Newsdate: February 14, 2025 11:30 am

An article in Emerging Infectious Diseases, January 2025 issue discusses evidence that Influenza A(H5N1) infection has been found in horses in Mongolia and suggests that with at the large population of horses in North America, it is important to monitor horses to prevent equine and avian (bird) viruses from affecting local horse populations.

A sick horse with a cough and respiratory illness.

Because H3N8 equine influenza virus is endemic in many countries, horses should be monitored to prevent reassortment between equine and avian influenza viruses with unknown consequences.
© 2017 by S. Hanusch New window.

According to researchers, recent outbreaks of influenza A(H5N1) have affected many mammal species. They have reported serologic evidence of H5N1 virus (bird flu) infection in horses in Mongolia. Because H3N8 equine influenza virus is endemic in many countries, horses should be monitored to prevent reassortment between equine and avian influenza viruses with unknown consequences.

Avian influenza viruses (AIVs) of the H5N1 subtype are a cause of concern because they are highly pathogenic in birds and various mammals. H5N1 AIVs have caused outbreaks in both wild and domestic avian species, leading to substantial biodiversity and economic losses from virus-induced deaths and culling interventions. Surveillance studies have shown an increased incidence of H5N1, particularly of clade 2.3.4.4b, in wild birds.

Horses are natural hosts of equine influenza virus (EIV). Two subtypes of EIV have emerged, including 2 H3N8 strains and 1 H7N7 strain. All EIVs are thought to have originated from AIVs.

In surveillance studies during July 2021–October 2023, researchers collected serum samples from 10 horses from 24 herds, 3 times per year. They recorded associated metadata including sex, approximate age, clinical status, and main use of the horse, as well as location of the herd.

The researchers noted that “Equids are clearly susceptible to infection by AIV H5N1. Abdel-Moneim et al. (7) described an outbreak of influenza in donkeys in Egypt in 2009; they isolated IAV H5N1 from nasal swabs and demonstrated that ≈26% donkeys sampled had been infected. In addition, H5 antibodies were detected in wild asses.”

The researchers findings show that horses are susceptible to infection by H5N1 viruses and that spillover events are likely frequent, highlighting the potential emergence of IAVs by reassortment between H3N8 EIV (the circulating subtype in horses) and H5N1 IAVs.

Researchers noted that ecologic conditions for reassortment are met in North America where 30% of the global horse population is located. EIV is endemic, avian influenza A(H5N1) clade B3.13 is spreading in cattle, and contact rates between cows and horses are likely to be high in agricultural settings.

“Consistent with our previous work showing that horses in Mongolia are commonly exposed to H3N8 AIVs in the absence of disease outbreaks (10), our results suggest that H5N1 infections in horses are likely to be subclinical, posing challenges to virus detection.”

Researchers recommend that serologic surveys in premises that keep horses should be done to aid in early virus detection.

See full article HERE

*reassortment:  a recombination of genomic material, especially as it occurs naturally in related viruses.

Press release by Emerging Infectious Diseases publication - Minor editing by EquiMed.com editor