06 May 2026
Researchers led by Emory and Colorado State University discovered that H5N1 influenza spreads through air and wastewater on dairy farms. These findings reveal critical new risks for cows and workers beyond contaminated milking equipment. Since highly pathogenic avian influenza (HPAI) H5N1 first emerged in U.S. dairy cattle in March 2024, the scientific consensus has largely focused on contaminated milking equipment as the primary driver of transmission. Because the virus is found in extremely high concentrations in raw milk, researchers believed direct contact was the main culprit. However, the exact mechanisms of how the virus moves between animals, and to the humans who care for them, remained alarmingly unclear.
To solve this puzzle, a multi-institutional team conducted extensive surveillance on 14 H5N1-positive dairy farms across two regions in California. Unlike previous studies that focused primarily on milk samples, this team used specialized air samplers and investigated "reclaimed" wastewater, water used to flush milk lines or clean housing pens that eventually sits in manure lagoons.
The findings, published in PLOS Biology, shift our understanding of farm biosecurity:
Infectious Aerosols: The team detected infectious H5N1 virus in the air within milking parlors. This suggests that the milking process itself can aerosolize the virus, creating a "breathable" risk for workers.
Contaminated Wastewater: Viral RNA was found at every stage of the farm’s waste stream, including manure lagoons used by migratory birds. Infectious virus was even recovered from wastewater sites, highlighting a potential environmental reservoir. The "Subclinical" Shedders: The study found that many infected cows are "subclinical," meaning they show no outward signs of illness like mastitis or a drop in milk production. These healthy-looking cows can still shed the virus, making them silent spreaders within the herd.
Perhaps most concerning were the results of the whole-genome sequencing. In air samples from one farm, researchers identified a specific mutation in the virus's hemagglutinin (HA) protein at position 189 (N189D in H3 numbering). While this specific variant's impact is still being studied, similar mutations are associated with an increased ability for the virus to bind to human-like receptors. This discovery suggests that the virus is actively evolving on dairy farms in ways that could increase its threat to human health.
Campbell AJ, et al. (2026) Surveillance on California dairy farms reveals multiple possible sources of H5N1 influenza virus transmission. PLoS Biol 24(5): e3003761
