Humans can catch bird flu, but not easily. What experts say about symptoms and risks

Farm workers in close contact with dairy cows and poultry are the most likely to catch bird flu because it has never spread between people, experts say.
Farm workers in close contact with dairy cows and poultry are the most likely to catch bird flu because it has never spread between people, experts say. (Joshua Lott/Reuters)

Three people in the United States — all of them dairy farm workers — have tested positive for bird flu this year amid the outbreak among dairy cattle, and the U.S. government is ramping up efforts to keep the virus from spreading further. Bird flu, also known as avian influenza, is hardly a new virus, but the latest cases in animals and humans have raised concerns about its spread.

For the record, the Centers for Disease Control and Prevention maintain that bird flu’s risk to the general public is “low.” But how do humans catch it in the first place, what symptoms does it cause and how can someone further reduce their risk? Here’s what we know so far.

Humans can catch bird flu, but not easily. Avian flu has been around and infecting wild birds and poultry since 1996. There have been nearly 1,000 known cases of bird flu in humans (889 between 2003 and May 3, 2024, according to the World Health Organization). But the virus has never spread between people, Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, tells Yahoo Life.

Instead, infections happen as a result of what scientists call “spillover” from humans to animals, Erin Sorrell, an associate professor at the Johns Hopkins Center for Health Security, tells Yahoo Life. People are only catching bird flu after having contact — most likely a lot of close, prolonged contact — with infected animals or the farm equipment and materials the animals were in close contact with.

The infected dairy farm workers were “at eye level with the udders in the milking process, so, if they don't have proper eye protection, that is a perfect route of infection,” Sorrell says. “Either their eyes had direct contact with drops of milk or their hands were contaminated and then they rubbed their eyes.” Eye tissue is also less equipped with the types of immune processes that help other parts of the body, including the respiratory tract, fight off infections, making the eyes somewhat more susceptible, Sorrell adds.

So far, there haven’t been mutations to the virus that would make it better at infecting people or spreading between humans. “For this virus to become adapted in a way that it can be transmitted by humans to humans is going to take a number of changes, and we have not seen those changes,” Osterholm says.

All three Americans infected in this year’s bird flu outbreak developed pink eye, also known conjunctivitis. For two dairy farm workers, eye irritation was the only symptom, while the latest person infected developed mild respiratory symptoms more typically expected in influenza. The worker had a cough, but no fever.

It’s not the first time that pink eye has been seen as a symptom of bird flu. The majority of people infected with a different strain of bird flu amid a 2003 outbreak in the Netherlands developed conjunctivitis. But in prior human cases of the strain of bird flu causing the current outbreak — H5N1 — people have mainly had high fevers and respiratory symptoms.

Anytime you have symptoms of flu, a cold or pink eye, you should see a health care provider, says Sorrell. But, unless you’ve been in close contact with wild birds, poultry or dairy cattle, it’s highly unlikely that those symptoms are due to bird flu.

“There are two different risk groups for risk of infection and infection,” and thus two different sets of advice to the public, she says. “For the general population there is a very low risk of exposure and infection. But, for individuals working with dairy cow, with poultry and for the agricultural workforce in general, there is a much higher risk of exposure.”

It’s crucial that those in this latter group have, and wear, proper personal protective equipment, including gloves, face masks or respirators, goggles or a face shield and aprons or coveralls, according to the CDC. If you work on a farm, especially one where there are infected or potentially infected dairy cows, chickens or other animals, you should monitor yourself for pink eye or any flu-like symptoms.

For most people who aren’t spending a lot of time in close contact with farm animals, there’s really only one thing you need to do to prevent possible infection with bird flu, Osterholm and Sorrell say: Don’t drink raw milk. There haven’t been any human cases resulting from raw milk (which has not gone through the pasteurization process that kills bird flu and other viruses), but high concentrations of virus have been found in it, and barn cats have died after drinking it.

However, the Michigan Health Department warned against field trips and visits to Michigan dairy and poultry farms and factories on June 4, CBS News reported — two of the three infected dairy farm workers were based in the state. “I think that’s a very proactive, cautious approach,” Sorrel says. She adds that everyone should stay up-to-date on the latest information from local and federal health authorities about how the outbreak is evolving. Limiting contact with possibly infected animals is important to keeping more people from getting sick, and to denying the bird flu virus the opportunity to evolve to be better at infecting humans, experts say.

In the past, Osterholm, who has traveled to areas with previous outbreaks to study bird flu, has been concerned it could become a pandemic, “and then nothing happened,” he says. “In influenza, you should surely expect the unexpected, but there’s no evidence right now” that bird flu has mutated in a way that is likely to cause a pandemic, he says. “I’ll start sleeping with one eye open when we have evidence of person-to-person spread.”