The bird flu virus that has beset dairy farms since early last year may be spreading through the air in so-called milking parlors and through contaminated wastewater, as well as from milking equipment, scientists have found.

The Department of Agriculture has said that the virus spreads primarily from milking equipment or is carried by dairy workers and vehicles traveling between farms.

But in the new study, scientists found live virus in the air of milking facilities, suggesting that cows and farmworkers might have become infected by inhaling the pathogen. The virus may also spread by water used to clean cattle barns or contaminated with discarded milk.

The study was posted online last week and has not been peer reviewed for publication. But the results are consistent with those from other teams who found that contaminated milking equipment might not explain all cases of bird flu observed on farms.

In the new study, researchers also found infected cows with no visible symptoms, suggesting that bird flu may spread from cows that appear healthy.

Just as people can become infected with the seasonal flu virus by touching a sick person, from contaminated surfaces or by inhalation, “there is no one way” that bird flu spreads, said Seema Lakdawala, a virologist at Emory University who led the work.

The bird flu virus, called H5N1, was first detected in dairy cows in March 2024. Since then, it has spread to more than 1,000 herds in 17 states, the majority of them in California, the nation’s leader in dairy production.

Bird flu has also infected dozens of people, hospitalizing a few and killing one.

In Asia, bird flu has been steadily causing new infections, including 14 in Cambodia this year alone. But in the United States, there has been little news of the virus in many months.

Federal officials have not held any briefings about the bird flu outbreak since President Trump took office in January.

That may change as the migratory season for birds picks up again in the fall, said Dr. Andrew Bowman, a veterinary influenza expert at Ohio State University who was not involved in the new study.

The infections in dairy cattle came as a surprise last year, as bird flu was not known to affect the animals. H5N1 appears to be particularly concentrated in the cows’ milk, turning it yellow and viscous.

Milking machines are often hooked up to many cows in quick succession and may not be thoroughly disinfected in between. The machines quickly became the suspected source of transmission on dairy farms.

The virus “replicates so darn well in the mammary gland, and we have these mechanical pieces of equipment that are touching lots of udders,” Dr. Bowman said.

But cows deliberately exposed to contaminated milking equipment did not become sick, he and his colleagues reported in June.

Dr. Lakdawala and her colleagues at first believed that milking machines would prove to be the main source. They collected samples from air, wastewater, milking machines and cows on 14 California dairy farms that reported bird flu outbreaks in late 2024 and early 2025.

They found the virus in the milk of sick cows and on milking equipment. To their surprise, the researchers also found infectious virus in air samples collected from parlors while cows were being milked.

A handful of exhaled breath samples collected directly from cows contained traces of viral genetic material but not live virus.

That suggests that the virus floating in the air of milking parlors came from airborne droplets of milk, Dr. Lakdawala said. “There’s tons of aerosols being generated during the process of milking,” she said.

Some farmworkers are thought to have become infected when milk splattered in their eyes or faces. Cats that drank contaminated milk on farms have also become severely ill. Many have died, a worrying hint that people who drink unpasteurized milk from infected cows might become sick.

It’s unclear how often the virus is spread through a particular route of transmission. In some cases, all may contribute to an infection in a cow or a person. What is clear is that milking parlors pose a high risk to farmworkers, underscoring the importance of wearing protective equipment in those areas.

“It’s not necessarily floating around where the cows are raised, but in the more confined environments where cows are being milked,” said Troy Sutton, a virologist at Pennsylvania State University who was not involved in the new work.

Milking parlors are notoriously hot, humid and poorly ventilated, dissuading workers from wearing masks. But even face shields, though imperfect, may greatly reduce risk, Dr. Lakdawala said.

The finding that some infected cows have no symptoms suggests that the virus may be able to spread undetected through farms. Each infection gives H5N1 new chances to evolve and to hit upon the mutations it needs to become deadlier to people.

“We’re going to have more human infections perpetually,” Dr. Lakdawala said.

“It could create something that is going to be quite dangerous in a few years,” she added.

So far, most infected Americans have not become dangerously ill. That may be because exposure to seasonal flu, and in particular to the H1N1 virus that has been circulating since 2009, protects against severe illness with H5N1, according to a recent study in ferrets led by Dr. Sutton.

Most people are assumed to have been exposed to at least one flu virus within the first five years of life. Children under 5, and those with weakened immunity because of age or other medical conditions, would be expected to be most vulnerable, Dr. Sutton said.