Study finds airborne and wastewater spread risks for H5N1 on California dairy farms
Researchers found infectious H5N1 in air, cows’ breath and wastewater on infected California dairy farms, pointing to more transmission routes than raw milk alone.
A new study suggests that H5N1 bird flu on California dairy farms may be moving through more routes than contact with contaminated milk alone. Insurance Journal, citing research published in PLOS Biology, reports that scientists detected the virus in air samples taken during milking, in cows’ breath and in farm wastewater. The findings point to a wider contamination picture on infected farms and a more complex transmission environment than previously assumed.
The study covered 14 infected dairy operations. Researchers reported infectious H5N1 in the air and at reclaimed wastewater sites, and they also found high viral loads and H5 antibodies in milk, including milk from cows that showed no obvious clinical signs. That matters operationally because asymptomatic infections can allow the virus to circulate within herds without immediate detection, delaying containment and raising the odds of worker exposure.
The paper arrives as California remains the centre of the outbreak in US cattle. According to the article, more than 700 dairy herds in the state have reported H5N1, and the virus has spread to at least 16 states since it was first identified in cattle in the United States in 2024. The latest livestock detections mentioned in the piece came from Idaho, which reported five infected herds in April.
Until now, raw milk has been the main focus in discussions about how the virus passes between cows and potentially to people. The new evidence does not rule that out, but it shows that milk is only part of the story. If infectious virus is also present in aerosols and waste streams, workers may face exposure through inhalation and through contact with contaminated surfaces, especially in milking parlours and milk-disposal areas.
The researchers say that biosecurity measures should now target those additional pressure points. The article highlights interventions in dairy parlours to reduce aerosols and the need to inactivate so-called sick milk before disposal. For the dairy sector, that implies a broader response: closer monitoring of animals without visible symptoms, tighter waste management, stronger protection for staff and updated farm protocols in locations where H5N1 has already become established.