Equine flu, as equine influenza is commonly known, is caused by various strains of the influenza virus that affect the upper and lower respiratory tract of horses, donkeys and mules. The virus is similar to the flu virus that affects people, but is not identical, so horses cannot catch human influenza or vice versa. Equine flu is found within the British horse population and is a major and economically important cause of acute respiratory disease throughout the world, with the exception of some island nations, such as Iceland and New Zealand.
Once the virus has been inhaled, it invades the lining (epithelium) of the airway, which becomes inflamed, producing a very sore throat and a nasty cough. This damage causes patches of the membranes lining the airways to ulcerate, which disrupts the clearance of mucus and debris from the airways causing a thick discharge from the horse’s nose. Bacteria then invade the damaged areas leading to further infections.