Flu Virus in Horses

Equine Influenza

Sometimes referred to as the horse flu, equine influenza is one of the most widespread infectious viral equine diseases in the world. In the U.S., it is most commonly seen in young horses around 2 to 3 years of age and frequently encountered at racetracks. Targeting the horse's respiratory system, the influenza virus damages the lining and mucous membranes in the animal's respiratory tract in a rather short period of time, with the incubation period being only one to three days after infection.


A horse with equine influenza has an abnormally high body temperature and nasal discharge, which is usually clear in color. Pneumonia is also a frequent secondary infection for horses with this condition, as their immune systems are compromised. This secondary infection can be deadly in foals. In severe cases of equine influenza, the horse may develop heart and liver complications. Other clinical signs include:

  • Dry deep cough
  • Anorexia
  • Depression
  • Occasionally muscle pain, with reluctance to move
  • Occasionally enlarged lymph nodes


There are several strains or subtypes of the equine influenza virus, but one of the most common flu viruses affecting horses is the Type A influenza. All flu viruses are airborne spreading from horse to horse, although some horses are only carriers of the virus, never showing any symptoms and appearing healthy. These horses, however, are still contagious. The virus acts destructively on the cells that line the upper respiratory tract as it replicates inside these cells.


The symptoms of equine influenza are usually enough to form at least a presumptive diagnosis, although signs of influenza are similar to other respiratory viral infections as well. This doesn’t matter, as all viral respiratory diseases will be treated the same.


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