Equine Influenza (Equine Flu)

A highly contagious respiratory infection that can spread rapidly between horses

Equine influenza — usually called equine flu — is a highly contagious viral infection affecting the horse's respiratory system.

It can spread very quickly wherever horses mix, including livery yards, competition venues, training yards, sales, studs and equestrian events. Horses can also potentially encounter infection without travelling themselves if infected horses, people or contaminated equipment introduce the virus to their yard.

A horse with influenza may develop a sudden fever, a harsh cough and appear generally unwell. However, not every horse shows exactly the same signs, and respiratory symptoms can have several different causes.

If equine influenza is suspected, isolate the horse from other horses and contact your veterinary surgeon.

What is equine influenza?

Equine influenza is caused by the equine influenza virus.

You may hear your vet describe it as:

Equine influenza
Equine flu
Influenza A
Viral respiratory disease
An infectious respiratory infection
An outbreak of respiratory disease

These descriptions can sound more complicated than they are.

In everyday terms, equine flu is a virus that infects the horse's airways and lungs and can pass very easily from one horse to another.

It is not the same disease as influenza in people.

What happens inside the horse?

When a horse becomes infected, the virus attacks cells lining the respiratory tract.

These cells and their normal protective mechanisms help the horse clear mucus, dust and other material from the airways. Influenza infection damages this lining and causes inflammation.

This helps explain why affected horses can develop such a characteristic cough.

The respiratory system then needs time to repair itself. A horse can appear brighter before its airways have completely recovered, which is one reason why an appropriate period of rest and a controlled return to exercise are important.

Trying to return a horse to strenuous work too quickly can interfere with recovery.

What might an owner notice?

Signs can appear quite suddenly.

They may include:

A dry, harsh or persistent cough
Raised temperature or fever
Nasal discharge
Reduced appetite
Dullness or lethargy
Enlarged or tender glands beneath the jaw
Faster breathing
Reduced willingness or ability to exercise
Muscle soreness or stiffness
Several horses on the same premises developing respiratory signs within a relatively short period

Some horses may show only mild signs, particularly where there is existing immunity, while others can be considerably more unwell.

What does “pyrexia” mean?

Your vet may say that a horse is pyrexic.

Pyrexia simply means that the horse has a raised body temperature — a fever.

A rise in temperature can sometimes be one of the earliest indications that an infectious disease is developing, before an obvious cough or nasal discharge appears.

Why does equine flu spread so easily?

An infected horse can release virus-containing droplets and particles when it coughs.

Other horses can become exposed through close contact, but direct nose-to-nose contact is not required.

Virus can potentially be transferred through contaminated:

Hands
Clothing
Tack
Headcollars
Grooming equipment
Buckets
Yard equipment
Horseboxes and trailers

This is why biosecurity becomes so important.

What does “biosecurity” actually mean?

Biosecurity simply means the practical measures we take to reduce the chance of disease entering a group of horses or spreading between them.

It includes actions such as isolation, hand hygiene, not sharing equipment, cleaning and disinfecting appropriately, controlling horse movements and monitoring temperatures.

Incubation period — what does that mean?

Your vet or an outbreak notice may refer to the incubation period.

This is simply the time between a horse becoming infected and signs of disease becoming apparent.

This matters because a horse may have been exposed before anybody realises there is a problem.

For this reason, identifying which horses have mixed, travelled or shared facilities can be extremely important during a suspected outbreak.

What should I do if I suspect equine flu?

Do not continue moving the horse around the yard or take it to an event, training session or another premises.

Separate the affected horse from other horses as effectively as possible and contact your veterinary surgeon for advice.

Where practical:

Stop unnecessary horse movements on and off the premises.
Avoid sharing buckets, tack, grooming equipment and other items.
Attend to healthy horses before attending to the suspected infected horse.
Wash hands and consider changing or cleaning footwear and outer clothing after handling an affected horse.
Monitor other horses closely.
Take temperatures if your vet advises you to do so.
Keep a record of temperatures, coughing and other changes.
Tell your vet about recent competitions, clinics, sales, transport, new arrivals or contact with horses from other premises.

If several horses are involved, your veterinary surgeon may recommend a more formal isolation and testing programme.

How is equine influenza diagnosed?

A vet will consider the horse's clinical signs and its recent history, including travel and contact with other horses.

Testing may then be recommended.

You may hear terms such as nasopharyngeal swab, PCR test or blood sample.

Nasopharyngeal swab

This is a sample collected from the horse's respiratory tract using a long swab passed through the nostril.

PCR

PCR is a laboratory technique used to look for genetic material from the virus.

In simple terms, the laboratory is looking for evidence that the influenza virus is present in the sample.

Testing can be particularly important during an outbreak because coughing alone cannot tell us which infectious disease is responsible.

A cough does not automatically mean influenza

Horses cough for many reasons.

Other possibilities can include:

Other infectious respiratory diseases
Equine herpesvirus
Strangles
Airway inflammation
Equine asthma
Dust or environmental irritation

This is why an unexplained cough — particularly when accompanied by fever, nasal discharge or illness, or when several horses are affected — should not simply be assumed to be a minor respiratory irritation.

Until an infectious cause has been considered, sensible biosecurity precautions protect the other horses around it.

How is equine influenza treated?

There is no simple medicine that immediately removes influenza virus from an infected horse.

Management is therefore generally supportive, allowing the horse's immune system to deal with the infection while reducing the risk of complications.

Your veterinary surgeon may recommend:

Rest
Careful monitoring
Appropriate hydration
Good-quality, palatable feed
Medication to control fever or inflammation where appropriate
Management of any complications
A planned return to exercise

Antibiotics do not treat influenza itself because influenza is caused by a virus rather than bacteria.

However, a veterinarian may prescribe antibiotics if a secondary bacterial infection develops.

What is a secondary infection?

Influenza can temporarily damage the respiratory system's normal defences.

This can make it easier for bacteria to cause an additional infection.

That bacterial problem is called a secondary infection — essentially, another infection developing while the respiratory system is already recovering from influenza.

Rest and returning to work

A horse may look much better before its respiratory system has completely recovered.

Rest is therefore part of recovery, not simply something required while the horse has a temperature.

Your veterinary surgeon can advise when exercise should begin again and how gradually workload should be increased.

The appropriate recovery period will depend on the individual horse, the severity of the infection and whether complications have occurred.

Do not use the disappearance of the cough alone as proof that the respiratory system has fully recovered.

Vaccination

Vaccination is an important part of reducing the impact and spread of equine influenza.

Vaccination cannot guarantee that a horse will never become infected, but vaccinated horses may have greater protection and disease can be less severe.

Competition and governing bodies may also have specific influenza vaccination requirements.

Your veterinary surgeon can advise on an appropriate vaccination programme for your horse.

It is important to distinguish between two different things:

Vaccination helps prepare the horse's immune system before exposure.

Biosecurity helps reduce the opportunities for the virus to spread.

Good disease control uses both.

New horses and travelling horses

Introducing a new horse is one of the ways infectious disease can enter an otherwise healthy group.

A horse returning from competition, training, sales or another premises may also have encountered horses from many different locations.

Where facilities allow, new or potentially exposed horses should be managed separately from the established group for an appropriate period.

Separate equipment and sensible hygiene procedures can further reduce the chance of introducing infection.

Your vet can help establish an isolation protocol appropriate for your premises.

When should I call the vet?

Contact your veterinary surgeon if your horse develops:

A raised temperature
A sudden or persistent cough
Significant nasal discharge
Difficulty breathing
Marked lethargy
Loss of appetite
Rapid deterioration
Respiratory signs following recent travel or mixing with other horses

Veterinary advice is particularly important if more than one horse develops similar signs.

In a suspected infectious-disease situation, contacting your vet early can help protect not only the affected horse but other horses on the premises and potentially the wider equine community.

The important thing to remember

Equine influenza is not simply “a horse with a cough”.

It is a highly contagious viral respiratory disease, and the movement of horses between yards and events gives it opportunities to spread.

Recognising the early signs, isolating a potentially infectious horse, seeking veterinary advice and applying good biosecurity can make an enormous difference.

Vaccination, sensible disease surveillance and good everyday biosecurity provide the strongest practical defence.

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