Influenza is by far the most common disease of horses.
In adult horses the symptoms are often undetected because they are so mild. Weanlings usually show the classic symptoms more than older horses. Yearlings can show these symptoms to a lesser degree and older horses may show fever, watery eyes, and a slight nasal discharge. If rested and sometimes treated with anti-fever medications, the symptoms usually disappear with 3 to 5 days. It can cause abortion which can be sporadic to epidemic and can cause tremendous economic loss.