Air and wastewater may help spread bird flu on California dairy farms, according to a study that points to more transmission routes than previously understood.
Researchers sampling 14 infected farms found the H5N1 virus in the air during milking, in cows’ breath and in farm wastewater, pointing to multiple possible routes of transmission beyond contact with contaminated milk.
The findings, published Tuesday in PLOS Biology, add to concerns that the outbreak in U.S. cattle could expose farm workers and other animals. H5N1 is spreading globally in birds and spilling into mammals including cattle, elephant seals and sea lions. Human cases remain uncommon and are largely tied to close contact with infected poultry and other farm animals. Scientists worry ongoing spread in mammals could help the virus evolve toward more efficient human transmission.
“Our data confirm the presence of infectious H5N1 virus in the air and reclaimed farm wastewater sites,” Seema Lakdawala, a microbiologist at Emory University who co-led the study, said in a statement. “In addition, we observed high viral loads and H5 antibodies in the milk of cows, including those without clinical signs, suggesting that multiple modes of H5N1 transmission likely exist on farms.”
The study also found many infected cows showed no obvious signs of illness, indicating the virus may be spreading undetected within herds.
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The H5N1 flu virus has been reported in more than 700 dairy herds in California alone and has spread to at least 16 states since it was first identified in cattle in the US in 2024. Idaho reported five infected herds in April — the latest detections in livestock.
Scientists have largely focused on raw milk as the main route of transmission between cows and to people. But the new research suggests that’s only part of the picture.
“Detection of infectious virus in the air and waste streams on farms was surprising but highlights that there is a considerable amount of infectious virus on farms and multiple sources of infection exist,” the authors said.
The results point to widespread contamination on affected farms, raising the risk of exposure for workers through inhalation or contact with contaminated surfaces.
“Targeted interventions in the dairy parlors to reduce the amount of aerosols in the air and inactivation of ‘sick milk’ prior to disposal will provide additional barriers to infection of farm workers and likely other farm animals,” the researchers said.
The study was based on environmental sampling rather than tracking infections directly, and more research is needed to confirm how the virus spreads, they said.
Copyright 2026 Bloomberg.
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