For months, discussions around H5N1 in dairy cattle have focused largely on infected milk and contaminated milking equipment. New research from California suggests the transmission picture may be far more complex.
In a study published this week in PLOS Biology, researchers investigating 14 H5N1-positive California dairies found evidence supporting several possible transmission pathways, including aerosols generated during milking and contamination within dairy wastewater systems. The study also identified signs of subclinical infection in some cows, raising new questions about how easily infected animals may be missed during outbreaks.
The findings add to growing evidence that the dairy environment itself may play a larger role in H5N1 transmission than previously recognized.
Infectious H5N1 Virus Detected in Dairy Parlor Air
One of the study’s most significant findings came from air sampling inside milking parlors.
Researchers collected aerosol samples during milking and detected not only viral RNA, but infectious H5N1 virus in some air samples. Viral material was also identified in exhaled breath collected from infected cattle.
Environmental sampling findings included:
- Infectious virus recovered from parlor air samples and wastewater systems
- Viral RNA identified in cow breath samples
- Evidence of infection in some cows without obvious clinical signs
The distinction between viral RNA and infectious virus is important. Detecting RNA alone does not confirm viable virus is present, while recovery of infectious virus suggests aerosolized particles could potentially contribute to transmission.
The authors stopped short of concluding that airborne spread is a primary transmission route on dairies. However, the findings raise new questions about respiratory exposure risks in enclosed milking environments.
Milking parlors routinely generate aerosols through animal movement, splashing fluids, equipment use and high-pressure cleaning systems. The study suggests those environments may warrant closer attention during outbreak investigations and biosecurity planning.
The findings also have implications for worker safety. Since the U.S. dairy outbreak began, human infections linked to dairy cattle exposure have generally been mild, with conjunctivitis among the most commonly reported symptoms. Aerosol exposure during milking has remained a persistent concern for occupational health experts.
Wastewater Systems are a Possible H5N1 Exposure Route
Researchers also detected widespread contamination throughout dairy wastewater systems.
H5N1 viral RNA was identified in parlor drains, wastewater sumps, lagoons and reclaimed water systems. Infectious virus was recovered from some wastewater-associated samples as well. This finding may be particularly relevant for modern dairy operations, where reclaimed water is frequently reused for flushing and other management purposes.
The authors noted wastewater systems could create additional opportunities for virus movement within the farm environment through splashing, aerosol generation, contaminated surfaces and possible wildlife exposure. Wild birds have already played a major role in the global spread of H5N1. Contaminated wastewater or standing water could represent another point of interaction between dairies and wildlife populations.
The study does not establish wastewater systems as a major driver of transmission. However, it does suggest environmental contamination pathways may deserve more attention as researchers continue investigating how the virus behaves in dairy systems.
Some Infected Cows Showed Few Clinical Signs
The study also identified evidence of subclinical infection.
Some cows tested positive for H5N1 despite showing limited or no obvious clinical illness. In several cases, cows produced H5N1-positive milk without severe visible mastitis signs. Researchers also detected antibodies in animals without previously recognized disease.
Infection patterns within udders added another layer of complexity. Researchers noted some patterns did not fully align with expectations if transmission were occurring solely through contaminated milking equipment.
If milking equipment was the only major transmission route, infections between udder quarters would likely appear more predictable. Instead, the findings suggested additional exposure pathways may be involved.
These subclinical infections could complicate surveillance and outbreak detection efforts. Farms relying primarily on visibly sick cows may miss infected animals, particularly during the early stages of transmission. That has implications for testing strategies, animal movement decisions and outbreak response planning.
The findings also highlight how differently H5N1 behaves in cattle compared to poultry, where highly pathogenic avian influenza often causes rapid and severe disease.
Biosecurity Implications Continue to Evolve
The authors emphasized more research is needed to determine which transmission pathways are most influential on commercial dairies. Still, the study broadens the conversation around H5N1 biosecurity.
Early outbreak discussions focused heavily on milk contamination and fomite transmission through milking equipment. This study suggests aerosols, wastewater systems, environmental contamination and subclinical infections may also play a role.
That could influence future discussions around:
- Parlor ventilation
- PPE use during milking
- Wastewater handling
- Environmental sanitation
- Surveillance strategies
- Monitoring apparently healthy cows
The paper also underscores how much remains unknown about H5N1 adaptation in dairy cattle. Researchers identified mutations in some environmental samples that have previously been associated with mammalian adaptation, though the significance of those findings remains unclear.
This study offers an updated look at a disease situation that continues to evolve rapidly — and suggests transmission on dairies may involve a broader network of environmental exposures than initially believed.
