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).
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.
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 106 °F [41 °C]), nasal discharge, have a dry, hacking cough, depression, loss of appetite and weakness. Secondary infections may include pneumonia.