted in raw milk, but has asked states that allow the sale of raw milk to restrict it as a precautionary measure.

“The fact that it’s in 20% of our milk supply should be disturbing to everyone because that means it’s gone around already,” said Xavier Becerra, U.S. Health and Humans Services secretary, during a speech to the Western Governors Association last week. “If it starts to jump, that’s when we really have to worry."

Why are officials concerned about human exposure to bird flu?

The great concern among federal agencies and researchers is that the virus will evolve and become more contagious among humans. For now, it’s hard for humans to contract the virus and it hasn't been passed from person to person.

Without stringent measures to avoid repeated viral transmission between wild and domestic animals, experts say the risks to people could grow. 

The Department of Agriculture and Centers for Disease Control and Prevention are urging farmers to step up efforts to prevent the spread by cleaning and disinfecting equipment, especially when the same equipment is used to handle manure and feed and to better protect farm employees.

Preventing farm to farm spread is “really critical,” agency officials said Thursday. 

How is this outbreak of avian flu different from previous ones? 

The outbreak is “unique in global expansion” and in the number of bird and mammal species that it’s infecting, concluded a study published earlier this year by Tufts University researchers Jonathan Runstadler and Wendy Puryear.  “Though the risk to humans remains low, this unexpected outbreak well illustrates the continued need for vigilance and further study," the study stated.

Avian influenza is spread globally among birds, particularly migratory waterfowl such as ducks. They’re natural reservoirs and migrate over long distances, traversing hemispheres in some cases. As they travel, infected birds shed the virus in mucus, saliva, and feces.

In previous outbreaks, wild birds would often get exposed and just carry the virus around with no symptoms, Lenoch said. With the current strain that has evolved, wild birds have been getting sick and dying in large groups. 

The virus raises many concerns, not only because of its impacts on human health and agriculture, but also because it's killing wildlife, such as seabirds, raptors and marine mammals, said Diann Prosser, a research wildlife biologist with the U.S. Geological Survey's Eastern Ecological Science Center.

The spillover into dairy cows is rare and officials aren’t certain how it started. One study released by federal officials this week said it likely started with wild birds infecting a cow in the Texas Panhandle. Cows from that herd, which weren’t showing any symptoms at the time, the report stated, were shipped to Michigan, where the virus quickly spread to other states. Investigations continue.

Typically, outbreaks eventually burn themselves out as wild birds build immunity and stop spreading the virus. That’s expected to occur this time, as well, but it’s taking longer than usual.

How is bird flu spreading?

Scientists don't fully understand all the methods of transmission, but most involve bird poop. 

Wild waterfowl are the main carriers, said Maurice Pitesky, an associate professor in cooperative extension, poultry health and food safety epidemiology at the University of California Davis.

The first four U.S. birds discovered with it were wild ducks taken by hunters in the Carolinas. Lenoch said the nation's duck hunters, who have their own suggested safety guidelines, have been invaluable at working with officials to get wild waterfowl tested.

Pitesky listed these examples of ways the bird flu virus may be transmitted:

• Virus spreads from wild bird poop in farm ponds or inside buildings.
• It can become aerosolized and passed in the air.
• A group of free-roaming cats died after contracting the virus from drinking rawmilk and showing neurological symptoms.
• Animals eat infected birds.
• Farm employees can track in shavings or dirt that may carry the virus from wetlands and farm fields. 
• Farm tractors and other equipment can carry infected materials between farms.

A visual guide:The bird flu outbreak

Could climate change play a role in this bird flu outbreak?

A key difference between this outbreak and previous ones is an increase in global average temperatures, especially since this outbreak started. Last year was the hottest year on record, and this year appears to be charting a similar course. 

The potential for warmer temperatures to alter the transmission of viruses and contribute to global pandemics has long been a concern to scientists studying potential links between climate change and the spread of viral outbreaks and pandemics. 

Prosser was a co-author on a study last fall that pointed out changing climate patterns and extreme weather events closely parallel this “unprecedented global spread of highly pathogenic avian influenza.”

There's no single answer to the link between climate change and the bird flu outbreak in the U.S. and further studies are needed, she said. On one hand, flu particles could degrade more quickly in warmer conditions which would decrease its ability to spread. However if heat waves cause changes in food resources or immune stress in humans or animals, the virus might be able to spread more easily, she said.

Is shrinking space for birds playing a role?

The warming climate isn’t the only thing altering the environment, Pitesky said. Human encroachment on agriculture and wild landscapes may be an even bigger potential factor.

Farms now increasingly are found near wild areas, especially wetlands, as agricultural areas expand and droughts dry up existing wetlands.

"We're producing more poultry than ever before and the more poultry we've produced, the less room for wild waterfowl, and less habitat for animals to roost and feed," Pitesky said. If these wild areas overlap with domestic poultry and cattle farms, it increases the potential for more disease transmission.

"In California we’ve lost 95% of our natural wetlands, primarily to agriculture,” he said. “There’s obviously benefits to that when it comes to feeding 8 billion (people)." But an unintended consequence is "waterfowl are using suboptimal habitats."

Dinah Voyles Pulver covers climate and the environment for USA TODAY. Reach her at [email protected] or @dinahvp