Enlarged submandibular lymph nodes in a horse with Strangles.

Strangles in horses is an upper respiratory infection caused by the highly infectious and contagious bacteria Streptococcus equi. Early clinical signs include fever, depression, and decreased appetite. Signs typically progress to include pharyngitis (sore throat), swollen lymph nodes that are firm and painful to palpation, and nasal discharge that may be clear initially but progresses to mucopurulent. Swollen lymph nodes often abscess and rupture especially in younger naïve horses whose clinical signs may be more severe than in older horses. The most commonly affected lymph nodes include the submandibular and the retropharyngeal. Horses of all ages are susceptible but clinical disease is most common in horses less than 5 years of age. In some cases the lymph node swelling is so severe that dysphagia (difficulty swallowing) and difficulty breathing may occur which is how the disease came to be known as Strangles.

Transmission is by the nasal or oral route and can occur directly by horse-to-horse contact or indirectly by the sharing of contaminated housing, feed and water buckets, tack, clothing and by fomites such as personnel and other animals like the farm dog or barn cat.

Endoscopic view of the guttural pouch showing retropharyngeal lymph node swelling and draining abscess in a horse with Strangles.

Diagnosis is based on history of possible exposure, clinical signs, and laboratory testing. Bloodwork abnormalities are nonspecific but may include increased neutrophil count, high fibrinogen, and mild anemia. Strep equi is not a normal inhabitant of the upper respiratory system so a definitive diagnosis can be made by identifying the organism on culture and qPCR testing by needle aspirate of an enlarged or abscessed lymph node, by saline lavage of the nasopharynx and/or guttural pouch, or by sampling nasal discharge. The bacteria rapidly invades the lymph nodes of infected horses so tests may be negative on nasal swabs or lavages in the early stages of disease.

It can take a few days to a few weeks to develop clinical signs after exposure. Nasal shedding of Strep equi usually begins 2-3 days after onset of fever and can persist for 2-6 weeks or longer. Supportive therapy with stall rest and feeding soft, moist food may be all the treatment required while the disease runs its course. Anti-inflammatory medications may be recommended for horses with more substantial clinical signs to reduce fever, pain, and swelling. Treatment with antibiotics is controversial but if disease is severe penicillin is the antibiotic of choice. Acutely affected horses with high fever prior to abscess formation, horses with severely enlarged lymph nodes and respiratory distress, and horses with guttural pouch infections being treated locally and systemically to eliminate the carrier state are some indications for antibiotic therapy. However, treatment with antibiotics may delay the maturation of abscesses or cause a recurrence of abscesses after antibiotics are discontinued and their use may reduce the development of immunity that occurs with recovery from natural infection leaving horses susceptible to reinfection.

Submandibular lymph node abscess in a horse with Strangles.

Due to the highly infectious nature of the organism, good hygiene and biosecurity are critical to containing an outbreak and preventing further spread of infection. This can be difficult at a farm where there is frequent movement of horses especially during show season. Any new arrivals to the farm should be isolated for at least 3 weeks. During an outbreak it is important to separate horses with clinical signs from those that are not sick preferably into separate barns and pastures to prevent horse-to-horse contact between groups. These horses’ need separate grooming supplies, stall cleaning supplies and feed and water sources to prevent cross contamination. It is necessary to adequately disinfect all potentially contaminated facilities and equipment. Fortunately, Strep equi is relatively susceptible to disinfection including with household bleach as long as organic material has been cleaned first. Exposure to direct sunlight has been shown to be beneficial, as cultured S. equi was shown to survive less than 24 hours on wood, rubber and metal surfaces when exposed to direct sunlight. Pastures containing infected horses should be rested for several weeks after animals are removed to allow for denaturation of S. equi through the effects of drying and direct sunlight.

Complications associated with Strangles are possible and have been reported to be as high as 20% in one study. Infection of the sinuses and guttural pouches (empyema or chondroids) may complicate treatment and lead to carrier states. “Bastard Strangles” or the development of infection at other anatomic sites including the lungs, mesentery, liver, spleen, kidneys and brain is possible. Potential immune-mediated complications consist of purpura hemorrhagica which is vasculitis caused by immune complex deposits, myositis or inflammation of the skeletal muscle, and myocarditis, an inflammatory disease of the heart.

Most horses that recover from natural infection develop strong immunity which may persist for 5 years or longer. A live, intranasal vaccine is available for horses but should only be administered to horses that are healthy without fever and nasal discharge. The initial series is 2 doses given 2-3 weeks apart and annually thereafter. The vaccine should not be administered to horses during an outbreak except to horses with no known contact with infected or exposed animals. Prior to vaccination, it is recommended to check Strep equi M protein ELISA (SeM ELISA) levels. Horses with titers of 1:3200 or greater should not be vaccinated due to increased risk of purpura hemorrhagica.

The information presented is a summary of the most current knowledge about S. equi that is available. Please contact your veterinarian or one of the veterinarians at Atlantic Equine Services if you have questions or would like more information.

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