Equine influenza (horse flu) is the disease caused by strains of influenza A that are enzootic in horse species. Equine influenza occurs globally, previously caused by two main strains of virus: equine-1 (H7N7) and equine-2 (H3N8). The OIE now considers H7N7 strains likely to be extinct since these strains have not been isolated for over 20 years. Predominant international circulating H3N8 strains are Florida sublineage of the American lineage; clade 1 predominates in the Americas and clade 2 in Europe. (Elton and Cullinane, 2013; Paillot, 2014; Slater et al., 2013). The disease has a nearly 100% infection rate in an unvaccinated horse population with no prior exposure to the virus. While 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. In modern times, though, the ramifications of equine influenza are most clear in the horse-racing industry. Clinical Signs Equine influenza is characterized by a very high rate of transmission among horses, and has a relatively short incubation time of one to three days. Clinical signs of equine influenza include fever (up to ), nasal discharge, have a dry, hacking cough, depression, loss of appetite and weakness. Secondary infections may include pneumonia. Horses that are mildly affected will recover within 2 to 3 weeks; however, it may take up to 6 months for recovery for severely affected horses. Horses that become immune may not show signs but will still shed the virus. An 1872 report on equine influenza describes the disease as: Causes Equine influenza is caused by several strains of the influenza A virus endemic to horses. Viruses that cause equine influenza were first isolated in 1956. The equine-1 virus affects heart muscle, while the equine-2 virus is much more severe and systemic. The virus is spread by infected, coughing horses in addition to contaminated buckets, brushes, tack and other stable equipment. The influenza virus causes symptoms by replicating within respiratory epithelial cells, resulting in destruction of tracheal and bronchial epithelium and cilia. Treatment When a horse contracts the equine influenza virus, rest and supportive care is advised so that complications do not occur. Veterinarians recommend at least one week of rest for every day that the fever persists with a minimum of three days' rest. This allows the damaged mucocilliary apparatus to regenerate. Non-steroidal anti-inflammatory drugs