In the Veterinary Record dated 16 June 2007, the Animal Health Trust reported four current pockets of infection spread across the Midlands, Kent and Hampshire. These outbreaks have been linked with the importation of non-vaccinated horses or those with an unknown vaccination history.

What is equine flu?

Equine influenza is a disease caused by various strains of a virus that affects the upper and lower respiratory tract of horses, donkeys and mules.

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 and this disrupts the clearance of mucus and debris from the airways. In turn these damaged areas are invaded by bacteria and further infections ensue.

As with human flu, the equine version is very infectious and, with an incubation period of one to three days, spreads rapidly. The disease is spread by the virus being released into the atmosphere by coughing and blowing. For this reason, equine flu is highly contagious within a group of horses, but is not airborne over long distances.

Signs of equine flu

  • A very high temperature of 39-41C (103-106F) which lasts for one to three days
  • A frequent harsh, dry cough that can last for several weeks
  • A clear, watery nasal discharge that may become thick and yellow or green
  • Enlarged glands under the lower jaw
  • Clear discharge from the eyes
  • Depression and loss of appetite
  • Filling of the lower limbs

If you suspect your horse has equine influenza, you should contact your vet. As soon as a horse shows any suspect signs, strict hygiene and isolation procedures should be adhered to.

Horses that have been in contact with an affected animal should be carefully monitored and should not attend shows. Exposure to the virus combined with the stress of travelling will make infection more likely.


An accurate diagnosis can be made by:

  • recognising the clinical signs and the rapid spread between horses
  • isolation of the virus through nasopharyngeal swabs
  • rising antibody levels in blood (serum) samples taken early in the course of the disease and two to three weeks later
  • history of recent contact with a confirmed case of the disease

Vaccinated horses may show signs, but these are usually milder than those experienced by unvaccinated horses.


Horses with respiratory infections should be given complete rest. Ideally they should not restart any strenuous exercise until two weeks after the signs have gone.

Frequently, the advice is that they should have a week off for every day they have had a raised temperature. But many riders find their horses are below par for longer, in the same way people can feel run down after flu.

Good stable ventilation and management is essential. Exposure to dust and spores should be minimised, as horses with respiratory infections are susceptible to allergic problems, including recurrent airway obstruction (RAO).

If hay is fed, it should be good quality and soaked. If weather permits, affected horses benefit from being turned out for at least part of the day once their temperatures have returned to normal. This is especially important in the recovery stages.

Antibiotics have no effect against a virus, but can be useful to control secondary bacterial invasion. This is a risk in foals, who can succumb to fatal pneumonia.

Some of the new antioxidant feed supplements marketed to help respiratory function may be of benefit, as well as other medications to help breathing.

If the disease occurs locally, it may be advisable to give a booster to any horse that has not been vaccinated in the previous six months. Consult your vet for advice.

Read more about equine flu