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This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.

Equine flu, as equine influenza is commonly known, is caused by various strains of the influenza virus that affect the upper and lower respiratory tract of horses, donkeys and mules. Equine flu is found within the British horse population and is a major and economically important cause of acute respiratory disease throughout the world, with the exception of some island nations, such as Iceland and New Zealand.

Once the virus has been inhaled, it invades the lining (epithelium) of the airway, which becomes inflamed, producing a very sore throat and a nasty cough. This damage causes patches of the membranes lining the airways to ulcerate, which disrupts the clearance of mucus and debris from the airways causing a thick discharge from the horse’s nose. Bacteria then invade the damaged areas leading to further infections.

Equine flu: Signs | How it spreads | Is it serious? | Diagnosis | Treatment | Prevention | During an outbreak

What are the signs of equine flu?

• A very high temperature of 39-41C (103-106F) which lasts for one to three days
• A frequent harsh, dry cough that can last for several weeks
• A clear, watery nasal discharge that may become thick and yellow or green
• Enlarged glands under the lower jaw
• Clear discharge from the eyes and redness around eyes
• Depression and loss of appetite
• Sometimes filling of the lower limbs, muscle stiffness and other signs such as weight loss

How do horses catch equine flu?

As with the human version, equine flu is very contagious. With an incubation period of one to five days, it spreads rapidly as the virus is released into the atmosphere by infected animals. It is mainly acquired through inhalation of virus from ill animals coughing and spluttering infected respiratory droplets. Indirect spread is also possible via buckets or grooms/handlers/nurses/vets. Unlike strangles and some other infections, the flu virus does not linger nor survive for long outside the horse, but it can be airborne, blowing over surprisingly long distances, certainly more than 1km.

How serious is equine flu: do I need to call the vet?

If your horse shows a raised temperature and/or any signs of equine influenza, strict hygiene and isolation procedures should be adhered to immediately and you should contact your vet.

Horses that have been in contact with an affected animal should be carefully monitored and should not attend shows or external clinics or training.

It is recommended that horses on a stable yard with an outbreak of equine flu do not leave the premises while the outbreak is ongoing.

Diagnosis of equine flu

An accurate diagnosis of equine flu can be made by:

• recognising the clinical signs and the history of rapid spread between horses
• isolation of the virus through nasal or nasopharyngeal swabs
• rising antibody levels in blood (serum) samples taken early in the course of the disease and two to three weeks later
• history of recent contact with a confirmed case of the disease

Vaccinated horses may show signs, but these are usually much milder than those seen in unvaccinated horses.

How do you treat equine flu?

Good basic nursing is key when treating a horse with equine flu. Good stable ventilation and dust-free management is essential. Exposure to dust should be minimised, as horses with respiratory infections are more susceptible to airway irritation. It is best to switch to dust-free bedding and feed-soaked hay, or better still haylage, from the floor.

If weather permits and risks to other animals are manageable, then affected horses benefit from being turned out for at least part of the day once their temperatures have returned to normal. This is especially important in the recovery stages. At the same time infected horses should be isolated.

Antibiotics have no effect against a virus, but can be useful to control secondary bacterial invasion. This is a risk in foals or elderly or otherwise unwell animals, which can succumb to fatal pneumonia. Medications to help breathing can prove beneficial.

How long does equine flu last?

Horses that catch equine flu typically start to show signs around one to five days after exposure and it may take three to six weeks before the signs pass, assuming they are receive appropriate care. Horses with a respiratory infection should be given complete rest and not restart any strenuous exercise until at least three to four weeks after the signs have gone.

How do you prevent flu in horses?

Equine flu is difficult to control, especially in horses that are frequently transported and mixed extensively, so regular vaccination is important. Outbreaks are most common when young susceptible horses are brought together at sales and shows, or for weaning and training. Surprisingly low numbers of the UK’s horse population are vaccinated, despite the fact that vaccination is the preferred method of control and is compulsory when competing under British Horseracing Authority, FEI and affiliated governing bodies’ rules in the UK.

Whenever you attend shows or training outings away from home, you should follow strict biosecurity and hygiene methods such as:

• Do not allow horses to touch or sniff other horses
• Riders/grooms should only handle their own horses and tack
• Do not share buckets or kit with other horses/riders
• Avoid using on-site water facilities. If this is impossible, avoid dunking the hose into the bucket and sanitise your hands before carrying the bucket to the horse
• Avoid hand grazing the horse at a venue
• Ensure show stables have been disinfected before and after use and all bedding removed and replaced with fresh
• Horse’s temperature should be taken before travel to an external venue
• Horses returning from external venues should be isolated if possible, with their temperature and their health closely monitored

How often are horses vaccinated for flu?

Many British equine sport governing bodies tightened up their vaccination rules following a significant increase in reported cases in 2019 involving the Florida Clade 1 virus, with all requiring the minimum of the first two vaccines to have been given prior to competition.

The British Horseracing Authority introduced new flu vaccination rules on 1 January 2022 to harmonise with other racing authorities in Europe. The new rules increased the frequency of boosters from 12 months to six months and reduced the time periods between the first and second vaccines from 21-92 days to 21-60 days, and between the second and third vaccines from 150-215 days to 120-180 days.

While there was a period in autumn 2022 when limited supply of the Proteq equine flu vaccine from Boehringer Ingelheim was available to UK vets, leading to British Equestrian’s emergency response group issuing “temporary guidelines to help minimise the impact of the shortage, the vaccine is now widely available once again resulting in a return to normal vaccination periods.

NB: We recommend all competition riders double check the latest version of the appropriate rulebook to ensure your horse’s vaccinations comply with their governing body’s requirements.

British Horseracing Authority: horses must have been vaccinated against equine flu within the past six months. Horses must not have been vaccinated less than seven days before racing.
FEI competitions: a booster vaccination must have been given no more than six months and 21 days prior to competing. A vaccination should not have been given within seven days of the competition or of entry into the FEI stables.
British Dressage: Since 30 June 2021 horses must have had a flu booster within six months and 21 days of a competition. British Dressage has an exemption policy for animals that experience severe reactions to being vaccinated.
British Eventing: The most recent booster injection must have been given within six calendar months and 21 days prior to the horse arriving at the competition. A horse may not compete if it has been given the booster on any of the seven days before it is to compete at an event.
British Showjumping: Horses to have received booster vaccinations not more than 365 days from their last vaccination.
British Riding Clubs: Horses must have had their initial course, plus annual (12 month) vacinations to attend British Riding Club (BRC) qualifiers and/or championships.

If a horse or pony is competing in one discipline for instance, showjumping but it is attending an event at a venue, such as a racecourse, it will be required to comply with the vaccination requirements under the rules of racing.

The vaccination record(s) in the horse’s passport, must be completed, signed and stamped line by line, by an appropriate veterinary surgeon (who is neither the owner nor the rider of the horse). The responsibility to comply with this rule lies with the owner, who should consult with their vet.

What to do during an outbreak?

If equine flu occurs locally, it may be advisable to give a booster vaccination to any healthy horse that has not been vaccinated in the previous six months. Consult your vet for advice.

You may wish to limit the situations where you expose your horse to others by reducing the competitions and training outings you attend, or choose only to visit venues that actively check the vaccination status of equines in attendance and require six-monthly boosters.

Can humans catch equine flu?

The equine influenza virus is similar to the flu virus that affects people, but is not identical, so horses cannot catch human influenza or vice versa.

References
Annual booster vaccination and the risk of equine influenza to Thoroughbred racehorses Equine veterinary journal, 21 November 2019

Update from the BHS on Equine influenza requirements BEVA website, 15 November 2019

Equine influenza vaccination update BEF website, 2 April 2020

British Riding Clubs changes flu rules BHS website, 10 September 2021

British Horseracing Authority: amendments to the vaccines code, October 2021

Equine Influenza vaccine shortage update, BEF website, 6 September 2022

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Our equine veterinary expert has more than 30 years experience in practice. She is an FEI vet and has attended the past three Olympic and two World Equestrian Games in her role. She has a particular interest in anaesthesia, pre-purchase examinations, ophthalmology and dermatology amongst other clinical concerns. She has been Horse & Hound’s official veterinary advisor since 1991. Karen is part of the Bell Equine team.