Horse Strangles | Streptococcus equi subsp equi infection in horses
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Horse Strangles | Streptococcus equi subsp equi infection in horses

by Ashley G. Boyle, DVM, DACVIM (LAIM)
Apr 7, 2020

What causes Strangles?

Strangles is a highly contagious upper respiratory infection of horses caused by the bacteria Streptococcus equi subspecies equi (S. equi). It is transmitted by inhalation of respiratory particles or direct contact with contaminated surfaces (for example horses sharing water buckets). The bacteria settle in the horse’s tonsils and pharynx (back of the throat) within hours of infection, and then infect the lymph nodes under and behind the jaw, resulting in abscesses forming in these structures days later. Horses develop a fever initially, but are typically not contagious during the initial 48-72 hours. 

Rarely, infection spreads to other parts of the body resulting in abscesses in other organs such as the intestines, kidneys, lungs, spleen or liver. This is often called “bastard strangles” or metastatic abscessation. A few horses may develop a hypersensitivity reaction to the bacteria with repeated exposure, either in the form of infection or vaccination, otherwise known as purpura hemorrhagica. Horses that develop classic clinical signs and are not treated with antibiotics can develop immune protection from the disease for up to five years.

How did the disease get its name?

The disease is commonly called “Strangles” because of the airway restriction caused by the disease. The swollen, infected lymph nodes can press on the back of the throat so much that the horse can’t get enough air through its trachea (airway).

What does Strangles look like?

Classic clinical signs include a fever (often >103°F or >39.5°C) first, followed by one or more of the following symptoms: 

  • Lethargy/lack of interest in normal activities
  • Thick white to yellow nasal discharge
  • Swelling under the jaw and/or in the throat latch region (from the infected lymph nodes)
  • White to yellow drainage under the jaw 

The abscessed lymph nodes may drain externally or into the guttural pouches (blind-end sacs connected to the throat in horses) resulting in nasal discharge. Horses that have been vaccinated for strangles or horses that have previous partial immunity may develop milder signs of upper respiratory tract infection.

Signs of bastard strangles can include:

  • Recurrent colic (rolling, pawing, flank-watching)
  • Fever
  • Weight loss 
  • Horses may or may not have a history of previous strangles disease or exposure. 

Signs of purpura hemorrhagica can include:

  • Edema (swelling) of the head, trunk, and/or legs, or patchy swelling on the body
  • Broken blood vessels or bruising of the gums, around the eyes and inside of the nose
  • Fever
  • Muscle tightness/soreness or loss of muscle
  • Severity can be mild to life-threatening

How will my veterinarian diagnose this disease?

Drainage from lymph node abscesses, throat washes or guttural pouch washes may provide a diagnosis by bacterial culture within 48 hours. Polymerase chain reaction (PCR, which identifies S. equi bacterial DNA) can provide a diagnosis the same day depending on proximity to a laboratory that provides the service. Horses do not shed the bacteria for up to 72 hours from the onset of fever, so tests may not be conclusive early in the disease. 

An increased white blood cell count, combined with anemia (low red blood cell count) and high proteins (inflammatory markers) can be suggestive of a bacterial disease, but do not specifically indicate strangles.

How is the disease treated?

Treatment goals are to control spread of the contagious disease and eliminate infection while providing future immunity to the disease. 

Uncomplicated cases require supportive care such as:

  • Soft food
  • Shelter
  • Anti-inflammatories
  • Hot packing and drainage of abscesses
  • Depending on the degree and length of the fever and severity of symptoms, horses may need antibiotics such as penicillin. 

Horses with bastard strangles typically require an average of 2 months of antibiotics. Horses with purpura hemorrhagica usually require antibiotics and treatment with corticosteroids for days to weeks to quiet the overactive immune reaction.

The enlarged lymph nodes sometimes block the airway, requiring emergency tracheostomy (hole created in the trachea to allow breathing). When the lymph nodes drain into the guttural pouch, repeated lavage (flushing out) might be required with the aid of an endoscope (camera) or an indwelling catheter. Removal with endoscopic equipment or surgery might be necessary to remove dried pus (chondroids). Local treatment of the guttural pouch with a gelatin/penicillin antibiotic mixture can be performed after removal of the material within the guttural pouch.

Biosecurity on the farm is necessary to prevent spread of disease and should involve the following:

  • Isolate new horses for three weeks prior to introducing them to the rest of the population.
  • Isolate any horse with a fever and signs of strangles.
  • Do not share tack or equipment between sick horses and others.
  • Perform twice daily monitoring of rectal temperatures of all horses in an outbreak to identify new cases.
  • Stop all movement of horses to and from the farm when strangles is identified.
  • Disinfect water buckets daily.
  • Use strict hygiene between horses to reduce spread of the disease.

How do I know when my horse is no longer contagious?

Ideally, examination of the guttural pouch with an endoscope is performed and a guttural pouch flush sample is obtained from recovering horses and any horses who were in contact with sick horses  and tested for S. equi subsp equi by PCR and culture. Identification of strangles bacteria in clinically recovered horses may mean the guttural pouches have retained some infection. 

Endoscopy of the guttural pouches allows us to see any pus or dried debris (chondroids) that harbor the bacteria. A small number of horses will recover from strangles and continue to shed bacteria from the guttural pouch, causing recurrent farm outbreaks. Detection and treatment of these “silent carriers” (S. equi bacteria in guttural pouches) via endoscopy and PCR is essential for preventing disease recurrence on a farm.

Discuss vaccination options with your veterinarian. Vaccination does not provide 100% immunity against S. equi infection. Vaccination is NOT recommended during or within one year of a strangles outbreak due to the increased risk of purpura hemorrhagica.

What is the prognosis for strangles infection?

Classic upper respiratory infection strangles cases have a good to excellent prognosis with proper supportive care. Cases of bastard strangles and purpura hemorrhagica have a fair to good prognosis with appropriate antibiotics and antibiotic/corticosteroid treatment, respectively.

Edited by:
Freya Stein, DVM, DACVIM (LAIM)
April, 2020

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