Over the past three years, bird flu — caused by the avian influenza A (H5N1) virus — has extended its reach around the U.S. and across many species. The virus arrived in wild birds, but it has now infected animals ranging from chickens and cows to mountain lions and harbor seals. Over 480 bird species and dozens of mammal species have been affected.

That includes humans. Seventy people in the U.S. have gotten sick with bird flu in the past year, nearly all exposed to the virus through birds or cattle. Although most of the cases were mild, one person died from their illness.

At a recent National Academies webinar, three experts gathered to offer answers to common questions about the disease — how it spreads, who is most at risk, and how the virus might evolve in the future. The conversation was the first in A Healthy Exchange webinar series about bird flu.

An old virus behaving in new ways

Bird flu is not a new virus, explained Gino Lorenzoni, associate professor of poultry science and avian health at Pennsylvania State University. It was first identified in Northern Italy in the late 1800s and known as the “fowl plague.” In 1996, the H5N1 lineage that is circulating today was first identified in geese in Guandong, China, and it has spread globally during the past 20 years, Lorenzoni said.

While bird flu isn’t new, the virus’s behavior is changing, said Nahid Bhadelia, founding director of Boston University’s Center on Emerging Infectious Diseases. “It’s acting in new ways and acting in surprising ways and infecting so many more animals that are closer to us than wild birds are,” she said. “That’s raising the alarm.”

The virus’s 2022 arrival in North American birds meant it had expanded to six continents, said Bhadelia. That same year saw the U.S.’s first human case of bird flu — an isolated case involving mild symptoms, contracted from poultry.

About a year ago, there was a transmission from birds into cows — the first time that has happened, said Bhadelia. The virus then started spreading in dairy herds across the U.S.

“We don’t all interact with wild birds, but many more of us interact with domestic animals, particularly dairy cows,” said Bhadelia. “Now you’re creating an environment where more humans are coming into contact with mammals who may have an infection and get infected — and that’s exactly what we’ve seen over the last year.”

About a year ago, the first mammal-to-human transmission of H5N1 occurred, spreading from a dairy cow to a person in Texas, she said. The 70 human cases of bird flu in the U.S. so far have been sporadic — scattered transmissions in which an individual was infected by an animal.

There is no sustained human-to-human transmission, Bhadelia said, though there have been rare cases where there may have been a transmission between humans.

“The way that this virus has currently evolved is that it is not adapted to the receptors in our airways,” she explained. “That makes it very hard for the virus to go from human to human.”

For a virus to become an epidemic or pandemic, it needs to be good at human-to-human transmission, and currently bird flu is not, said Bhadelia.

Who is at risk?

Right now, said Bhadelia, the CDC’s assessment is that the risk to the general public from bird flu is low.

The people at highest risk of contracting H5N1 right now are those whose occupations involve contact with animals that may have the virus, such as poultry farm workers and dairy farm workers.

“That’s where we’ve seen the majority of infections,” she said. “That’s why the use of personal protective equipment and biosecurity on farms is one of the important things that has been stressed in trying to control this outbreak on those farms, as well as trying to diagnose workers who may present with symptoms.”

While over the past 20 years H5N1 in humans globally has generally been severe — with a fatality rate of 40 to 50 percent — most of the recent U.S. cases have had milder symptoms, said Bhadelia. We also have tools to diagnose and treat the disease, she said — including antivirals such as Tamiflu that have been shown to be quite effective against the current version of the virus. Candidate vaccines against the current virus are in development as well.

Although the risk to the general public from bird flu is low, it’s not as low as it was two years ago, Bhadelia said, because there are more birds and mammals around us that have the virus. “We have to be more aware of our environment,” she said.

For example, having a backyard poultry flock could raise your risk of exposure to a diseased animal, she added; so might having a bird feeder in an area where there is lots of H5N1, in the event a bird sickens or dies in your yard.

Lorenzoni explained how infected migrating birds can leave virus in their droppings around bodies of water where their flocks stop. A person out walking their dog or hunting near one of these reservoirs, for example, could track that infected fecal matter on their boots back to a domestic farm and animals.

One possible route of human exposure is drinking raw milk, which carries risks not only of H5N1 but also other pathogens such as salmonella and listeria, said Bhadelia. Drinking pasteurized milk is safe, she emphasized.

Are our pets vulnerable to bird flu? Dogs do not seem to be at risk for disease from this virus, said Treana Mayer, a postdoctoral fellow in microbiology at Colorado State University. Cats, on the other hand, are susceptible, she said. Indoor-outdoor cats, barn cats, and feral cats could potentially contract the virus by hunting and coming into contact with infected wild birds. Cats could also be exposed to the virus if fed raw poultry or raw milk. When cats do get H5N1, they don’t seem to pass it on to humans or other species, she added.

A still-evolving virus

Asked about the path the virus is likely to take in the future, the speakers stressed the many unknowns. “The only thing we know for certain is that this virus is going to continue to change, and we don’t know in what direction,” said Mayer.

Bhadelia described the more worrisome possibilities — that the virus could evolve so that human-to-human transmission is easier, or to be deadlier, or to evade the treatments we have. “Those are the worst-case scenarios,” she said. “I can’t tell you if it will happen six months from now, a year from now, or never. I hope never.”

Surveillance for the disease in humans is important, Bhadelia said, because it would detect if there were clusters of infections that didn’t come from animal transmission, suggesting possible human-to-human transmission.

Mayer pointed to a small, yet hopeful development — studies showing that a lot more raptors, which once suffered high mortality rates from H5N1, are now surviving infection and mounting immunity. “That gives me a tiny bit of hope that potentially this virus might become less lethal in some animal groups,” she said.

One crucial way to lessen the odds of a bad trajectory, Bhadelia stressed, is to give the virus fewer chances to infect people, and therefore fewer chances to mutate.

“The one way we can prevent the virus from evolving further to adapt to humans is to reduce its chances to cause sporadic infections in humans,” said Bhadelia. “Which is why protecting the workers — particularly those from vulnerable populations, who have language barriers, farmworkers who are migrants who may not have access to health care — ensuring they are tested and get access to care [is important] … Because the more humans this virus affects, it’s like Russian roulette — it could give it chances to adapt.”

Watch webinar recording and view additional resources.

Bird flu and the egg supply

Bird flu has infected around 70 million laying hens in the U.S., and unfortunately infected animals need to be culled, explained Lorenzoni. Fewer hens means fewer eggs, and the relative scarcity drives up prices. Higher egg prices also reflect the higher cost of doing business, as poultry farms have had to implement PPE and other biosecurity measures.

The effects of H5N1 on the supply and price of eggs will be among the subjects discussed at the next webinar in the series, on June 3. The conversation will also explore how farmers are coping with the challenge of H5N1, and the role played by state and local officials.