Strangles 

The Disease

Strangles is a widespread, highly infectious, debilitating disease of horses, which is endemic in the U.K. It is caused by a bacterium called Streptococcus equi. Strangles is considered to be the most commonly diagnosed infectious disease affecting horses of all sizes, breeds, types and ages, although some groups are higher risk than others and may be the worst affected - young horses (< 5 years old), sick horses, horses with compromised immune systems, stud farms, racing stables, livery yards and riding school horses. This is due to the way the disease is spread. Direct contact between horses is the most common factor, but spread of the disease is also via contaminated food, drinking vessels or equipment including peoples’ clothing. The disease can be spread by inhalation but this is less common.

Strangles is not a notifiable disease, but yards with confirmed outbreaks are strongly recommended to implement stringent biosecurity precautions to prevent further spread, and to notify neighbouring premises.

Clinical Signs

The severity of clinical signs can vary depending on the age and condition of the horse. Clinical signs are usually seen between 3-7 days after the horse has been in contact with the bacterium, but can take as long as 14 days to show.

Clinical signs include:

• Loss of appetite
• Difficulty in swallowing
• Nasal discharge (often thick green or yellow in nature)
• Depression and dullness
• Development of a cough
• Fever
• Swelling of the lymph nodes (glands) under the jaw or lower down the neck approximately a week after the onset of clinical signs.
• Complications seen in a small number of rare cases include:
• Abscesses placing pressure on the airways, causing the horse to have difficulty breathing – hence the term ‘Strangles’
  • ‘Bastard Strangles’ causes abscessation in other areas of the body such as the brain, lungs and/or intestines. This can result in organ failure due to the space occupying nature of the abscesses. Rupture of these abscesses can prove to be fatal.
  • Purpura haemorrhagica, which causes bleeding into the skin, gums and organs such as the lungs and can also prove fatal.

When the abscesses in the lymph nodes burst they discharge highly infectious, thick creamy-yellow pus.

Atypical Strangles is now a commonly recognised condition of infected horses that display less severe clinical signs. Such infected animals may show a mild respiratory infection but without developing abscesses. These horses may not be recognized, as owners may think that their horse just has a “cold” and therefore these cases go untreated. It is unknown just how many horses are infected in this way and it may be that they represent a significant majority of the total number of horses affected. Most horses infected with Strangles recover uneventfully over a period of 3-4 weeks but this depends on the severity of the symptoms, and the immune system of the affected horse. More severe cases may take longer to make a full clinical recovery. Up to 10% of infected horses can remain as infectious carriers for many years even though they appear to have made a full recovery.

Spread of Infection

The disease is spread when the nasal discharge or material from the draining abscess is passed from direct contact between horses or contaminates the environment

For example, the infection can be spread:

• by nose to nose contact between horses
• via equipment shared with infected horses, such as:
• water troughs where the bacterium can survive for long periods
• feed buckets
• brushes
• tack

Horses that appear healthy are easily overlooked when investigating a case of Strangles. It is important to remember that

• There is a delay of between 3 and 14 days between horses becoming infected and showing clinical signs
• Horses can shed disease (i.e. pass it to others) before showing clinical signs of the disease
• Horses with atypical Strangles may only show very mild clinical signs but are still infectious to others
• Horses recovering from the disease can still be infectious for many weeks. Untreated horses (not given antibiotics) will shed the bacteria on average for 2-6 weeks after infection.
• Carriers can remain infectious for months or years

Diagnosis

The diagnosis is relatively straightforward in horses that develop classic signs and is confirmed by taking a swab from the back of the horse’s nasal cavity (nasopharynx) or by directly swabbing a draining abscess. The more challenging cases are those, which only develop a nasal discharge without the classic lymph node enlargement, as this can resemble other, less serious bacterial respiratory diseases.

We recommend isolation of any horse that develops a thick nasal discharge and taking nasopharyngeal swabs for culture to check for Strangles bacteria. A blood test exists which can establish if new horses entering a yard have previously been exposed to the infection. This blood test detects antibodies, which are produced in response to infection with the bacteria, which causes Strangles. This is particularly useful to identify carrier horses. The blood test can also be used to assess which horses have been exposed to the bacteria at the end of an outbreak. In addition if the disease is suspected in a case where there is not a huge amount of nasal discharge e.g. in an atypical case of Strangles, then paired blood samples taken 10-14 days apart can be used to see if there are rising antibody levels, which would indicate exposure to the disease.

There are currently several tests available, which can be carried out by your veterinary surgeon:

 

1. Blood testing: The blood test identifies antibodies to the S. equi antigens (Strangles bacteria) in the horse. In practice the blood test is used to identify both carriers and recently infected horses with a 90.9% sensitivity. If the test is positive the horse is likely to have been in contact with Strangles in the recent past. Further tests are then required to confirm if the horse is recovering from the disease or is a carrier. It is important to note that it takes approximately two weeks for a horse to develop antibodies against each antigen and so it may not be possible to accurately identify horses that are incubating the disease or are in the early stages. The test is particularly useful as a screening tool prior to movement or introduction of a horse into a new yard, and in the identification of potential carriers at the end of a Strangles outbreak.


2. Guttural pouch endoscopy including polymerase chain reaction (PCR) and bacteriological analysis of the guttural pouch washes: This test is useful to determine the true disease status of a horse, which may have had a positive result on a blood test, or it can be used to assess whether or not a horse which has recently suffered from Strangles infection has fully recovered from the disease. These days most practices have access to an endoscope for performing guttural pouch washes. However, if an endoscope is not available, then a series of three nasopharyngeal swabs, taken at weekly intervals can be used to test for the presence of the bacteria, which causes Strangles. However, the disadvantage of this technique compared to the guttural pouch wash is that a series of three clear nasopharyngeal swabs must be obtained to ensure that the horse is disease free. Obviously this adds another three weeks on to the “recovery” period. In short a guttural pouch wash submitted for PCR +/- bacteriology will give a faster result at the end of an outbreak.

Carriers

Approximately 10% of horses, following infection with the disease and apparent recovery, can become chronic carriers. These horses harbour a reservoir of the bacteria in their guttural pouches, usually in the form of chrondroids (balls of dried pus). Such horses show no clinical signs of the disease and appear perfectly normal and healthy.

It is interesting to note that many outbreaks of Strangles occur after the introduction of apparently healthy animals (which are carriers) to a yard (hence the importance of the Strangles blood test and an appropriate period of isolation). It is unknown how long a horse may remain a carrier, although trials have shown the bacteria can survive for more than five years.

Prevention

Should Strangles infection be identified or if indeed it is suspected while awaiting a diagnosis, then it is important to separate the horses into the following groups using a “traffic light” type system:

• Infected horses (must be isolated) (Red Group)
• In contact horses (kept separate from infected horses and from “safe/clean” horses) (Amber Group)
• “Safe/clean” or disease free horses which have not been in contact with either of the above (Green Group)

Isolation is a critical step in preventing further disease spread. The ‘Strategy to Eradicate and Prevent Strangles (STEPS)’ provides detailed information on isolation and can be found here.

Yard owners should implement a strict quarantine procedure for new arrivals to the yard, not just for Strangles risks, but also for worming, influenza, and ectoparasites. A clear blood test (for Strangles) can be requested before accepting a new horse on to the premises, and on some yards a further blood sample may be requested at the end of the 14 day isolation period (so from Day 15 onwards) to ensure that the horse has not become infected in the time in between taking the first blood sample and moving to the new yard.

Yard owners are also strongly advised to consider biosecurity management plans in the event of an outbreak. It is critical that rapid isolation of infected and at risk animals takes place to prevent further spread of the disease. A delay whilst this is investigated could mean the difference between movement restrictions on the yard for a few weeks or for many months. Strict hygiene and biosecurity plays an essential role in preventing and controlling this disease.

Treatment

Treatment varies for each individual case, but in the majority of cases treatment is aimed at keeping the horse comfortable by using anti-inflammatory drugs such as phenylbutazone (Bute) and encouraging drainage of pus from the abscesses. The disease then has to run its course, with guttural pouch endoscopy and washes or nasopharyngeal swabbing being performed at a minimum of 30 days after the cessation of clinical symptoms. Individual treatment protocols should be discussed and agreed with the attending veterinary surgeon. The speed and success of resolving cases often hinges on the strict management of isolation, quarantine, minimising movements (of people and horses) and disinfection.

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Strangles FAQs

A horse on our yard has been diagnosed with strangles. It has been isolated but is there a risk to my horse?

If your horse has had any direct nose to nose or indirect (people, equipment, tack) contact with the infected horse then there is a risk that your horse may develop disease. Rectal temperatures should be taken twice daily from ‘in contact’ horses and those displaying clinical signs. Horses with a raised temperature >38.5C should be isolated and presumed infected until the results of further testing prove otherwise.

How do I know if my horse is a carrier of the disease?

Approximately 10% of recovered horses become carriers of the disease and harbour the bacteria in their guttural pouches (pouches in the throat region). These horses are often symptom free and appear outwardly healthy although they can spread the disease to susceptible horses for months or even years. It is these carrier horses, which perpetuate the problem in the U.K. A blood test is available which will identify potential carriers (see information above). Further investigation of positive cases should then follow by employing either a series of nasopharyngeal swabs (three swabs taken a week apart) or preferably by endoscopic examination of the guttural pouches with PCR +/- bacteriology of the fluid collected from flushing the guttural pouches. Horses can be treated through removal of infected material and flushing of the pouches.

How long should a field be left empty of horses after a contaminated horse has been grazing the field?

The bacteria are unlikely to survive longer than 3-4 days in the environment; however, water troughs carry the greatest risk and can harbor the infection for up to 30 days. It is advisable to empty troughs and disinfect them before moving horses back onto the pasture.

How long should a yard remain closed after a case of strangles has been diagnosed?

This depends on the specific situation and how well isolation procedures have been followed. Ideally a yard should remain closed to all horse movements until all horses have been identified as free from disease on diagnostic testing. Screening for carriers after an outbreak will usually commence around 35 days after the last clinical case has stopped showing symptoms. Once all carriers are identified, treated and confirmed free from disease a yard can be re-opened.

How can I prevent my horse from getting strangles?

A good understanding of biosecurity is essential in preventing your horse from becoming infected with strangles (see biosecurity fact sheet). Good biosecurity principles should be applied in both the yard situation and when out and about at shows and competitions.

Is there a vaccine available?

Yes. A live attenuated vaccine is now available in the UK to aid in the management and prevention of strangles. It can be used in horses from four months of age and is administered by submucosal injection into the upper lip. The initial vaccination course consists of two vaccinations given 4 weeks apart.

Can vaccination give my horse strangles?

No. Although the vaccine is live, the bacteria have been altered so that they only survive long enough to stimulate the immune system and will not cause disease.

My horse is the only horse vaccinated on the yard, is there any benefit at all?

The best way to control strangles through vaccination is to vaccinate all horses on a yard to reduce the level of challenge. Individual horses will still benefit from vaccination, as their immunity will be higher than any unvaccinated horses.

Can my horse be vaccinated if there is an outbreak on the yard?

Boosters should be given at 3 monthly intervals to maintain immunity. However, a priming response is maintained for up to six months after basic vaccination. Therefore, only a single dose of vaccine is needed to restore immunity. If your horse has not been vaccinated or vaccinations have lapsed then it is recommended to wait until the yard becomes ‘disease free’ through diagnostic testing before initiating a vaccination programme.

For further information regarding Strangles and biosecurity measures to help prevent Strangles please contact Central Equine Vets on: 0131 664 5606.

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