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Respiratory Disease – Equine Influenza and Strangles

Respiratory Disease – Equine Influenza and Strangles

Respiratory Disease – Equine Influenza and Strangles

Credits to Dr Andrew Waller, Head of Bacteriology at the Animal Health Trust & Adam Rash, Equine Influenza research team at the Animal Health Trust

Equine influenza

Equine influenza, or flu is an infectious disease caused by the influenza virus and can be passed very easily from one horse to another. The virus replicates in the nose, throat and windpipe (respiratory tract) and is spread when the horse breathes, coughs or sneezes. The virus can also be passed on by direct contact between horses and by handlers, so good hygiene is very important.

Respiratory Disease – Equine Influenza and Strangles

Respiratory Disease – Equine Influenza and Strangles


Strangles is caused by a bacterium called Streptococcus equi. Strangles typically causes lymph node abscesses in horses (see Table of clinical signs/what to look out for), but not all horses may show typical signs. Some horses have very mild reaction to the disease, either because they were not exposed to a high dose of bacteria, they may be naturally more resistant to the bug, or because they may have had Strangles in the past. Some horses can develop very severe clinical signs and Strangles kills around 1 to 2% of horses that become infected.

Strangles can also persistently infect recovered horses, which are called ‘carriers’. These carrier horses look and are completely healthy, but they can shed the bacteria into drinking water or onto food and other objects, enabling the bacteria to spread to other horses, triggering new outbreaks. On the plus side, the Strangles bacteria are much bigger and heavier than influenza virus and require closer contact in order to spread from one animal to another. However, this could just be as slight as a person touching an area that has been contaminated by the bacteria from an infected horse and then touching the nose, food or drinking water of another horse. So washing hands and avoiding nose-to-nose contact of horses is very important to prevent the spread of Strangles.

Both flu and Strangles can spread at events where horses come together in large groups and are under stress from travelling or competing, such as shows, drag hunts and sales.

What to look for - See table
What to do

If your horse is showing signs of flu or Strangles then you should call your vet as soon as possible. Your vet will be able to take swab and blood samples from your horse to confirm the infection. Your vet will then be able to advise you on the most appropriate treatment and on the long-term recovery of your horse. They can also help you identify other susceptible horses and suggest strategies to limit the spread of the disease.


Flu - It takes time for the lining of the respiratory tract to fully recover after infection with flu. During this time the horse should not be put under 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 your horse had a raised temperature they will need at least a week off from exercise.

Strangles - this can last for several weeks and some horses can take months to recover. One month after clinical signs have resolved it is important that your vet checks to see if any of the horses that had strangles have become carriers. They may blood test all horses on the yard that remained healthy to see if any of them may have been exposed to the infection. Exposed horses and those that have recovered can then be examined by guttural pouch endoscopy to confirm whether they have completed cleared the infection or require further treatment. This process can take time, but is essential if further outbreaks of strangles are to be prevented.


Some horses can fall ill again after appearing to recover from Strangles. Signs such as weight loss and depression could indicate a rare complication, called ‘bastard Strangles’, where the bug has spread to other lymph nodes around the body. This should be investigated further by your vet.

How to prevent infection and spread

Equine influenza and Strangles are endemic in the UK so most horses will come into contact with these bugs during their lifetime.

The main way of controlling equine flu is achieved through regular vaccination, which is given either once or twice a year. There are risk factors associated with influenza infection, which include being compromised by injury or illness, being very old or very young, and not being regularly vaccinated. Vaccinated horses may not show signs of flu themselves, but they are still able to carry the virus and pass it on to other horses. For this reason vaccination strategies for companion ponies or older horses should be carefully considered as even if they don’t leave the yard themselves, they are at risk of being infected by other vaccinated horses that do travel.

If you suspect a horse has influenza (flu), it is important to isolate it from other horses. The virus spreads very quickly and horses can be infected before they show any signs, so if it is appropriate it is sensible to also isolate horses that have been in contact with the infected individual. The virus can survive outside of the host for limited periods and so objects such as head collars, tack, grooming kits and feed and water buckets can also be sources of infection. However, most disinfectants are sufficient to destroy the virus. As horses can be infected before showing any signs of disease it is good practice to isolate or quarantine new arrivals to the yard for two weeks. Any signs of infection should become apparent during this time and will therefore reduce the risk of spreading the disease to other resident horses.

Strangles is best prevented by good hygiene, quarantining new arrivals and testing them to make sure they are not carriers of Strangles. Being vigilant when attending equine events (avoid your horse sharing drinking water with another horse and nose-to-nose contact) can also help to reduce the risk of bringing Strangles back to your yard. A drop of pus from an infected horse contains as many as two million bacteria. Around one thousand bacteria are enough to set up an infection in a horse that could, in time, lead to the development of Strangles, so one drop, or cough could in theory infect two thousand horses! However Therefore, good hygiene can greatly reduce the risk of passing on strangles to other horses.

As with influenza, the spread of Strangles can also be controlled by isolating infected horses and using simple good hygiene measures. For further details please see the STEPS guidelines produced by the British Horse Society ( The strangles bug can survive on grass, tack and fence posts for only a couple of days, less if the sun is shining and more if the weather is cold and wet. However, the bacterium can survive for up to a month in drinking water. Therefore, also be wary of public water troughs for horses whilst riding or travelling to events as the strangles bug might still be present one month after an infected horse last had a drink from it.


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Table - What to look out for 




Harsh dry cough

Raised temperature

Laboured breathing

Clear or white nasal discharge

Purulent yellow nasal discharge


Enlarged lymph nodes in the throat

Abscessed lymph nodes


Quiet or depressed


Loss of appetite

Spreads rapidly between horses

Possible, depending on management of horses


The Animal Health Trust (AHT)

What we do - At the AHT we run a surveillance scheme for equine influenza where we are able to offer free testing for the virus with funding from the Horserace Betting Levy Board. We use the samples that are submitted by vets to find out where and how frequently outbreaks of flu are occurring in the UK. We also monitor how the virus is changing over time compared to current vaccine strains. Together with surveillance data from around the world, this information is used to recommend suitable strains to be included in vaccines.

In the last two years there have been 52 recorded outbreaks of equine influenza in the UK, however this may just be the tip of the iceberg and the actual number of outbreaks may be much higher. If you would like to find out more about equine influenza, and to tell your vet about our surveillance scheme, please visit

Our Strangles research at the AHT has investigated the emergence of the causative bacteria through studying its DNA. We have shown that the current strains of S. equi date back to the 19th or early 20th century. This period corresponds to a time when horses were a major mode of transport and played important roles in a number of global conflicts such as World War I, where an estimated eight million horses died on the battlefield. At its peak 1,000 horses per day were imported to the UK from the USA and horses from all around the world were called into action. The mixing of these horses, and their replacement with young animals on an unprecedented scale, through initiatives like the formation of the National Stud, would have provided ideal conditions for the emergence and spread of the fittest strain of S. equi from which today’s global population has emerged.

We have used this information to develop more sensitive diagnostic tests that detect the bug’s DNA or antibodies produced by horses exposed to the Strangles bug. These tests are being used around the World to prevent strangles, most recently clearing strangles from over 2,000 horses in Australia!

We are also working towards developing new vaccines to prevent Strangles. There are over 600 outbreaks per year in the UK alone, with many thousands of horses affected. Over time we hope to be able to protect horses from Strangles in the same manner that horses are protected from flu today and we are working tirelessly towards this goal.

With thanks to the Animal Health Trust 



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