Equine influenza (horse flu) is the disease caused by strains of influenza A that are enzootic in horse species.Equine influenza occurs globally, and is caused by two main strains of virus: equine-1 (H7N7) and equine-2 (H3N8).hile equine influenza is historically not known to affect humans, impacts of past outbreaks have been devastating due to the economic reliance on horses for communication (postal service), military (cavalry), and general transportation.