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Strangles (Streptococcus equi infection)


Synonym(s): Streptococcus equi subspecies equi infection, S. equi infection


  • CauseStreptoccocus equi Streptococcus spp subspecies equi, a beta-hemolytic streptococcus.
  • Signs: acute onset depression, pyrexia, inappetence, nasal discharge, pain and swelling in the pharyngeal area, ruptured/discharging abscesses.
  • Diagnosis: nasopharyngeal swab; isolation and identification of Streptococcus equi subspecies equi, polymerase chain reaction, serology.
  • Treatment: isolation, intensive nursing and early use of antibiotics, especially high-dose penicillin, in most cases. Antibiotic use in more established cases is controversial.
  • Prognosis: most horses have a full recovery and are no longer infectious 6 weeks after apparent recovery and disappearance of symptoms. Around 10% of infected animals become carriers of the bacteria within their guttural pouches. Serious complications/sequelae ('bastard strangles', purpura hemorrhagica Purpura hemorrhagica and guttural pouch empyema Guttural pouch: empyema) are rare but in some outbreaks may occur in up to 20% of cases.
Print off the Owner factsheet on Strangles (Streptococcus equi infection) to give to your clients.



  • Streptococcus equi subspecies equi Streptococcus spp.
  • Lancefield group C (beta-hemolytic).
  • Hyaluronic acid capsule variation in pathogenicity. There is a variant of the organism that lacks as much hyaluronic acid in its capsule so it appears as a matt colony and is less pathogenic than the classic organism. This has been used in experimental vaccines.
  • Cell wall M-protein (SeM) antiphagocytic. This acts with hyaluronic acid to inhibit phagocytosis.
  • SeM is an important immunogen and virulence factor.
  • Pyrogenic mitogens.

Predisposing factors


  • New individual introduced to a pre-existing group.
  • Young horses but all age of horses can be infected..
  • Herding horses together in close proximity, eg stud farm, racing stables, riding school.
  • Poor condition due to inadequate feeding, teeth problems.
  • Concurrent parasitic infestation.
  • Poor general management.
  • General stress factors.


  • Inhalation of the bacteria → adhesion to the upper respiratory tract epithelium → colonization of pharyngeal mucosa and tonsils → translocates to the local lymphatics and colonizes the draining (head) lymph nodes. The bacteria resists phagocytosis by neutrophils → abscess formation.
  • Release of toxins and enzymes → severe inflammation.
  • Rarely see a bacteremia and involvement of multiple organs.
  • Organism evades the normal immune response (phagocytosis) by use of the hyaluronate and M-protein-containing capsule.
  • In the pharynx, toxins and enzymes → severe tissue damage, especially in the local lymph nodes → lymphadenitis and abscessation.
  • Toxins and enzyme release → pyrexia, severe dullness, depression and anorexia.
  • Bacteremia → organism disseminated throughout the body → 'bastard/metastatic' strangles.
  • Mortality: rare but can be up to 8-10% in individual outbreaks.
  • Morbidity: up to 100% in susceptible populations.


  • Incubation period: 2-14 days.
  • Elimination of infection: about 4 weeks after the end of clinical symptoms. Most horses eliminate the infection within a couple of weeks but some horses will shed the bacteria for up to 6 weeks after the end of clinical symptoms.
  • Around 10% of affected animals go on to carry the bacteria in their guttural pouches for months to years. Rupture of the retrophyrangeal lymph nodes into the guttural pouch causes empyaema and eventually chondroid formation and this is the likely source of these carriers.

Grossly normal guttural pouches can still have a biofilm of bacteria and the lack of chondroids or empyaema does not preclude the horse being a carrier.


  • Common in young horses, especially where groups of animals are herded together.
  • Highly contagious.
  • S. equi var. equi Streptococcus spp can survive for long periods either in the horse or in the environment. This survival is mainly dependent on the substrate and temperature.
  • Infection requires close contact with infected individuals or contaminated fomites, including flies.
  • Aerosol transmission is not important.
  • Around 10% of exposed horses become carriers and can intermittently shed S. equi for months or years.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • North S E et al (2014) Development of a real-time PCR to detect Streptococcus equi subspecies equiEquine Vet J 46 (1), 56-59 PubMed.
  • Pusterla N et al (2011) Surveillance programme for important equine infectious respiratory pathogens in the USA. Vet Rec 169 (1), 12 PubMed.
  • Pimenta E L M et al (2011) Comparative study between atropine and hyoscine-N-butylbromide for reversal of detomidine induced bradycardia in horses. Equine Vet J 43 (3), 332-340 PubMed.
  • Ijaz M et al (2010) Prevalence and serum protein values of strangles (Streptococcus equi) affected mules at Remount Depot, Saragodha (Pakistan). Equine Vet Educ 22 (4), 196-198 VetMedResource.
  • Sweeney C R et al (2009) Streptococcus equi infections in horses: guidelines for treatment, control and prevention of strangles. J Vet Inter Med 19 (1), 123-134 PubMed.
  • Waller A S & Jolley K A (2009) Getting a grip on strangles: Recent progress towards improved diagnostics and vaccines. Vet J 174 (3), 492-501 VetMedResource.
  • Whelchel D D, Arnold C E & Chaffin M K (2009) Subscapular lymph node abscessation as a result of metastatic Streptococcus equi subspecies equi infection: An atypical presentation of bastard strangles in a mare. Equine Vet Educ 21 (3), 131-134 VetMedResource.
  • Whelchel D D & Chaffin M K (2009) Sequelae and complications of Streptococcus equi subspecies equi infections in the horse. Equine Vet Educ 21 (3), 135-141 VetMedResource.
  • Albini S et al (2008) Mandibular lymphadenopathy caused by Actinomyces denticolens mimicking strangles in three horses. Vet Rec 162 (5), 158-159 PubMed.
  • Timoney J F & Kumar P (2008) Early pathogenesis of equine Streptococcus equi infection (strangles). Equine Vet J 40 (7), 637-642 VetMedResource.
  • Kemp-Symonds J, Kemble T & Waller A (2007) Modified live Streptococcus equi ('strangles') vaccination followed by clinically adverse reactions associated with bacterial replication. Equine Vet J 39 (3), 284-286 PubMed.
  • Ramey D (2007) Does early antibiotic use in horses with 'strangles' cause metastatic Streptococcus equi bacterial infections? Equine Vet Educ 19 (1), 14-15 VetMedResource.
  • Timoney J F (2007) Strangles vaccine in trouble again. Equine Vet J 39 (3), 196 PubMed.
  • Moller Gronbaek L et al (2006) Evaluation of a nested PCR test and bacterial culture of swabs from the nasal passages and from abscesses in relation to diagnosis of Streptococcus equi infection (strangles). Equine Vet J 38 (1), 59-63 PubMed.
  • Sponseller B T et al (2005) Severe acute rhabdomyolysis associated with Streptococcus equi infection in four horses. JAVMA 227 (11), 1800-1807 PubMed.
  • Brazil T (2005) Strangles in the horse: management and complications. In Pract 27 (7), 338-347 VetMedResource.
  • Fintl C et al (2000) Endoscopic and bacteriological findings in a chronic outbreak of strangles. Vet Rec 147, 480-484 PubMed.
  • Jacobs A A C et al (2000) Investigations towards an efficacious and safe strangles vaccine; submucosal vaccination with a live attenuated Streptococcus equiVet Rec 147, 563-567 PubMed.
  • Newton J R et al (2000) Control of strangles outbreaks by isolation of guttural pouch carriers identified using PCR and culture of Streptococcus equiEquine Vet J 32 (6), 515-525 PubMed.
  • Verheyen K et al (2000) Elimination of guttural pouch infection and inflammation in asymptomatic carriers of Streptococcus equiEquine Vet J 31 (6), 527-531 PubMed.
  • Rush B (1998) How do I control a strangles outbreak in my barn? Comp Cont Educ 20, 844-845 VetMedResource.
  • Newton J R, Wood J L N, Dunn K A, DeBrauwere M N & Chanter N (1997) Naturally occurring persistent and asymptomatic infection of the guttural pouches of horses with Streptococcus equiVet Rec 140, 84 PubMed.
  • Sweeney C R et al (1987) Complications associated with Streptococcus equi infection on a horse farm. JAVMA 191 (11), 1446-1448 VetMedResource.

Other sources of information

  • Horserace Betting Levy Board (2016) Codes of Practice. 5th Floor, 21 Bloomsbury Street, London WC1B 3HF, UK. Tel: +44 (0)207 333 0043; Fax: +44 (0)207 333 0041; Email:; Website:
  • Newton J R (1999) Strangles - diagnosis and treatment of sequlae. In: Proc 38th BEVA Congress. pp 120-121.
  • Rose R J & Hodgson D R (1993) Manual of Equine Practice. W B Saunders Company. pp 165-167. ISBN 0-7216-3739-6.
  • Hoffman A M (1988) Strangles. In: Proc World Equine Airway Symposium.

Related Images


Abdomen: peritonitis

Anesthesia: analgesia - overview

Behavior: headshaking


Blood: biochemistry - alkaline phosphatase (SAP, ALP)

Blood: biochemistry - aspartate amino transferase (AST)

Blood: biochemistry - creatine kinase

Blood: biochemistry - creatinine

Blood: biochemistry - total bilirubin

Blood: biochemistry - total protein

Blood: biochemistry - urea

Blood: leukocytes

Blood: neutrophils

Bone: osteochondrosis



Conjunctivitis: bacterial

Conjunctivitis: overview


CSF: cell count and differential

CSF: cytology


Esophagus: megaesophagus

Esophagus: obstruction

Gastrointestinal: nasogastric intubation


Guttural pouch: empyema

Guttural pouch: tympany

Hard/soft palate: cleft

Head: fractures

Head: radiography

International disease outbreaks


Joint: immune-mediated polysynovitis


Kidney: renal failure

Larynx: hemiplegia

Lung: abscess

Lung: pleuropneumonia - bacterial (pleuritis)

Lung: pneumonia - bacterial

Musculoskeletal: rectal palpation

Nose: nasal discharge

Parascaris equorum infestation

Penis: paralysis / priapism

Pharynx: nasopharyngeal swab

Plasma: fibrinogen

Polymerase chain reaction (PCR)

Post mortem examination


Purpura hemorrhagica

Respiratory: EHV infection

Respiratory: endoscopy

Respiratory: neoplasia

Rhodococcus equi infection


Streptococcus equi subspecies equi ELISA test

Streptococcus equi vaccine

Streptococcus spp

Sulfadoxine with Trimethoprim

Therapeutics: anti-inflammatory drugs

Therapeutics: antimicrobials

Therapeutics: immune system

Thorax: radiography

Toxicity: snakebite

Trachea: stenosis

Trachea: tracheostomy - permanent

Ultrasonography: overview

Uveitis: bacterial


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