Can Bird Flu Spread Through The Air?

The study, which is a preprint and is yet to be peer-reviewed, focused on the mysterious spread of H5N1 between three farms in the Czech Republic in February 2024. It began at a duck farm on February 4, with the sudden death of around 800 of its birds. By February 7, the decision was made to kill the entire flock, and a protection and surveillance zone was established around the farm.

How then, did genetically identical strains of H5N1 end up at two unrelated chicken farms, both 8 kilometers (5 miles) away and with high biosecurity standards, a week later?

Researchers from the State Veterinary Institute Prague set to find out – and the first thing they eliminated with the possibility that humans or wild birds had carried the virus over. Interviews with managers revealed no possible connections between the farms, and since there was no significant bodies of water near the chicken farms, it was deemed “highly unlikely” that it had been transmitted by wild birds.

Instead, the virus appeared to have spread by the wind; examination of meteorological data uncovered that there had been near-perfect weather conditions for the virus, emitted in a contaminated plume at the duck farm, to have been carried by a steady breeze to the chicken farms. Cloud cover stopped ultraviolet light from killing the virus particles, while cool temperatures allowed them to thrive.

It’s worth pointing out that the researchers didn’t conduct any air sampling as part of the study to confirm the presence of the virus, but nonetheless, other experts not involved in the research have supported the findings.

“Meteorological conditions, timing of infection, housing conditions of the animals, susceptibility of the animal populations that became infected and the lack of other epidemiological links between the premises are supportive of airborne transmission in this case,” Dr Montserrat Torremorell, chair of the Department of Veterinary Population Medicine at the University of Minnesota, who has previously conducted air sampling studies during bird flu outbreaks in the US, told CIDRAP.

Equally, others have said the results should be interpreted with a degree of caution.

 “It basically says that it could have happened, and I would not dispute that,” Dr David Stallknecht, professor emeritus at the University of Georgia’s College of Veterinary Medicine, told CIDRAP. “But to actually come down with concrete proof like you would in an experimental controlled experiment, there’s too much going on.”

With the US having seen human cases of H5N1 infection too, the possibility of windborne transmission could also have some people concerned about what this means public health. 

Dr Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, told CNN that while it’s quite unlikely someone could get infected this way, it could explain the few cases where the source of infection was unknown.

“I think it’s very low-risk for humans to be infected with the virus like that, but I think it happens.”

The study has been posted to the preprint server bioRxiv and is yet to be peer-reviewed.