Influenza ('flu)

These information sheets are provided for your interest. They should not replace veterinary advice from your veterinary surgeon.

Whilst every effort is taken to ensure the accuracy and completeness of the information provided, your specific circumstances must be discussed before advice can be given.

Fri, 20/05/2022 - 12:12 -- Stephanie Fitzg...

Equine influenza originates from the same family of viruses as swine flu, avian flu and even human influenza. Horses experience nasty flu- like symptoms just like we do!

So what are the clinical signs in horses exactly?
Horses can experience a variety of clinical signs including:

  • Fever
  • Coughing
  • Mucopurulent discharge (pus coming from the nostrils)
  • Malaise
  • Inappetence
  • Dyspnoea (struggling to breathe)

Unfortunately, symptoms may not become obvious until 2 days after exposure. Fevers can reach up to 41OC and can remain high for up to 10 days!

Resolution of clinical disease can take up to 3 weeks.

Complications are rare but not unheard of! The virus effects the cells lining the respiratory system; this makes the effected horse more vulnerable to developing secondary infections including pneumonia and pleural pneumonias.

How do we definitively diagnose equine influenza?

  • We will always be suspicious of influenza if a horse or horses (especially those in high risk groups- young or unvaccinated horses) present with sudden onset respiratory signs. We should treat all query cases with caution until proven otherwise! Always better to be safe than sorry!
  • Nasopharyngeal swabs paired with blood samples help to confirm our diagnosis!

Treatment and control:

Similarly, to when humans get the flu there is no magic cure! We can help alleviate the symptoms (controlling fevers) and try to limit secondary complications and complications but there is medication to “treat” equine flu.

Unlike a lot of other diseases influenza is highly contagious. It is passed from horse to horse to horse via direct contact with nasal secretions, via the aerosol route- i.e. coughing or sneezing; any animals in the same air space are at risk and contact with recently contaminated fomites including feed, water, equipment, and clothing. Luckily the virus is not very stable and doesn’t survive for very long in the environment. So, our main method of control/ prevention is to isolate any suspect cases and limit movement of horses. Always remember that horses might not show symptoms immediately, or at all, after exposure but they can still pass the infection onto other horses.

This is why when we are experiencing an outbreak there are often movement restrictions and competitions cancelled. Annoying but it does help.


Several companies have developed vaccines against equine influenza. The aim of vaccination is the same of that in people; to help prime the immune system so that if the virus is encountered the immune system is better able to handle the infection.

The standard primary course of vaccinations consists of an initial vaccination followed by a 2nd vaccination 4 weeks later and a 3rd vaccination 5 months after that. Usually at this point horses can then move to annual boosters- receiving one within every 365 days.

Vaccination helps to dramatically reduce the spread and severity of the disease. Due to the everchanging nature of equine flu it is not 100% preventative. The majority of “vaccinated” horses that become effected have incomplete vaccination histories or have had gaps in their vaccinations.