Potential airborne transmission of avian influenza observed on dairy farms, according to researchers' findings
The H5N1 bird flu virus, first detected in dairy cows in March 2024, has spread to over 1,000 herds in 17 states, with the majority in California, the nation's leader in dairy production. Recent research on dairy farms in California has shed light on the multiple pathways through which the virus spreads.
These pathways include airborne transmission within milking parlors and contamination via wastewater or water used for cleaning barns.
Airborne spread is a significant concern. Studies have detected H5N1 viral RNA and infectious virus in the air of milking parlors during milking operations. This indicates that both cattle and farm workers can be exposed by inhaling virus-laden particles. Air samples collected with different air-sampling devices, including those simulating worker breathing zones, confirmed the presence of the virus airborne on affected farms.
Contaminated wastewater and cleaning water also pose a risk. The virus has been found throughout wastewater systems on dairy farms, including in manure lagoons that are accessible to migratory birds. Water used to clean milking equipment and parlors also tested positive for the virus, suggesting it can act as a vector for virus spread within the farm environment.
The milking environment itself is a critical hotspot. The airborne virus and contaminated surfaces or water converge in this area, facilitating transmission to both cattle and handlers.
These findings represent a shift from initial beliefs that transmission occurred mainly through direct contact, milk contamination, or movement of infected animals and equipment. Instead, airborne virus in milking parlors and contaminated wastewater systems are now recognized as important contributors to ongoing H5N1 transmission on dairy farms.
Although H5N1 viruses in dairy cattle retain avian virus characteristics and show limited ability to infect humans via airborne transmission, close contact with infected animals remains a risk, especially in these contaminated farm environments.
The virus has also been found in the milk of sick cows and on milking equipment. The bird flu virus appears to be particularly concentrated in the cows' milk, turning it yellow and viscous.
Moreover, some infected cows have no visible symptoms, suggesting that the virus may spread undetected through farms. This is concerning as it could allow the virus to evolve and potentially become deadlier to people.
Some farmworkers are thought to have become infected when milk splattered in their eyes or faces. Cats that drank contaminated milk on farms have become severely ill, with many dying.
Given these findings, it is crucial for farmworkers to wear protective equipment in milking parlors and for milk to be thoroughly pasteurised before consumption.
The research was conducted as recently as August 2025, highlighting the ongoing threat of the H5N1 bird flu virus on dairy farms.
- The environment in milking parlors, where the airborne virus and contaminated surfaces or water converge, is a critical hotspot for the transmission of the H5N1 bird flu virus among both cattle and farm workers.
- Contaminated wastewater systems on dairy farms, including manure lagoons and water used to clean milking equipment and parlors, are significant contributors to the ongoing spread of the H5N1 bird flu virus.
- Recent research suggests that the H5N1 bird flu virus can spread undetected through farms, as some infected cows show no visible symptoms, potentially allowing the virus to evolve and become deadlier to humans.
