Strangles is a highly contagious upper respiratory infection of horses caused by the bacterium Streptococcus equi subspecies equi. It is one of the most common infectious diseases of horses worldwide and is characterized by abscessation of the lymph nodes of the head and neck, particularly the submandibular and retropharyngeal lymph nodes. The disease gets its name from the swollen lymph nodes that can compress the airway, though true airway obstruction is uncommon. Strangles outbreaks can shut down barns and competition venues, making biosecurity and prompt identification critical.

Overview

Streptococcus equi is an obligate pathogen of horses that has evolved sophisticated mechanisms to evade the equine immune system. The bacteria are spread through direct contact with infected horses or their nasal discharge, as well as through contaminated equipment, water sources, hands, and clothing. After exposure, bacteria colonize the tonsillar tissue and lymphoid tissue of the pharynx, then spread to regional lymph nodes where they cause abscess formation. The incubation period is typically 3-14 days.

A key epidemiological concern is the carrier state: approximately 10% of horses that recover from strangles become persistent carriers, harboring the bacteria in the guttural pouches (air-filled pouches unique to horses) for months or even years. These carriers appear healthy but intermittently shed bacteria, seeding new outbreaks.

Causes & Risk Factors

  • Direct contact with infected horses or their nasal/abscess discharge
  • Fomite transmission — Shared water troughs, buckets, twitches, halters, grooming tools, and human hands/clothing
  • Young horses (under 5 years) are most susceptible, though horses of any age can be affected
  • Overcrowding and commingling of horses from different sources
  • Stress — Transport, competition, weaning
  • Immunocompromised or naive horses — Those without prior exposure or vaccination
  • Introduction of new horses without quarantine

Symptoms

  • Fever — Often the first sign (102.5-106 degrees F / 39.2-41.1 degrees C), frequently biphasic
  • Depression and lethargy
  • Loss of appetite (may be painful to swallow)
  • Nasal discharge — Initially serous, becoming thick, mucopurulent, and copious
  • Swollen, painful lymph nodes — Submandibular and retropharyngeal lymph nodes become enlarged, firm, and eventually fluctuant as abscesses mature
  • Extended head and neck posture (due to pharyngeal pain)
  • Difficulty swallowing (dysphagia)
  • Abscess rupture — Draining thick, creamy pus, often from the submandibular area
  • Cough
  • Labored breathing (if retropharyngeal abscesses compress the airway)

Most horses recover uneventfully after abscess drainage. However, complications including “bastard strangles” (metastatic abscessation in distant organs), purpura hemorrhagica (immune-mediated vasculitis), and guttural pouch empyema require prompt veterinary intervention.

Diagnosis

  • Clinical signs and history — Typical presentation in a horse with known exposure or during an outbreak
  • Nasal or abscess swab culture — Culture of Streptococcus equi from nasal discharge or abscess aspirate
  • PCR testing — More sensitive than culture for detecting S. equi DNA from nasopharyngeal washes or guttural pouch lavage
  • Guttural pouch endoscopy and lavage — Essential for identifying carriers. Allows visual inspection for chondroids (inspissated pus) and collection of samples for culture and PCR
  • Serology (SeM antibody, Streptolysin S) — Can help determine exposure history and immune status but does not distinguish current from past infection
  • Blood tests — Elevated white blood cell count (leukocytosis) and increased fibrinogen

Treatment & Medications

Treatment strategy depends on the stage of disease:

Uncomplicated Strangles (Standard Approach)

  • Supportive care — Rest, soft or soaked feed, NSAIDs for fever and pain
  • Hot packing or poulticing of abscesses — To encourage maturation and rupture, which hastens recovery
  • Abscess drainage — Veterinary lance and drainage of mature, fluctuant abscesses
  • Anti-inflammatory therapy — Flunixin meglumine (Banamine) or phenylbutazone for fever control and pain
  • Antibiotics are CONTROVERSIAL in uncomplicated strangles — Antibiotic therapy can delay abscess maturation and may increase the risk of developing the carrier state or complications. Most veterinarians do NOT recommend antibiotics for routine strangles cases.

When Antibiotics ARE Indicated

  • Horses in the very early febrile stage (before abscess development) may be treated with penicillin to abort the infection
  • Horses with complications: bastard strangles, severe dysphagia, airway compromise
  • Penicillin G (Procaine penicillin, IM) is the first-line antibiotic; S. equi is consistently susceptible
  • Trimethoprim-sulfa (oral) is an alternative for long-term treatment of complications

Complications Requiring Specific Treatment

  • Guttural pouch empyema — Lavage of the guttural pouches to remove inspissated pus and chondroids; may require endoscopic-guided procedures
  • Bastard strangles — Metastatic abscesses in the abdomen, lungs, brain, or other sites. Requires prolonged antibiotic therapy and carries a guarded prognosis.
  • Purpura hemorrhagica — An immune-mediated vasculitis causing limb edema, petechial hemorrhages, and skin necrosis. Treated with corticosteroids and antibiotics.

Prognosis

  • Uncomplicated strangles carries an excellent prognosis; most horses recover within 3-4 weeks after abscess drainage
  • Approximately 75% of recovered horses develop strong immunity lasting 5+ years
  • Approximately 10% of recovered horses become silent carriers
  • Complications (bastard strangles, purpura hemorrhagica) carry a more guarded prognosis
  • Mortality from uncomplicated strangles is low (1-2%) but rises with complications

Frequently Asked Questions

Should new horses be quarantined before joining my barn? Yes. A minimum 2-3 week quarantine with temperature monitoring is recommended. Ideally, new horses should be tested (nasopharyngeal wash or guttural pouch lavage for PCR) before introduction. This is the single most effective measure to prevent strangles outbreaks.

Can my horse get strangles even if vaccinated? Yes. Vaccines reduce severity and shedding but do not completely prevent infection. Both intranasal and intramuscular vaccines are available. Intranasal vaccines provide stronger mucosal immunity but carry a small risk of abscessation at the site if inadvertently injected.

How long is a horse with strangles contagious? Horses are contagious from the onset of nasal discharge until 3-6 weeks after clinical signs resolve. Three consecutive negative nasopharyngeal PCR tests, each one week apart, are recommended before ending isolation.

How do I disinfect my barn after a strangles outbreak? S. equi can survive in the environment for weeks, especially in water and organic material. Clean and disinfect all surfaces, buckets, and equipment with a phenol-based or accelerated hydrogen peroxide disinfectant. Allow stalls to dry thoroughly before reuse.

This information is for educational purposes only and does not replace professional veterinary advice. Strangles is a reportable disease in some jurisdictions. Contact your equine veterinarian immediately if you suspect strangles in your horse.