Strangles

Strangles is an acute, highly contagious upper respiratory infection of horses caused by Streptococcus equi subsp. equi. It is one of the most common infectious diseases of horses worldwide.

Transmission: Direct contact with infected horses, contaminated fomites (tack, buckets, human hands), aerosolized purulent material from ruptured abscesses. Incubation 3–14 days.

Pathophysiology: Bacteria enter via nasal/oral mucosa → drain to retropharyngeal and submandibular lymph nodes → abscess formation → rupture (external or into guttural pouch/pharynx). Some horses become guttural pouch carriers with persistent infection.

Signalment: Young horses (weanlings, yearlings, 2–3 year-olds) most susceptible; any age can be affected. Stress (transport, weaning, surgery) increases susceptibility.

Clinical signs:

• Fever (39–41°C / 102–106°F)
• Bilateral mucopurulent nasal discharge
• Submandibular and retropharyngeal lymphadenopathy → abscessation
• Dysphagia, anorexia, depression
• Respiratory stridor (pharyngeal compression)

Bastard strangles (metastatic strangles):

• S. equi spreads hematogenously to distant lymph nodes and organs
• Abscesses in mesentery, lungs, brain, liver, kidney, spleen
• High mortality (~40%); do NOT attempt to lance internal abscesses
• Treatment: Prolonged antibiotics (penicillin) + drainage if accessible

Purpura hemorrhagica:

• Immune-mediated vasculitis; Type III hypersensitivity to S. equi M-protein
• Edema of limbs, head, ventral abdomen; petechiation; weight loss
• Treatment: Corticosteroids (dexamethasone) + penicillin

Guttural pouch empyema:

• Most common sequela; pus accumulates in guttural pouch (medial compartment)
• Can become chronic carrier state; chondroids may form
• Treatment: Lavage through endoscope; surgical drainage if refractory

Other: Laryngeal hemiplegia (arytenoid chondritis from abscess), myocarditis, anemia.

Diagnosis:

• Culture: Nasopharyngeal swab or wash; low sensitivity in early/subclinical cases
• PCR: Higher sensitivity; detects carrier states; can use guttural pouch lavage
• Serology: ELISA (SeM protein antibody); useful for screening exposed/asymptomatic horses; does not differentiate vaccination from infection

Treatment:

• Supportive: NSAIDs for fever/pain, soft/palatable feed, warm compresses to abscesses
• Antibiotics: Controversial in uncomplicated cases (may delay abscess maturation). Indicated in: bastard strangles, purpura hemorrhagica, severe dyspnea/dysphagia, very young/valuable horses. Penicillin (procaine penicillin 22,000 IU/kg IM q12h or potassium penicillin IV) is first-line.
• Abscess drainage: Allow maturation and spontaneous rupture; surgical lancing if mature and accessible externally

Biosecurity:

• Quarantine: Isolate confirmed/suspected cases for 4–6 weeks minimum
• New arrivals: Quarantine 2–3 weeks; test (PCR/culture) before introduction
• Carrier clearance: Three negative nasopharyngeal PCR/cultures 1–2 weeks apart + guttural pouch endoscopy
• Vaccination: Modified live intranasal vaccine (Pinnacle IN); killed IM vaccines available but less effective. Vaccinate 2–3 weeks before anticipated exposure.