A new study has shown that horses have been infected with H5N1 – avian flu – in Asia and that infections have been occurring in horses undetected.
The latest research, led by the MRC-University of Glasgow Centre for Virus Research in collaboration with an international team of scientists, including groups from the US and Mongolia, comes as the recent emergence and rapid spread of avian flu in cows in the US has raised concerns for wider viral spread.
Avian flu, also commonly referred to as bird flu, primarily spreads among wild birds such as ducks and gulls and can also infect farmed and domestic birds such as chickens, turkeys and quails. While the disease mainly affects birds, it has been known to spill over into other species, including mammals, and in rare cases, humans. While H5N1 infections in humans are rare, when they do occur, they have the potential to be extremely serious.
To carry out this study, the research team undertook a series of H5N1 antibody tests in horse herds in two Mongolian regions: the first, a wetland area which hosts a large population of wild birds, and the second, a dry area with low bird density.
The results, which are published in the journal Emerging Infectious Diseases, show conclusively that horses have been infected with avian flu. The researchers found that horses had been infected with the virus in a number of different areas of Mongolia over multiple years. Importantly, there were no prior reports of outbreaks of respiratory disease in the herds studied, suggesting the animals that tested positive for H5N1 antibodies may have experienced asymptomatic infections.
Since March 2023, H5N1 influenza has been spreading rapidly in dairy cattle over several US states. Concerningly, North America is home to 30% of the global equine population, where many horses are likely to be exposed to H5N1 infected animals in agricultural settings.
Additionally, horses are already known to carry equine flu – H3N8 – and researchers say that now, with the knowledge that they can also be infected with avian flu, these animals should be monitored to help prevent against viruses combining, with unknown consequences.
Professor Pablo Murcia, lead author of the study from the MRC-University of Glasgow Centre for Virus Research, said: “Influenza emergence is a rare event. Pandemics were triggered by the reassortment of viruses from different species. For instance, the 2009 influenza pandemic was caused by a swine-origin virus that contained genetic material from avian, human, and swine influenza viruses. This resulted from a complex series of events, including coinfections in pigs with these different viruses, followed by natural selection.
“Horses, too, have their own influenza strain, known as equine influenza, which is endemic in North America. With the rapid spread of avian influenza among cattle in the US, the likelihood of horses coming into contact with infected cows, and the chances of horses becoming coinfected with both avian and equine influenza viruses may increase. This raises the risk of generating novel viruses with unknown pathogenicity, potentially posing a threat not only to horses but to other mammals, including humans.”
The study, ‘Evidence of Influenza A(H5N1) Spillover Infections in Horses, Mongolia’ is published in Emerging Infectious Diseases.
**Enquiries: ali.howard@glasgow.ac.uk or elizabeth.mcmeekin@glasgow.ac.uk**
**First published: 12 December 2024**