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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.