Just 10 viral particles of the H5N1 bird flu that caused hundreds of influenza outbreaks in U.S. dairy cattle can cause infection in cows, a new study shows, according to research published in Nature Communications and announced on June 15.
The study suggests that the virus’s preference for cow mammary glands rather than airways may be complicating efforts to contain and prevent the disease. Tests examining possible transmission among cows through milk machinery or feeding of calves, as well as between birds and cattle via shared indoor air, did not show evidence of disease spread. Researchers said they are still seeking answers about how the virus is transmitted from cow to cow.
"How it spreads from cow to cow becomes a very important question. We need to understand if there's a way to change milking practices or farming practices, whatever it is, to limit cow-to-cow transmission because we think spillover is going to happen again. It's just a matter of time," said senior author Andrew Bowman, professor of veterinary preventive medicine at The Ohio State University. "And right now we don't have a great way to prevent either that spillover or cow-to-cow transmission once it happens."
Highly pathogenic H5N1 avian influenza A viruses are typically associated with wild birds and poultry but have circulated globally in mammals since 2021 and were first reported in U.S. dairy cattle in March 2024. To date, there have been 1,053 confirmed outbreaks involving the B3.13 genotype across herds in 17 states; federal data indicates small numbers of infections continue in Idaho, Utah and Texas.
Bowman and colleagues previously detected H5N1 particles in retail milk supplies earlier this year when 36.3% of samples tested positive for the virus: "Pasteurization is inactivating it. But once a cow's infected, they produce high-viral-titer milk for a week-plus," Bowman said.
Experiments showed that even at the smallest dose—10 viral particles—injected into individual teats resulted in productive infection with high concentrations of viral particles shed into milk but fewer clinical signs compared with higher doses.
Transmission tests using contaminated milking equipment failed to infect healthy cows over two weeks despite exposure; bottle-feeding calves infected milk led only to minimal detection; intranasal dosing did not make lactating cows sick nor result in significant virus presence except for some immune response markers found post-mortem; co-housed chickens remained healthy after exposure alongside infected cows for over two weeks.
Bowman said biocontainment conditions might differ from farm settings: "I think the milking equipment is something we still have to consider... The straight line between two points would say the milking equipment still seems to be the likely route of transmission between cows," he said. He also raised questions about how spillover occurs from wild birds into cattle mammary glands.
