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Equine Influenza ('Flu)

Equine Influenza, 'flu', is caused by an influenza virus similar to the ones that effect people (but horses cannot be infected by human influenza or vice versa). Viruses are tiny infectious particles that can produce disease, although they can only replicate inside another cell.  All influenza viruses undergo frequent genetic changes to produce different strains, which make it tricky and expensive to produce up-to-date commercial vaccines against the current strain.  As a result, the vaccines that are available may not provide full protection against any new strain of the virus, however, vaccinated horses are much less severely affected by the disease, so vaccination is extremely worthwhile.

Equine flu is highly contagious and is acquired mainly by the inhalation of the virus, via aerosols, from other infected horses or via virus-containing droplets in the horse's environment. In short, affected horse's coughing and spluttering will spread the virus.

Indirect spread is also possible via feed or water buckets, or grooms / handlers / nurses / vets or anyone who has contact with an affected horse. Unlike strangles and some other infections, the flu virus does not linger nor survive for long outside the horse.

Once a horse has breathed in the virus, it invades the lining epithelium of the airway, which becomes swollen and inflamed producing a very sore throat and a nasty cough. The virus replicates mainly in the upper respiratory tract and the viral damage causes the surface membranes lining the airways to ulcerate.  Clinical signs are due to this local tissue damage and the whole body's inflammatory and immune response to the infection.  The damage of the surface layers of tissue in the repiratory tract can disrupt the clearance of mucus and other debris from the airways. In turn these damaged areas end up being invaded by bacteria and further infections ensue.

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

  • Equine Influenza ('flu') is a disease that affects the upper and lower respiratory tract of horses, donkeys and mules. It is caused by several strains of the equine influenza virus. The disease is very infectious and spreads rapidly through groups of horses. The incubation period is 1 – 3 days.
  • Equine flu in not a worldwide disease. Some countries are free of the disease due to strict quarantine measures.
  • The equine flu virus has been scientifically shown to be transmittable from horses to dogs The virus has caused outbreaks of the disease in racing greyhounds and in pet dogs, and is suspected to have affected some hound packs.
  • Efforts have been made to pre-empt an equine flu epidemic by vets investigating and identifying acute respiratory disease in horses. If your vet suggests taking a few blood tests or swabs, it may be very useful to help prevent further cases of respiratory illness.
  • In late 2018 and 2019, there has been a large outbreak of equine flu in Europe and the UK. While some vaccinated horses developed mild clinical signs, severe disease (including death) has occurred in unvaccinated horses. 
  • A vast number of horses and ponies are not vaccinated and are thus at risk of contracting flu and increasing the spread of the disease.

Clinical signs of equine flu in horses are variable, but include the following:

  • a high temperature of 39-41°C (103-106°F) lasting for 1-3 days
  • a frequent harsh, dry cough that can last for several weeks
  • a clear, watery nasal discharge that may become thicker and yellow or green after 4-5 days
  • enlarged glands under the lower jaw
  • conjunctivitis and ocular discharge
  • depression and loss of appetite
  • filling of the lower limb

Equine flu in general has a has a fairly short incubation period, with clinical signs usually appearing within 1-5 days after exposure.  It is generally a self-limiting disease resolving within 3-6 weeks, provided that horses are well managed.  Horses may also experience persistent poor performance syndromes after the initial acute phase of infection.

All horses with respiratory infections should be given complete rest. Ideally, they should not recommence any strenuous exercise until two weeks after the signs have gone. Frequently the advice is given that they should have a week off for every day that they have had a fever (raised temperature), but many riders will find that their horses are below par for longer, in the same way the people can feel run down after flu.

Good stable ventilation and management is essential. Exposure to dust and spores should be minimized as horses with respiratory infections are susceptible to developing further airway inflammation. If hay is fed, it should be of good quality and soaked. If weather conditions permit, affected horses benefit from being turned out into a small paddock for at least part of the day once their temperatures have returned to normal. This is especially important in the recovery stages.

Some of the new antioxidant feed supplements on the market to help respiratory function, may well be of benefit, as well as some other medications to help breathing. It is best to consult your own vet for the appropriate treatment of individual cases.


An accurate diagnosis can be made by:

  • recognising the clinical signs and the rapid spread between horses
  • PCR: (detection of flu virus genetic material) of nasopharyngeal swabs
  • virology: demonstration of the virus in nasal or nasopharyngeal swabs collected early on in infection (within 7-10 days of exposure) 
  • blood tests provide clues, such as abnormal white blood cell pattern
  • rising antibody levels in blood (serum) samples taken early in the course of the disease and 2-3 weeks later confirms exposure to the virus
  • a high antibody titre on a single sample is also considered diagnosis of the disease
  • history of recent contact with a confirmed or suspected case of the disease.

As soon as a horse shows any suspect signs, strict hygiene and isolation procedures should be applied. Any horses that have been in contact with the affected horse should be carefully monitored and ideally should not attend shows or any other competitions as they may be incubating the disease. Exposure to the virus combined with the stress of travelling will make infection more likely.

The disease is spread by inhalation of virus released into the atmosphere as an aerosol by coughing and blowing, essentially one horse coughing over another. For this reason equine flu is highly contagious within a group of horses.

If you suspect your horse has equine influenza you should contact your vet. Steps can then be taken to stop the spread of the disease. One of the most crucial is identifying the infection accurately; if you know what you are dealing with, then you can control it.


  • Isolate
  • Contact your vet and get your horse seen as soon as possible
  • Rest with restricted turn out if appropriate (following veterinary guidance)
  • Switch to dust-free bedding and feed soaked hay, or better still, haylage, and feed from the floor
  • Antibiotics are sometimes necessary to treat secondary bacterial infection (but antibiotics are ineffective against the virus itself)
  • Other drugs are occasionally required to improve the removal of discharge from the airways (eg mucolytics) and help manage the coughing. Painkillers such as Equipalazone ('bute') can also help.

Equine flu is difficult to control, especially in horses that are frequently transported and mixed extensively. Outbreaks are most common when young, susceptable horses are brought together at sales, shows or for training. Regular vaccination is the key to the prevention and control of outbreaks of equine influenza and is compulsory under British Horseracing Authority (BHA) and International Equestrian Federation (FEI) rules in the UK.

Various equestrian regulatory bodies have slightly different rules regarding vaccination. Please chesck with your own regulatory body to ensure that your horse's fly vaccinations comply. Broadly speaking the regulations are:

  • 1st vaccine
  • 2nd vaccine (21 - 92 days after the first)
  • 3rd vaccine (150 - 215 days after the second)
  • Followed by annual vaccinations

To compete, horses require booster vaccinations not more than 365 days from their last vaccination.  Horses must not have been vaccinated less than 7 days before a competition. Horses competing under FEI rules must have received a booster vaccination not more than 6 months + 21 days prior to competing.

Please check that all vaccinations are up to date. In the face of a local outbreak, it may be advisable to give a booster to any horse that has not been vaccinated in the previous 6 months. Maximum immunity is not reached until about 2 weeks after the vaccination.

If you have any questions or concerns, please speak to one of the vets at BELL EQUINE on 01622 813700.

During the course of the 2018-19 equine influenza outbreak, we had many questions about equine influenza vaccines. Influenza does occur from time to time every year, and is most commonly reported in unvaccinated horses.

The detection of influenza in a small number of vaccinated horses led some people to question whether vaccination is worthwhile.

Following expert advice, we believe that vaccination is essential to controlling equine flu and is particularly important during an outbreak. All horses’ immune systems respond slightly differently to the flu vaccine and flu can vary a little from strain to strain, so predicting the exact response to an infection can be difficult but vaccination remains by far the most important way of controlling this disease

Experience from recent outbreaks shows that even in situations where vaccine protection is not 100% the vaccines are still helpful by reducing the severity of the symptoms, reducing the time that the horse is ill for and reducing spread of disease.

Spread of flu through unvaccinated groups of horses is often rapid, horses generally have more severe symptoms and take longer to recover. In rare cases it can be fatal.

How vaccination works:

Vaccination works by mimicking the effects of infection using dead/deactivated virus or parts of the virus. The horse’s own immune system then comes to recognise the virus and is primed and ready if it meets a real flu virus. The vaccines are produced and updated using expert scientific guidance to cover the strains of flu that they think our horses are most likely to be exposed to.

By boosting vaccination the immune system remains primed. The frequency of boosters will be determined by the level of risk the horse is exposed to. At present we advise vaccination for all unvaccinated horses and booster vaccination for any horse that has not had a vaccination within the last 6 months. Please feel free to call the clinic should you wish to discuss your horse’s vaccinations and individual situation.

The outbreak of equine ‘flu in North Kent in early 2019 follows the outbreak of cases in the later part of 2018 and 2019, which caused the British Racing Authority (BHA) to cancel racing across the UK on 7th February 2019 for the first time in 18 years. BBC Sport posted an article regarding this decision which you can read HERE‚Äč.  

The Animal Health Trust (AHT) informs us on the most recent outbreaks of equine influenza. They regularly update their dedicated equine ‘flu page EQUIFLUNET with the most up to date information as it comes through. 

The British Equestrian Federation (BEF) also has a very useful GUIDE on biosecurity and Q&A on equine flu.

We would like to take this opportunity to encourage everyone to be vigilant to minimise further disease spread and to protect your horse from the disease in the future. Please ensure that all horses are up to date with their vaccinations and have any lapsed or unvaccinated horses vaccinated as soon as possible. While annual vaccination should protect the majority of horses for at least one year, we recommend a booster vaccination if your horse has not received a vaccination in the last 6 months, especially for certain at risk individuals.

Please discuss your horse’s individual risk with your vet and be on the lookout for clinical signs of fever and signs of respiratory disease (nasal discharge, coughing and fast breathing).

Do not hesitate to contact us on 01622 813700 if you have any questions or concerns. Our vets will be happy to help.

Useful tips to reduce infectious disease risk:

  • When away from home avoid direct nose to nose contact with other horses and shared water/ feed buckets
  • Quarantine new arrivals on yards for 21 days
  • Contact your vet if your horse shows signs of infection in particular a cough or runny nose
  • Vaccinate horses for equine influenza. 

We advise that the frequency of vaccination should depend on the risk to the horse. All horses should be vaccinated at least yearly. Horses at higher risk, in particular those that mix for competition or are stabled on yards with frequent new arrivals should receive boosters every 6 months.


for example:

British eventing: please click HERE to go to their website.

The Pony Club: please click HERE to go to their website.