Potentially deadly bird flu has been detected in the air, raising the risk of outbreaks, researchers have warned.
Bird flu, also known as H5N1, typically infects wild birds and domestic poultry, as well as herds of dairy cows. This leads to the virus usually being detected in unpasteurized milk.
It is typically spread via contact with saliva, mucus and feces – not in the air.
However, researchers studying California dairy farms found that during active bird flu outbreaks, the virus could be detected in the air of milking rooms, enclosed spaces where farm workers could be exposed.
The findings suggest contact with birds and contaminated milk and milking equipment may not be the only cause of bird flu’s spread, raising the risk of humans becoming infected.
Instead, the virus may be able to spread in the air through droplets of milk that get released during the milking process.
Of the 71 Americans who have been infected with bird flu since 2024 – which includes two deaths – the majority were farm workers exposed to infected animals.
Additionally, the researchers found cows that seemed healthy and did not show signs of H5N1 still carried the virus antibodies, suggesting prior infection that previous testing had not detected.
The scientists said the findings show a need for increased and more extensive testing on farms for H5N1.
Researchers in a new study have found H5N1, also known as ‘bird flu,’ may spread in the air (Pictured above: USDA workers disinfect a work crew at a turkey farm in Minnesota, 2015. File photo)
‘Dairy parlors, which are often enclosed spaces and where aerosolization of milk occurs, pose the greatest threat from inhalation of the virus to dairy farm workers compared to the open-air housing pens,’ the study authors wrote in PLOS Biology.
Bird flu has infected 180 million farmed birds since 2022 and more than 1,000 herds of dairy cows since early 2024.
In January 2025, an unidentified person older than 65 with underlying health conditions became the first US bird flu death after they were hospitalized with severe respiratory symptoms.
While almost all bird flu patients had direct contact with infected birds or cattle, a patient in Missouri became the first to be infected without any exposure to these animals last year.
It’s still unclear how that patient became ill.
Symptoms in humans include eye redness and irritation (conjunctivitis), mild fever, cough, sore throat, runny or stuffy nose, body aches, headaches and fatigue, according to the CDC.
In rare but severe cases, patients may suffer pneumonia, respiratory failure, kidney injuries, organ failure, sepsis and inflammation of the brain (meningoencephalitis).
In the new study, researchers conducted air sampling on California dairy farms during active H5N1 outbreaks between October 2024 and April 2025.
These farms included five in California’s Central Valley between October and December 2024. Subsequently, the researchers looked at seven southern California and two Central Valley farms between February and April 2025.
Data from the California Department of Public Health shows the state has detected 38 human cases of bird flu since 2024, along with more than 700 dairy herds.
Researchers tested collection devices in milking rooms and housing areas, including one worn in a backpack to mimic the exposure a farm worker would have.
Air samples were collected from exhaled breath from individual cows and rows of cows in housing pens, as well as from milking parlors during milking and areas with wastewater such as fields.
Cows are pictured grazing in a California field in December 2024. Researchers have found that cows that appear healthy may still have H5N1 antibodies, a sign of prior infection (stock image)
A total of 71 air samples were collected and analyzed in the first phase of the experiment for the presence of H5N1, and six samples came back positive, which included those from breathing zones of the rows of cows.
In the second phase, 35 air samples were collected in milking rooms, with 21 coming back positive. In four of the samples, the virus was still alive, making it capable of causing infections.
The researchers believe the milking process sends fine droplets of milk into the air, and during an outbreak, those droplets may carry H5N1.
Additionally, the team found live H5N1 in two wastewater samples taken from one farm.
On one farm, researchers also looked at cows that had recovered from an outbreak, those that showed a temporary drop in milk production and those that never had any sign of illness.
When the researchers tested milk from all groups, every recovered cow tested positive for antibodies, which are produced after an H5N1 infection. Six out of the 10 cows that showed no signs of illness also had antibodies, indicating prior exposure that the farm had not previously detected.
On a separate farm, seven cows tested positive for H5N1 in their milk without showing mastitis, inflammation of their utters that is a primary warning sign of the bird flu virus in dairy cattle.
‘Together, these results highlight the extensive environmental contamination of H5N1 on affected dairy farms and identify additional sources of viral exposure for cows, peridomestic wildlife, and humans,’ the researchers wrote.