United States: In an intriguing revelation, cows have been shown to contract the highly pathogenic avian influenza H5N1 through inhalation of aerosolized virus particles, as per a preprint on bioRxiv. Nonetheless, scientists posit that this transmission route is likely not the principal driver behind the current bovine outbreak in the United States.
First identified in Texan cattle in March 2024, H5N1 has since spread to 157 herds and infected four individuals in direct contact with cattle across 13 states. Investigations indicate that contaminated milk, leading to infected milking equipment, is the predominant transmission pathway among cows, according to the reports by the Nature.
Prior to this outbreak, the transmission of influenza A viruses such as H5N1 in bovines was not well-documented. The potential for respiratory spread raises concerns about control measures and human infection risks due to the close interaction between cows and humans.
To elucidate the infection mechanisms in cattle, several research teams undertook challenge studies, deliberately infecting animals with H5N1. Initial findings from these studies, posted on July 13, remain unreviewed.
Airborne Transmission Experiments
Amy Baker and her team at the USDA in Ames, Iowa, conducted experiments where they exposed four one-year-old calves to virus-laden mist via masks covering their noses and mouths. The infected animals developed neutralizing antibodies, confirming infection. The calves exhibited mild symptoms, and the infectious virus was isolated from the upper respiratory tracts of the two calves, suggesting that respiratory disease could possibly spread in densely populated environments, as per the Nature.
However, Thomas Peacock, a virologist at Imperial College London, notes that the low levels of viral shedding imply that respiratory transmission is not a significant spread mechanism. He suggests indirect aerosol infection from the udder as a possible, albeit minor, route.
Wendy Barclay, also from Imperial College London, concurs, highlighting that the inefficiency of airborne transmission cannot account for the current outbreak’s dynamics. Instead, the study reinforces the theory that infected milk remains the main vector, with “udder-to-udder” transmission being the most probable route. However, Peacock warns that the virus’s behavior could change if the outbreak persists, necessitating vigilant monitoring.
Jürgen Richt, a veterinary virologist at Kansas State University, adds that while the study focused on infection susceptibility rather than transmission, it aligns with observations in agricultural settings. Richt’s own research on bovine infection, conducted with German colleagues, is expected to be published soon, as per the reports.
Milk Contamination and Viral Spread
Baker’s team also explored transmission through direct insertion of the virus into the mammary glands of lactating cows. Two days post-infection, the cows exhibited mammary gland inflammation and reduced milk production, with the milk turning yellow and thickening. The infectious virus persisted in the mammary glands throughout the three-week study and in the milk for up to two weeks.
These findings implicate the mammary gland and milk as primary vectors for H5N1 spread within and between dairy herds. Although respiratory transmission cannot be entirely ruled out, the evidence strongly points to milk contamination as the central concern, the reports outlined.
This research underscores the need for continued vigilance and investigation into H5N1 transmission dynamics to better manage and mitigate outbreak impacts in cattle populations.