Welcome to Equiflunet. The information below will help you identify the signs of equine influenza (‘horse flu’) and inform you on the best course of action if you suspect your horse may be infected.

You will also find some useful information about the nature of this infection and how you can vaccinate to protect against influenza in your horses.

Horse owners – if you suspect flu, please speak to your vet. We are unable to offer you specific advice as every situation has so many varying factors. Your vet is best placed to give you relevant and useful advice. Thank you.

Download our ‘dealing with equine flu’ helpsheet here

What is equine flu?

Equine influenza, sometimes referred to as equine flu or horse flu, is a highly contagious respiratory infection.

An infected horse will show clinical signs similar to those of human flu and will be infectious for about a week. Tens of thousands of horses can be affected by one outbreak. Although rarely fatal, it can have a huge impact on competition and breeding due to restriction of horse movements.

Signs of infection

Unvaccinated horses

In unvaccinated horses we tend to see certain ‘cardinal’ signs. The virus targets the upper respiratory tract where the cough receptors are positioned so a very harsh dry cough is typical. Often horses will develop a raised temperature which will last around 7-10 days, during this time they may be quiet, off their food and generally sluggish, they may also have a small amount of clear or white nasal discharge and enlarged lymph nodes in their throat.

Vaccinated horses

Horses that have been regularly vaccinated often show no clinical signs, but they may still shed enough virus to infect other horses. This is how the outbreak in Australia in 2007 started.

Horses that have been vaccinated but only have partial protection, e.g. because they haven’t been vaccinated frequently enough or because the vaccine type used was not updated, may show varied signs of mild non specific respiratory disease. This can include mild lethargy, nasal discharge and possibly a cough.

Call your vet if you suspect that your horse may have equine influenza

Dr Debra Elton
Dr Debra Elton, Head of Virology


There are a number of long term complications that follow infection with equine flu, these tend to be poorly recognised and therefore their importance is often underestimated.

The development of secondary bacterial infections after having flu is a very important complication for horses, both in terms of the degree and longevity of the illness and the financial implications to the owner. For non-sports horses it is probably the most important part of the whole flu syndrome.

Risk Factors

Secondary infection may occur if the horse has a compromised immune system, either due to age, stress or illness or because the lining of the respiratory tract has not fully recovered and the horse is over exercised.

Clinical Signs

Typical signs include a thick yellow/green nasal discharge (shown below), and a wet, often productive cough. The horse may develop a high temperature and be dull and off its food. These signs will usually last for much longer than they would with infection of flu alone. Horses who are very compromised may develop particularly severe infections, leading to pneumonia or even death.

What to do if your horse is showing signs of equine flu…


If your horse is showing signs of flu then the best thing to do is call your vet as soon as possible. If they also suspect flu, they will then be able to take a swab from inside your horse’s nose and send it to the AHT where we will test it for flu free of charge.

Horses shed the most virus in the first few days of infection, so the sooner your vet takes a sample, the more likely they are to get a positive test result. Paired blood samples are also useful for making a diagnosis, especially if your horse is not sampled for several days after it first shows signs of infection.

Diagnosis and treatment

Once your vet has received the result, they will be in a better position to advise you on the most appropriate treatment and on the long term recovery for your horse. They will also help you identify other susceptible animals and suggest isolation strategies to limit the spread of the disease.


It takes between 50-100 days for the lining of the respiratory tract to fully recover after a horse has been infected with flu. During this period they should not undergo any stress or strenuous exercise as they will be predisposed to developing other respiratory infections.

A good rule of thumb is for every day that they had a temperature they will need at least a week off from exercise.

Prevention of equine flu

Horse Flu is endemic in the UK, which means that most horses will come into regular contact with the virus during their lifetime. The control of horse flu in the UK is based on limiting the signs of infection in horses that have been exposed to the virus, rather than trying to prevent exposure itself. This is primarily achieved through regular vaccination which is given either once or twice a year.

There are a number of vaccines available in the UK, which differ in their formulation and also the strains of virus used, click here for more information about products available. Vaccination is compulsory for racing and some competitions.

All viruses increase their numbers by making copies of themselves whilst inside the animal they have infected, a process called replication. An important feature of influenza viruses is that during replication it regularly ‘makes mistakes’ so over time the virus will change. This poses a problem for vaccines because they need to be regularly updated to ensure they stay effective against the virus as it changes. We are already familiar with this situation in people where the flu vaccine strains are reviewed annually. A similar process is carried out for equine influenza, for more information see ‘Surveillance of equine flu in the UK’ below.

Risk factors for transmission of equine flu

Horse flu is an infectious disease and can be passed very easily from one horse to another. As the virus replicates in the nose, throat and windpipe it is spread via aerosols when the horse breathes, coughs or sneezes.

The virus can also be passed on by direct contact and by handlers, so good hygiene is very important.

Horses that travel or live on large yards are particularly likely to encounter the virus. When horses are in close contact, flu can spread extremely quickly. Shows or drag hunts where horses come together in large groups and are under stress from travelling or competing can provide the perfect environment for the virus to spread. For this reason outbreaks of flu are often traced back to such events.

Vaccinated horses may show no signs of flu themselves but are still able carry the influenza virus around and pass it on to other horses. For this reason vaccination protocols for companion ponies and older horses should be carefully considered: even if they don’t leave the yard themselves they can be infected by other vaccinated horses who do travel.

Major risk factors include being compromised by injury or illness (such as Cushings disease or COPD), being very old or very young and not being regularly vaccinated. The consequence of infection in compromised unvaccinated horses may be more severe. Please see the Complications section for more information.

Surveillance of equine flu in the UK

The Animal Health Trust runs a surveillance scheme for equine influenza in the UK. We have set up a Surveillance Scheme, which has participants from equine veterinary practices throughout the UK, to encourage submission of nasal swab and blood samples from horses with suspected flu.

These samples are vitally important for us to find out where and how frequently outbreaks of flu are occurring, we can also use them to monitor any changes that occur in viruses circulating in the UK.

We work together with many other collaborators in Europe, the USA, Argentina, China, Japan and India and support other laboratories that carry out this type of work. The information that is collected every year is used to select suitable vaccine strains for recommendation to vaccine manufacturers click here for more information.

The surveillance scheme is funded by the Horserace Betting Levy Board, and is free for any veterinary practice to join. Members of the scheme can send in samples for diagnosis, free of charge. They are also notified of any outbreaks of influenza in their area. If you would like to tell your vet about this scheme, please contact us.




Supported by Horserace Betting Levy Board