The H7N7 and H3N8 subtypes of the equine influenza virus are the two that cause equine influenza. Although the H7N7 subtype caused outbreaks in the middle of the 20th century, it has been thought to be extinct since it hasn’t been isolated in almost 20 years. Since its emergence in the late 1950s, the H3N8 subtype has been the primary cause of influenza in horses. H3N8 has undergone several sublineages of mutation following decades of evolution, including American and Eurasian lineages. There are three more subgroups within the American lineages: Florida, Kentucky/94, and Newmarket/1/93. In North America and Europe, the Florida subfamilies’ Clades 1 and 2 are most common. Antigenic drift in equine influenza viruses causes new strains to emerge, necessitating frequent strain updates for vaccines.
