Equine Influenza Virus

Equine influenza virus (EIV) that causes equine influenza (EI) is the most important equine respiratory viral pathogen and has the greatest economic impact internationally, because it has a very high morbidity rate during outbreaks leading to disruption of major equestrian events. EIV can also infect other equids such as donkeys, mules, and zebras. EIV is a member of the family Orthomyxoviridae and belongs to the genus Influenzavirus A (influenza A type). The first strain of EIV isolated in 1956 was of H7N7 configuration (subtype 1) and designated influenza virus A/equine/Prague/56, and it caused epidemics during the 1960s and 1970s. The last confirmed outbreak caused by an H7N7 subtype in horses was recorded in 1979 and therefore, the H7N7 subtype is thought to be extinct or possibly still circulating at a very low level in nature. A second EIV subtype (subtype 2), H3N8, emerged in 1963 and was designated as influenza virus A/equine/Miami/63. This subtype has been associated with all confirmed outbreaks of EI since 1980. Extensive antigenic drift has been detected in this virus over the years (see later).