select search filters
briefings
roundups & rapid reactions
Fiona fox's blog
A paper published in PLOS Biology looks at surveillance on California dairy farms that reveals multiple sources of H5N1 virus transmission.
Dr Joseph Neary, Senior Lecturer in Livestock Health and Welfare, University of Liverpool, said:
“The research undertaken by Campbell and colleagues confirms that H5N1 is not just a milk-borne disease. The identification of infectious virus in the air of milking parlours points to a source of possible zoonotic spread as farm workers working in the parlour are likely breathing in the virus.
“The presence of subclinical carriers – animals that appear healthy but are shedding the virus – means that H5N1 is likely more widespread than initially thought. The silent spread among subclinical cows makes biocontainment very difficult.
“The discovery of the N189D mutation in environmental samples is a sobering sign of real-time viral evolution, enhancing binding to human-type receptors and bringing a bovine outbreak closer to a potential human health threat.
“Infectious H5N1 in farm wastewater and manure lagoons transforms these areas into persistent environmental sources of the virus. Migratory birds visiting the lagoons can easily spread it to new locations, maintaining a cycle of infection that is very difficult to break.”
Prof Ed Hutchinson, Professor of Molecular and Cellular Virology, MRC-University of Glasgow Centre for Virus Research (MRC CVR), said:
“One of the most concerning aspects of the ongoing H5N1 bird flu outbreak is the fact that the virus has adapted to grow well in cattle, providing this avian virus with opportunities to become better adapted to mammals, and creating repeated opportunities for it to infect humans. For more than two years it has been circulating in dairy cattle in the USA. So far none of the human cases of cow flu have led to onwards transmission of the virus, but the situation remains very concerning as a potential source of the next pandemic.
“H5N1 influenza viruses had not previously been known to infect cattle, and although we are now over two years into the USA dairy outbreak there is a lot about how this virus spreads among cows that we still don’t understand. In this study, researchers carried out the difficult task of studying the virus within dairy farms, rather than in a controlled laboratory environment. They found that during H5N1 outbreaks the virus could be detected in cow’s milk, in the wastewater from the milking parlours and in the air, with aerosols probably arising both from viruses in the cow’s breath and from aerosolised milk. By identifying cows which had developed an immune response to the virus, the researchers showed that subclinical infections were frequent. Viruses that are infected but do not have obvious symptoms make the virus much harder to contain without widespread rapid testing.
“Although this study increases our understanding of how H5N1 spreads around the environment on a dairy farm, it also highlights a major gap in our understanding. We know how cattle can shed this virus, but it is still surprisingly unclear how they get infected by it. Although transmission from udder to udder by contaminated milking equipment is a plausible route of transmission, this study is consistent with other work in suggesting that multiple routes of transmission may be happening at the same time. More generally, it is still unclear why the outbreak of H5N1 in dairy cattle has only occurred in the continental USA. H5N1 avian influenza is globally distributed, it is clearly an extremely effective virus in dairy cows, and there have now been multiple confirmed introductions of the virus from birds into cattle, including a recent case in the Netherlands. However, it is only in the USA that the virus has established an ongoing outbreak. Studies like this one, which engage with both the science of viral transmission and the actual practice of dairy farming, are needed to help us better understand how to contain the virus within the USA and also to understand and control the risks is might pose to dairy farming, and to humans, elsewhere in the world.”
‘Surveillance on California dairy farms reveals multiple possible sources of H5N1 influenza virus transmission’ by A. J. Campbell et al. was published in PLOS Biology at 19:00 UK Time on Tuesday the 5th of May 2026.
DOI: https://doi.org/10.1371/journal. pbio.3003761
Declared interests
Dr Joseph Neary: “I don’t have any industry links related to this paper or other academic conflicts.”
Prof Ed Hutchinson: “I have received honoraria for work in a steering group of the Centre for Open Science (Open Practices in Influenza Research; 2021-2022) and on an advisory board for Seqirus (2022). I have unpaid positions on the board of the European Scientific Working group on Influenza and other respiratory viruses (ESWI) and as a scientific adviser to PinPoint Medical. I have carried out work on H5N1 influenza A viruses funded by UKRI and DEFRA.”