Equine Influenza (EI) is a highly contagious, though rarely fatal, acute respiratory infection of horses, donkeys, mules, and other equidae (domestic and wild) that replicates in respiratory epithelial cells.
Outbreaks in domestic horses can severely impact agricultural industries, and outbreaks in wild horses can spread to domesticated horses if contact is permitted.
EI is caused by two subtypes of Influenzavirus A (family Orthomyxoviridae): H7N7 and H3N8. They are enveloped and have a segmented, negative-sense single-stranded RNA genome. They are related to, but are distinct from the viruses that cause human and avian influenza. Equine influenza viruses of both subtypes are considered to be of avian ancestry, and highly pathogenic avian H5N1 has been associated with an outbreak of respiratory disease in donkeys in Egypt.
EI is an OIE-notifiable disease in domesticated equids as indicated in the Terrestrial Animal Health Code, and voluntarily reportable in wildlife.