Streptococcus equi subsp. equi

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Causes: Strangles


Strangles is highly contagious upper respiratory disease of equids worldwide. It is caused by the very pathogenic bacteria Streptococcus equi subsp. equi resulting in rhinitis, pharyngitis, lymphadenitis and myositis. The bacteria are haemolytic streptococci of Lancefield group C and are common inhabitants of the equine nasopharynx. They multiply extra-cellularly.

The disease often occurs where there are high numbers of young horses that are under stress, such as in racing yards. Most animals affected are under 6 years old though the disease is prevalent in all ages.

Transmission is via purulent exudate discharging from upper respiratory tract or from lymph nodes, which is then transmitted to the other horse by direct contact e.g. on stable doors, tack or feed buckets or by inhalation. A chronic carrier state of the disease can occur when bacteria are in the guttural pouch. The organism remains viable in the environment for months.

There will be 100% morbidity; 5% mortality in most cases as the disease is so contagious.

Clinical Signs

Classic Strangles

After infection, there is an incubation period 3 to 6 days followed by pyrexia, depression, anorexia, purulent bilateral nasal discharge, and swelling or abscessation of regional lymph nodes, especially the submandibular nodes. The lymph nodes may rupture. There may also be guttural pouch empyema.

Death can occur from pneumonia, breathing difficulties from swollen lymph nodes or purpura haemorrhagica (an immune-mediated disease).

S. equi may also be involved in cutaneous lesions.

Atypical Strangles

The clinical signs of atypical strangles are milder. There is only mild inflammation of the upper respiratory tract and minimal nasal discharge. A cough and pyrexia are still present but the lymphadenopathy is self-limiting. Whether this form of strangles occurs depends on the strain of bacteria, the existing immunity of the horse and genetics.

Bastard Strangles

This may occasionally occur, with abscessation in many organs of the body. It will be difficult to diagnose as clinical signs are systemic and variable, although history of having strangles in the normal form is presumptive.


History and clinical signs are presumptive. Deep nasopharyngeal swabs, a sample of abscess content or guttural pouch washings should be taken and sent off for culture. If present, you will see mucoid colonies with beta-haemolysis. Sugar fermentation allows differentiation of S. equi from S. zooepidemicus and S. equisimilis as the last two are commensals of the upper respiratory tract. PCR can be used to detect asymptomatic carriers.

To diagnose carrier animals, either three swabs from the nasopharynx taken one week apart or a single guttural pouch washing are needed. This picks up the majority of carrier animals, but not all of them.


In the case of an outbreak, penicillin should be administered to the affected animal and to in-contact animals. If abcesses are already present antibiotics should not be administered as this is shown to slow the recovery. Instead abcesses should be poulticed and drained. Additional supportive care includes feeding soft food, giving anti-inflammatories to reduce pyrexia and good nursing care.

It should be noted that horses treated with penicillin will not develop immunity to Strangles and are therefore susceptible to the disease if re-exposed.

Animals with abdominal abscesses (Bastard Strangles) require long term antibiotic therapy (up to 6 weeks). Horses presenting with purpura haemorrhagica have a guarded prognosis and require treatment with penicillin, immunosuppressive doses of steroids and analgesics.

Guttural pouch empyema is treated by lavage and/or surgical drainage.


Isolation of affected animals is required immediately. The yard needs to be shut and no animals are allowed in or out. Strict control needs to be enforced on all personnel in the yard and them made aware they are not to visit another horse or take their vehicle to other yards. Disinfection of all tack, stables, vehicles, boots and any other equipment that has come into contact with infected animals needs to be performed. Water troughs should be emptied and disinfected regularly to reduce the exposure of in-contact horses to the pathogens. Before horses are taken out of isolation, a negative culture from three consecutive swabs of the nasopharynx (each taken one week apart) or a single guttural pouch washing are needed to confirm freedom from disease.


Prevention methods include quarantine of all in-coming animals, ideally for 3-4 weeks. Infected horses should demonstrate some clinical signs over this time, and any suspicious horses should be tested. It is important to try to reduce stress on the yard or within groups of horses turned out together and it is necessary to avoid overcrowding and mixing different age groups.

Immunity occurs in animals that have been infected, however it is short-lived. A vaccine exists but it is currently not available in the UK due to fears of adverse reactions.

Streptococcus equi subsp. equi Learning Resources
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Test your knowledge using flashcard type questions
Equine Internal Medicine Q&A 06


Interview with Professors Josh Slater and Ken Smith providing an interesting insight into the pathogenesis, prevalence and possible prevention of Streptococcus equi infections in horses - listen to Strangles podcast

Brown, C.M, Bertone, J.J. (2002) The 5-Minute Veterinary Consult - Equine, Lippincott, Williams & Wilkin

Horserace Betting Levy Board (2011) Codes of Practise 2011 HBLB

Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students University of Liverpool

Mair, TS & Divers, TJ (1997) Self-Assessment Colour Review Equine Internal Medicine Manson Publishing Ltd

RVC staff (2009) Respiratory System RVC Intergrated BVetMed Course, Royal Veterinary College

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