The recent discovery of the H5N1 “bird flu” virus among horses living in Mongolia may have implications for equine and human populations around the globe.

Avian influenza viruses primarily spread among wild birds, but certain strains can infect other species, including mammals. In fact, the H5N1 strain, which is considered highly pathogenic, has been detected in some U.S. dairy cattle herds since March 2023. H5N1 infections in people are rare but can cause severe illness.

Natural hosts

Horses are of particular interest to researchers monitoring the emergence and spread of new influenza strains, says Professor Pablo Murcia, DVM, PhD, of the University of Glasgow Center for Virus Research. “Horses are natural hosts of influenza viruses (IAVs). In a previous study, we found that horses are routinely exposed to less pathogenic H3N8 avian influenza viruses,” explains Murcia, who was part of the international research team that detected the H5N1 virus in the Mongolian herd. “Wild migratory birds spread the viruses to different geographical regions and there are two overlapping flyways in Mongolia, which is also home to a large horse population.”

How exactly the horses acquired H5N1 isn’t clear. “Birds shed avian influenza virus in feces. We know that horses and wild birds spend a lot of time in lakes in the summer, sharing water sources,” says Murcia. “We speculate that horses get infected from those water sources.”

Possible implications

Although none of the Mongolian horses carrying H5N1 antibodies showed signs of illness, this discovery raises several concerns. The first is a potential for “spillover events,” in which a virus moves from one species to another, says Murcia. “For example, if a human gets infected with H5N1 from an animal, the person might get sick or even die. But it is unlikely that they will transmit the virus to another human. With viruses that evolve very fast, like influenza virus, the more spillover infections, the more opportunities the virus has to adapt to a new host and transmit from individual to individual in that species.”

The chance of a person catching H5N1 from an infected horse is low but not zero, says Murcia. “We know that horses can act as source of new influenza viruses for other species. For example, equine influenza virus (H3N8) jumped into dogs in early 2000s and established as canine influenza virus.”

In addition, Murcia says the threat to horses themselves shouldn’t be minimized. “While a lot of attention is on H5N1’s potential to cause pandemics in humans, I would like to highlight that influenza is an important equine disease. If a horse is infected with both the H3N8 equine influenza virus and an H5N1 avian influenza virus, there is a possibility that the viruses swap their genetic material and generate novel influenza viruses with unknown consequences.”

Would current equine influenza vaccines protect horses from H5N1? “In our study we showed that antibodies against equine influenza virus in horse serum do not neutralize H5N1 avian influenza virus in vitro,” says Murica. “Based on this, we assume that the equine influenza vaccine is unlikely to protect against H5N1 influenza.”

Head start on control

The good news is that by detecting the infection in the Mongolian horses, the researchers have a head start on controlling the virus if it does spread to other populations. “In general, we find out about emerging viruses after they cause outbreaks of disease,” he says. “Now we are a step ahead of the virus and can put measures in place to try to prevent the emergence of new influenza viruses in horses or control the virus effectively if it starts infecting horses.”

Reference: Damdinjav, B., Raveendran, S., Mojsiejczuk, L., Ankhanbaatar, U., Yang, J., Sadeyen, J.-R., Iqbal, M., Perez, D. R., Rajao, D. S., Park, A., Viana, M., & Murcia, P. R. (2024). “Evidence of Influenza A(H5N1) Spillover Infections in Horses, Mongolia.” Emerging Infectious Diseases, 31(1). https://doi.org/10.3201/eid3101.241266, January 2025