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Equine Influenza

Updated: Feb 22, 2023

Author: Lucy Carmichael BVSc MRCVS

horse with a cough

What is equine influenza?

Equine influenza, as with human influenza, is caused by a virus. This virus is highly contagious and constantly evolving, leading to the development of different viral strains. It is spread through inhalation- usually via aerosols from other horses, or from breathing in virus containing droplets which can survive in the horse’s environment, on handlers and on equipment. It has even been proven, in certain situations, that the wind can spread the virus up to 2km.

Clinical signs

1) A snotty nose and a cough. This is due to the virus attacking the epithelium which lines the upper airways, resulting in swelling and inflammation. The damage to the tissue prevents normal drainage of secretions, so fluid builds up in the airways. In some cases this can result in a secondary bacterial infection, and even pneumonia

2) High temperature (39°C-41°C)

3) Dullness, depression and sometimes a reluctance to move due to muscle soreness

4) Loss of appetite

5) Enlarged glands under the jaw

6) Filling of the lower limbs

Normally, there will be an incubation period of 1-5 days after the horse encounters the virus, before clinical signs develop, and the severity of these symptoms depends on the strain of the virus. Symptoms usually last for 2-10 days, however, it can take several weeks for the horse to return to full health, and secondary complications may arise in some cases.


Your vet will normally have a suspicion of equine influenza based on the your horse’s history and symptoms. This suspicion will of course be raised further if your horse is known to have been in contact with a positive case of influenza.

Your vet will likely take a nasopharyngeal swab- just like the swabs we are familiar with for COVID, but much larger! They might also take blood sample to confirm the diagnosis.

Equine nasal swab
Image by Rossdales Veterinary Surgeons via


Horse in a stable
  • Due to the contagious nature of the virus, affected horses must be completely isolated

  • There are no specific antiviral medications that have been shown to be efficacious against equine influenza therefore treatment is primarily based around good nursing care. This should include:

    • Plenty of rest

    • Encouraging the horse to eat and drink. Apple juice or molasses can be added to water if there are not drinking enough. Equally if it is cold, some horses will prefer to drink warmer water

    • Feed soaked hay or haylage from the floor

    • Provision of dust free bedding

  • Anti-inflammatories such as ‘bute’ may be given to decrease the horse’s temperature and reduce muscle soreness

  • Mucolytics may be indicated to aid removal of mucus from the airways

  • Antibiotics are not indicated for the treatment of viruses, however they may be required in some cases if there is a secondary bacterial infection

Vaccination protocols

Vet injecting horse

Vaccination aims to prevent horses from developing clinical signs if they are exposed to the influenza virus, however they may develop a mild cough and clear nasal discharge. It may also be possible for them to spread the virus to depending on which strains of influenza virus they were vaccinated against and the length of time between vaccination and infection. Therefore, vaccinated horses with these symptoms should be investigated.

Vaccination is certainly better than cure, as equine influenza can be very challenging to control, and can be fatal for unvaccinated horses in some cases. Following an initial course of three vaccinations, horses should receive a booster vaccine annually. In higher risk populations, 6 monthly vaccinations can be given. Please speak with your veterinarian to discuss the appropriate vaccination intervals for your horse.


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