An infected cow that looks fine keeps spreading the virus
Across California's vast dairy landscape, a pathogen is moving in ways that confound the visible logic of illness — infecting cows that appear healthy, drifting through the air of milking parlors, and flowing outward through wastewater and milk. A new study from Emory and Colorado State universities has mapped these hidden transmission routes of H5N1 on fourteen infected farms, revealing that the virus is far more mobile and far less detectable than previously understood. With over seven hundred herds now affected, the findings reframe the risk not as a crisis of sick animals, but as a crisis of invisible presence — one that touches the workers, the water, and the wildlife that share these spaces.
- H5N1 is circulating silently through California dairy farms, carried by cows that show no symptoms yet exhale infectious virus and produce contaminated milk.
- Researchers discovered the virus spreading through three previously underestimated routes — airborne particles in milking parlors, farm wastewater, and raw milk — each a distinct pathway to human or wildlife exposure.
- Farm workers face compounding risks they cannot easily see or avoid: breathing potentially infectious air, handling virus-laden equipment, and working alongside animals that betray no outward sign of infection.
- Scientists are calling for targeted interventions — aerosol reduction in dairy parlors and treatment of infected milk before disposal — to interrupt the multiple transmission chains now confirmed.
- The study's scope was limited to fourteen cows over a few months, meaning the true scale and consistency of these routes across hundreds of infected herds remains an open and urgent question.
More than seven hundred dairy herds across California are now infected with H5N1, and a new study published in PLOS Biology has fundamentally changed how scientists understand the virus's movement through these farms. Researchers from Emory University and Colorado State University spent several months collecting air, wastewater, and milk samples from fourteen infected farms across two California regions, testing each for viral presence and sequencing genetic material to track how the pathogen was evolving.
What they found was unsettling in its breadth. The virus was airborne — exhaled by infected cows and lingering in the dairy parlors where milking takes place. It was present in farm wastewater. And it appeared in milk, sometimes at high concentrations, from cows that looked and behaved entirely healthy. Antibody testing confirmed these asymptomatic animals had been infected, meaning the farm could be contagious without any visible warning.
Each of these pathways — air, water, milk, equipment — represents a potential exposure point for farm workers and for wildlife that might encounter contaminated runoff or aerosols. The lead researchers noted that finding infectious virus in both air and wastewater was unexpected, and that the sheer variety of transmission routes means no single intervention will be sufficient.
For California's dairy workers, the implications are immediate and personal. They are breathing air that may carry the virus, handling equipment potentially coated in high-viral-load milk, and caring for animals whose infections are invisible. The researchers acknowledge their sample was limited and that broader, longer-term study is needed — but the picture that has already emerged is clear enough: H5N1 is far more present on these farms than anyone could previously see.
More than seven hundred dairy herds across California—the nation's largest milk-producing state—are now infected with H5N1, the highly pathogenic avian influenza strain. A new study published in PLOS Biology this week reveals that the virus spreads through routes researchers did not previously understand, and that many infected cows never show any signs of sickness at all.
Scientists led by Seema Lakdawala at Emory University and Jason Lombard at Colorado State University set out to map how the virus actually moves through a dairy farm. Between October 2024 and January 2025, they collected air samples, wastewater samples, and milk samples from fourteen infected farms in two different California regions. They tested everything for the presence of virus and sequenced the genetic material to look for variants and mutations that might tell them how the pathogen was evolving.
What they found was sobering. The virus was floating in the air—exhaled from the breath of infected cows and lingering in the dairy parlor where milking happens. It was present in farm wastewater. And it was in the milk itself, sometimes in high concentrations, even from cows that appeared perfectly healthy. The researchers detected H5N1 antibodies in these asymptomatic animals, confirming they had been infected but mounted no visible response.
This matters because it means the virus is far more mobile than previously thought. A cow can be infected and contagious without anyone knowing it. The contaminated milk can travel through equipment. The virus can be breathed in. The wastewater can carry it to other parts of the farm or beyond. Each of these pathways represents a potential point of exposure—not just for other cows, but for the farm workers who handle the animals and the equipment, and for wildlife that might come into contact with contaminated water or air.
Lakdawala noted in the study that the presence of infectious virus in both air and wastewater was unexpected. The sheer amount of virus on these farms, and the number of different ways it can spread, suggests that preventing transmission will require more than one approach. Reducing aerosols in the dairy parlor—the room where milking occurs—could help. So could treating sick milk before it is disposed of, which would prevent it from contaminating water systems or exposing workers.
The researchers acknowledge that their work has limits. They sampled only fourteen individual cows over a relatively short window. To truly understand how widespread these transmission routes are, and how consistently they operate, scientists would need to sample more farms over a longer period. But the findings are clear enough that they point toward a new understanding of how H5N1 moves through the places where humans and animals work side by side.
For farm workers in California's dairy industry, the implications are immediate. They are breathing air that may contain infectious virus. They are handling equipment that may be contaminated with milk carrying high viral loads. They are working in an environment where infected animals may show no signs of illness. The virus, it turns out, is far more present than anyone could see.
Notable Quotes
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.— Study authors, PLOS Biology
Our data confirm the presence of infectious H5N1 virus in the air and reclaimed farm wastewater sites, suggesting that multiple modes of H5N1 transmission likely exist on farms.— Dr. Seema Lakdawala, Emory University
The Hearth Conversation Another angle on the story
Why does it matter that some infected cows don't show symptoms?
Because a sick animal is visible. You can isolate it, treat it, keep it away from others. An infected cow that looks fine keeps spreading the virus—through the air it breathes, through its milk, through the farm's water systems—and nobody knows to stop it.
The study sampled only fourteen cows. Doesn't that make the findings unreliable?
It makes them preliminary. The researchers are honest about that. But they found the virus in air, wastewater, and milk on every farm they tested. That's not a fluke. It's a signal that something real is happening. They're saying: this is what we found in a small sample. Now we need to look at this much more broadly.
What would actually reduce transmission in a dairy parlor?
The study suggests two things: reduce the aerosols—the tiny droplets floating in the air where milking happens—and treat the milk from sick animals before it goes into the waste stream. Both are interventions that could happen right now, without waiting for a vaccine or a cure.
Are farm workers in immediate danger?
They're in a situation where the risk is real and not fully understood. They're breathing air with infectious virus. They're handling contaminated milk. They're working with animals that may be infected but look fine. That's not the same as saying everyone will get sick. But it's a situation where precautions matter.
What happens to the wastewater?
That's part of the puzzle. The virus is in it. Where does it go from there? Into the soil? Into water systems? Into contact with wildlife? The study doesn't answer that, but it raises the question. And that question matters for everyone downstream.