Scientists have discovered that as few as ten particles of the H5N1 bird flu virus are sufficient to cause infection in dairy cows, a finding that raises serious concerns about how difficult the disease may be to contain on farms.
The research was carried out by scientists at Ohio State University and published in Nature Communications. It also sheds some light on why outbreaks have been so baffling for scientists, farmers, and livestock handlers.The key complication appears to be that the virus has a particular affinity for cow mammary glands rather than the airways, which is not how influenza typically behaves.
Since the virus was first reported in American dairy cattle in March 2024, more than 1,000 outbreaks have been confirmed across 17 US states. The outbreaks have been brought under control through a national milk testing strategy that stopped the movement of herds whose milk tested positive for the virus.
In their experiments, researchers introduced the virus directly into individual cow teats and found that even the smallest dose of ten viral particles was enough to trigger a productive infection, with infected cows subsequently producing milk containing high concentrations of the virus.
However, the study threw up several surprises about how the disease does not appear to spread. Tests involving contaminated milking equipment transferring the virus from infected cows to healthy ones twice daily for a fortnight showed no sign of infection in the healthy animals. Bottle-feeding calves with milk from infected cows also failed to establish clear infection. And lactating cows given the virus through the nose did not fall ill, with little viral presence found in nasal swabs and none in milk.
The lead researcher, Professor Andrew Bowman, cautioned that the biosafety conditions required to carry out such experiments may have been too controlled to reflect real farm life, particularly regarding airborne spread. He noted that the milking equipment route still cannot be ruled out, given the high viral concentrations in milk and the shared contact between equipment and animals.
A further puzzle surrounds how the virus originally jumped from birds to cows at all. In waterfowl, H5N1 replicates in the gut, and how it ends up establishing itself in a cow's mammary gland is, in Professor Bowman's words, "a head scratcher."
For now, scientists are unable to offer farmers clear, evidence-based guidance on how to prevent the disease from spreading within herds. "We think spillover is going to happen again. It's just a matter of time," Professor Bowman said. "And right now we don't have a great way to prevent either that spillover or cow-to-cow transmission once it happens."
