An Introduction to Strangles

Strangles is a highly contagious respiratory infection that affects horses worldwide, which is caused by the bacterium Streptococcus equi subspecies equi, and is characterised by fever and abscessation of the lymph nodes of the head and neck.

The disease, which makes horses struggle to  breathe, can affect any age, sex or breed of horse and is endemic in the horse population, meaning it is always circulating at any given time.

Worldwide prevalence of strangles

Strangles is one of the most frequently diagnosed infectious diseases in horses, donkeys and ponies. There are typically between five and 100 times more outbreaks of this respiratory tract infection than of equine influenza.1, 2, 3

Large outbreaks of strangles can affect hundreds of horses and have a high economic and welfare impact.

Only Iceland remains free of strangles by virtue of a ban on the import of horses, which has been in force for over 1,000 years.

Horses travel around the world to attend equine events or sales, which creates a risk of transmission of infectious diseases worldwide. Recently, a global platform for genomic surveillance of Strep. equi was created and 670 isolates from 19 countries were examined. This unique project highlighted numerous examples of the transmission of Strep. equi around the world and the importance of biosecurity, diagnostic testing and vaccination for the prevention of disease transmission. The study identified six broad types of Strep. equi (BAPStypes), with emergence and dominance of BAPS2 in Europe.4

The same variant of Strep. equi was recovered from horses in Argentina, the United Arab Emirates and the UK, highlighting one example of the international transmission of Strep. equi.

 

 

 

What do you need to know about strangles?

Watch the video to see how horses become infected with Strep. equi and how the bacterium behaves once inside the horse’s body.

 

 

  • Fever (body temperature >38.5 oC)
  • Purulent (pus) nasal discharge
  • Soft, moist cough
  • Depression
  • Inappetence
  • Abscessation of the lymph nodes in the head and neck (and other body sites in rare cases)
  • Respiratory noises due to impeded airflow
  • Up to 100% morbidity and 10% mortality (if high infectious dose)

Diagnosis is initially based on recognition of common clinical signs including pyrexia, lethargy, anorexia, nasal discharge, enlarged painful lymph nodes and eventual lymph node rupture.

Occasionally the bacteria may travel via the lymphatic system or blood vessels to other lymphoid tissue or other organs in the body and cause variable clinical signs.

  • All horses have two guttural pouches, one either side of the head, which are extensions of the middle ear 
  • Strep. equi enters the guttural pouch during disease and can persist here in around 10% of recovered ‘carrier’ horses5, 6 
  • Carriers look healthy, but can spread strangles to other horses 

How to best prevent strangles

Given the impact a strangles outbreak can have from a welfare and economical perspective, preventing the bacterial infection is much more attractive.

Biosecurity and vaccination are the two important aspects of prevention and they go hand-in-hand.

Watch the video below to learn about preventative measures for controlling strangles:

 

  • Quarantine new arrivals to a yard for a minimum of three weeks, so that they have no direct contact with other horses, or indirect contact via equipment used for other horses 
  • Test horses using the strangles blood test and guttural pouch endoscopy – ideally before arrival or while in quarantine to identify horses exposed to Strep. equi
  • Do not allow your horse to share drinking water and avoid direct contact with other horses while attending equine events 
  • Regularly clean and disinfect all food and water containers, clothing, stabling and equipment at your yard 
  • Clean and disinfect horseboxes/trailers before and after collecting any new horses 
  • Wash hands between handling different horses 
  • Reduced severity of clinical signs 
  • Reduced number of abscesses and spread of the disease 
  • Improved protection at an individual horse level as well as on a yard level 
  • Reduced risk of horses contracting strangles while travelling to other premises or attending equine events 

Vaccination aims to maximise herd immunity, which is achieved when a high proportion of a population is vaccinated. As a result, it becomes difficult for a contagious agent to spread, because there are not many animals left that can become infected and transmit the disease. 

DIVA is short for: Differentiate Infected from Vaccinated Animals.

Recently, a new intramuscular strangles vaccine has become available that does not contain live or killed Strep. equi cells, and instead targets the equine immune response to eight important proteins, to give the best protection. These proteins are different to those used in the diagnostic blood test for exposure to strangles.

Horses vaccinated with a DIVA vaccine can attend events and sales, or move yards normally without triggering positive tests, unless they have been exposed to Strep. equi

Click here if you want to learn more about the new strangles vaccine.

FAQs about vaccination against strangles

 

Young and old horses are at the highest risk of severe disease if they become infected with Strep. equi. Vaccination protects these susceptible horses from potential introductions of Strep. equi. Vaccination is also highly recommended for: 

  • Horses before competition, sales or possible exposure events 
  • Horses in an area with known outbreaks of strangles 

Vaccinating all horses on the yard raises herd immunity, and decreases the risk of the disease spreading. 

There are some guidelines with respect to vaccination in an outbreak or high-risk situation:

  • Horses with clinical signs and those in contact should not be vaccinated 
  • Vaccinated horses that have not been in contact with cases: 
    • Last dose >two months: booster doses will maximise immunity
    • Last dose <two months: no booster required 
  • Unvaccinated horses: start primary course – this will offer partial protection from two weeks after second dose

This will depend on the biosecurity policy of the yard and whether the horse has also been vaccinated against other equine infectious diseases like equine influenza and equine herpes disease. With respect to strangles, the risk will be significantly reduced. 

This will depend on the type of vaccine that has been used. If a vaccine with DIVA capability has been used, then the horse will not test positive, unless it has been infected with Strep. equi previously. 

It is generally recommended not to exercise horses in the two to three days following vaccination. This also depends on whether the horse shows a rise in body temperature as a reaction to vaccination or not. Temporary fever after vaccination is not uncommon as this is part of the immune system activation.

How to confirm strangles by diagnostic testing

Culture test, PCR and ELISA

Clinical samples (aspirates from abscesses, nasal swabs, nasopharyngeal swabs/washes or guttural pouch washes) are spread onto nutrient plates to grow live Strep. equi bacteria over 24 hours. Bacterial colonies are then grown in nutrient media containing lactose, sorbitol or trehalose. Strep. equi will not ferment any of these sugars (media does not turn acidic and pH indicator remains purple). Other bacteria, such as Strep. zooepidemicus will ferment lactose and/or sorbitol, turning the media yellow.

Pros

  • Demonstrates the presence of viable Strep. equi bacteria

Cons

  • Takes several days
  • Poor sensitivity (60%)
  • Confounded by contamination
  • Requires shedding of Strep. equi
  • A negative fermentation result (i.e. no acidification of sugar media) = a positive result for Strep. equi. Therefore, contamination with any other bacteria is a problem for this test.
  • Need to recover Strep. equi and separate it from contaminating bacteria that are present in the sample taken from the horse

Uses

  • Confirm strangles using aspirates from abscessed lymph nodes

Clinical samples as listed above are centrifuged to recover a cell pellet. DNA is purified from the cell pellet and PCR is used to detect one or more genes that are specific to Strep. equi

Pros

  • Rapid (same-day results – test can be completed within one hour) 
  • >90% sensitive 
  • >95% specific = gold standard test 

Cons

  • Contamination of equipment (e.g. endoscopes) can lead to false positives 
  • Requires shedding of Strep. equi 

Uses

  • Confirmation of strangles for acute and persistent cases

Blood sample is allowed to clot and the serum is removed. Serum is diluted 1:800 and added to test plates. Antibodies against the two test proteins from Strep. equi bind to the test plates and are detected using an anti-horse antibody to give a yellow colour in the test. The higher the intensity of yellow, the more antibodies against the two Strep. equi test proteins that were present.  Test results of 0.5 or above indicate a positive test.

Pros

  • Convenient sampling (1 ml blood sera is sufficient) 
  • Rapid (same-day results) 
  • >90% sensitive for recent exposure 
  • >95% specific to strangles 

Cons

  • Does not detect all exposed horses (two weeks to seroconvert following exposure) 

Uses

  • Screening tool to identify horses requiring further investigation

How to deal with an outbreak situation and how to treat strangles

  • Take immediate action!  
  • Stop all horses moving on or off the yard  
  • Use isolation as a precaution for all horses while you speak to your vet and arrange for testing 
  • Use the ‘traffic light’ system to segregate horses into groups and minimise the risk to other horses on the yard and the surrounding area 
  • Spot the clinical signs of strangles and take each horse’s temperature at least twice a day  
  • Move horses with signs into the ‘red’ group to minimise risk of transmission to other horses 
  • After an outbreak, make sure to disinfect stables, paddocks and equipment to make them free of infection 
  • Look for carriers and treat them to resolve persistent infection and prevent new outbreaks 

Treatment of acute cases  

Most infected horses will recover from strangles without treatment, but the recovery period is relatively long and usually takes three to six weeks if no complications occur. 

Early treatment of exposed horses revolves around good nursing care to alleviate clinical signs and may include NSAIDs.

Antibiotics (penicillin) may prevent abscess formation but should only be considered very early in the disease process, if at all.

However: 

  • Lymph node abscesses quickly become so large that antibiotics cannot penetrate to sufficient levels 
  • Treatment may be required for several weeks and the infection could still flare up again when treatment stops, extending the recovery period 
  • Antibiotic resistance is emerging (12.5% of UK isolates in one recent study)7 
  • Antibiotic treatment may interfere with natural immunity8 

Severe cases require emergency treatment with antibiotics, corticosteroids and/or surgical intervention (tracheostomy).

Treatment of carriers 

  • Physical removal of chondroids from the guttural pouches 
  • Procaine benzylpenicillin administered into guttural pouches 
  • Systemic antibiotics for two weeks may be warranted 
  • Repeat endoscopy and lavage to confirm infection-free status 

Keen to learn more about strangles and vaccination?

Please visit Dechra Academy for a series of interesting presentations on the subjects.

Strangles’ diagnosis, control and vaccination by Andrew Waller;

Conservation of antigen sequences across a global population of Strep. equi by Sara Frosth

Functional activities of antibody responses following vaccination of ponies with a multicomponent subunit vaccine against strangles by Francesco Righetti.

Click here to view the comprehensive module with all three presentations

Additional information

Additional information about strangles disease, treating the respiratory infection, outbreak management and disease control, can be found on:

  1. Pusterla et al. Surveillance programme for important equine infectious respiratory pathogens in the USA. Vet Rec 2011;169:12. 

  2. Defra quarterly disease surveillance reports. 

  3. Boyle et al. Streptococcus equi infections in the horse: guidelines for treatment, control, and prevention of strangles – Revised Consensus statement. J Vet Intern Med 2018;32:633-647. 

  4. Mitchell et al. Globetrotting strangles: the unbridled national and international transmission of Streptococcus equi between horses. Microb Genom 2021;7:1-14. 

  5. Newton et al. Detection and treatment of asymptomatic carriers of Streptococcus equi following strangles outbreaks in the UK. Equine Infectious Diseases VIII: Proceedings of the Eighth International Conference, Dubai, March 1998 

  6. Newton et al. Control of strangles outbreaks by isolation of guttural pouch carriers identified using PCR and culture of Streptococcus equi. Equine Vet J 2000; 32:515–526. 

  7. Fonseca et al. Antibiotic resistance in bacteria associated with equine respiratory disease in the United Kingdom. Vet Rec 2020;187:189. 

  8. Pringle et al. Influence of penicillin treatment of horses with strangles on seropositivity to streptococcus equi ssp. Equi – specific antibodies. J Vet Intern Med 2020;34:294-299. 

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