Recent outbreaks of influenza A(H5N1) have affected many mammal species. We report serologic evidence of H5N1 virus 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 (1), which coincides with growing reports of infections in mammal hosts including skunks, raccoons, bears, and foxes (2). In such studies, affected animals were believed to be dead-end hosts, which is consistent with previous perceptions that AIV H5N1 exhibits no or poor transmissibility in mammals. That perception changed in 2022, when outbreaks of H5N1 clade 2.3.4.4b were reported in fur farms in Europe breeding minks and foxes (3,4) and in populations of pinnipeds (e.g., seals and sea lions) in South America (5). In early 2024, an outbreak of AIV caused by genotype B3.13 H5N1, a descendant of H5N1 2.3.4.4b, was reported in dairy cattle in the United States (6). At the time, infection was also reported in cats, mice, and farm workers, but direct transmission from cattle could not be confirmed. Cow-to-cow transmission was later confirmed by sequencing data and epidemiologic information.