Equine Flu 101

What is equine flu

What is equine flu?

Equine flu, or EI as it is often called, is a highly contagious respiratory disease that affects horses, donkeys and mules.

What causes it?

It’s caused by several strains of the influenza A virus endemic to horses. Viruses that cause equine flu were first isolated in 1956. Keeping it simple, the equine-1 virus affects the heart muscle, while the equine-2 virus is much more severe and systemic.

How long has it been around and where has it come from?

The first official cases of the disease were reported from Ontario, Canada in 1872. All the streetcar horses and major livery stables were affected within only three days. By the middle of October of this year, the disease had reached Montreal, Detroit, and New England.

However, a comprehensive report compiled following this outbreak in North America, mentioned the earliest record of it dating as far back as Hippocrates, c. 460- c. 370 BC.

Nowadays the disease can be found regularly in many countries. In fact, the spread of equine flu has been so rapid that today very few countries are free from infection. Currently New Zealand and Iceland hold a disease-free status. However due to the nature of the disease and the global transport of horses, these countries remain at high risk of introduction of disease if quarantine measures fail.

 How is it spread?

The disease is spread by the virus being released into the atmosphere by infected animals. It’s mainly acquired through inhalation of the virus from ill animals coughing and spluttering.

How will I know if my horse is infected?

Once a horse becomes infected with equine flu, the symptoms usually appear within 1-3 days and infected horses can spread the virus for 5 days thereafter.

What are the symptoms of equine flu?

Clinical signs vary greatly depending on the immunity and health of the infected horse. Symptoms include:

  • a high temperature of 39-41°C, lasting for 1-5 days
  • a dry, harsh-sounding cough that may linger for several weeks
  • clear nasal discharge that may turn green or yellow as secondary infections develop
  • swollen lymph nodes under the jaw
  • a clear eye discharge
  • Loss of appetite and lethargy.

What should I do if I suspect that my horse has it?

If you suspect your horse has equine flu, you should:

  • Isolate him from others
  • Monitor other horses that he has come in to contact with
  • contact your vet
  • Do not attend any shows or competitions ideally.

How is it diagnosed?

An accurate diagnosis can be made by a vet by:

  • Taking swabs from the nose or naval cavity early on in infection, i.e. within 7-10 days of exposure
  • Doing blood tests
  • Knowing if there has been recent contact with a confirmed or suspected case of the disease.

How long does equine flu last?

The disease has a very short incubation of 2-6 days and clinical signs usually resolve in 1-3 weeks. Horses with the disease remain infectious for up to 7-10 days. The dry, harsh-sounding cough may linger for several weeks. 

How is it treated?

Treatment is largely based on supportive care:

  • Isolate the infected horse
  • Rest, e.g. 1 week for every day of fever and with a minimum of 3 week’s rest. Do not resume work until the horse’s coughing has subsided completely. A return to work that is too early will increase the likelihood of secondary complications
  • Possibly restricted turn out depending on your vet’s advice
  • Use dust-free bedding
  • Feed soaked hay, or better still, haylage, and feed from the floor.

Antibiotics are sometimes necessary to treat any secondary bacterial infections that develop. However, they are ineffective against the virus itself.

Non-steroidal anti-inflammatory drugs (NSAIDs) are recommended for horses with a fever greater than 40°C. NSAIDs are medications widely used to relieve pain, reduce inflammation, and bring down a high temperature, i.e. fever.

What is the prognosis?

For uncomplicated cases, horses should completely recover and return to work within 3-6 weeks of infection. In more severe cases horses might require up to 100 days of rest. Horses that develop secondary bacterial infections will require a longer period of recovery. Whilst the virus is generally not thought to be life-threatening, it does limit the competitive capability of horses.

Can it be prevented?

Prevention is based on a combination of hygienic management practices and vaccination. Suggested management practices that reduce the exposure of horses to disease include:

  • Adequate nutrition
  • Stress reduction
  • Isolated newly introduced horses for 2-3 weeks
  • Separation of susceptible animals, e.g. foals, mares, yearlings and older horses
  • Separate travelling horses from resident horse population if possible.

Vaccination of at-risk horses is vital for the individual animal and for the control of the spread of disease. Consult your vet for advice on correct vaccination timing and regimes.

Can humans get it?

Humans can’t get infected with equine flu. However, humans can physically carry the virus on their skin, hair, clothing and shoes, and can therefore transfer the virus to other horses.

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Kim Horton

Co-Founder of EQUUS and a keen equestrian, when Kim's not at her desk she's with her horse, Waldo.


  • Sheila Urquhart posted on February 12 2019 at 08:02 PM

    Thanks for the information. Our three horses rarely come into contact with other horses, but its still reassuring to know the facts.

  • Catherine posted on February 14 2019 at 08:02 AM

    Very interesting and informative. I have my horses vaccinated annually- does this not prevent EI?

  • Sandra Price posted on February 14 2019 at 08:02 AM

    Thank you…. this was really useful information 😀

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