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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Acute upper respiratory tract infection characterized by fever, lethargy, purulent rhinitis, and regional lymph node abscessation.

Pathophysiology!!navigator!!

  • Streptococcus equi ssp. equi is inhaled or ingested after direct contact with mucopurulent discharge from infected horses or contaminated equipment
  • Adheres to the epithelial cells of the buccal and nasal mucosa
  • Spreads to the regional lymph nodes, such as the submandibular, submaxillary, and retropharyngeal lymph nodes (with likely rupture into the GPs)
  • Fever occurs 3–14 days after exposure
  • Nasal shedding occurs 1–2 days after the onset of fever, persisting for at least 2–3 weeks
  • Asymptomatic carrier horses are responsible for maintaining the infection in affected herds and can shed from GPs for many years

Systems Affected!!navigator!!

  • Respiratory
  • Hemic/lymphatic/immune

Incidence/Prevalence!!navigator!!

Disease occurs sporadically on farms. Morbidity rates will depend on age (range 32–100%). Mortality rates are considered low in uncomplicated cases (<2%).

Geographic Distribution!!navigator!!

Occurs worldwide.

Signalment!!navigator!!

Any age group, 1–5 years are predisposed. No breed or sex predilection.

Signs!!navigator!!

  • Fever of >39.5°C (>103°F)
  • Depression and listlessness
  • Lymphadenopathy and abscessation of retropharyngeal and submandibular lymph nodes (rarely parotid and cranial cervical lymph nodes)
  • Bilateral mucopurulent nasal discharge
  • GP empyema
  • Respiratory stridor
  • Dysphagia, anorexia, cough, and neck extension
  • Ocular discharge

Causes!!navigator!!

S. equi ssp. equi, a Gram-positive coccus.

Risk Factors!!navigator!!

The immunologically naive, young equine population housed in highly concentrated and transient populations.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

For Nasal Discharge

  • Influenza
  • EHV-1 and EHV-4
  • Equine rhinitis virus
  • Adenovirus
  • Reovirus
  • EHV-2
  • Pharyngitis
  • Chronic pharyngeal lymphoid hyperplasia
  • Nasal/paranasal sinus infection/cysts/polyps/tumors
  • Early bacterial pneumonia/pleuritis
  • GP infection/mycosis
  • Overflow of nasolacrimal ducts
  • Severe equine asthma (heaves)

For Fever

Any disease that causes inflammation.

For Lymphadenopathy and Abscessation

  • Lymphoma
  • Upper respiratory tract infection
  • Corynebacterium pseudotuberculosis lymphadenitis
  • Bacterial endocarditis
  • Ulcerative/epizootic/sporadic lymphadenitis
  • Glanders
  • Plasma cell myeloma
  • Tuberculosis
  • Hemolytic/uremic-like syndrome

CBC/Biochemistry/Urinalysis!!navigator!!

  • Hyperfibrinogenemia, leukocytosis characterized by a neutrophilia, and possibly anemia
  • Serum biochemistry and urinalysis abnormalities may indicate complications

Other Laboratory Tests!!navigator!!

  • Samples—obtain from mature abscess aspirates, nasopharyngeal washes (have been shown to be higher yield than swabs), and GP washes. Rostral nasal swabs are not recommended
  • Testing:
    • Cytology—Gram-positive extracellular cocci in long chains support suspicion
    • Culture—useful in animals with active disease, but sensitivity is very low in animals with low numbers of bacteria (early in disease, convalescent horses, and long-term carriers)
    • PCR—fast (2 h) and high sensitivity with ability to detect small numbers of bacteria
    • Serology—(1) SeM ELISA: may detect recent but not current infection, need for vaccination (do not vaccinate if ≥1:3200 owing to risk of PH), and support clinical diagnosis of S. equi PH and metastatic abscessation; (2) combined SeM and SEQ2190 serology (currently available in Europe) can help detect infection as recent as 2 weeks and used to screen new horses for further testing via GP endoscopy and PCR

Imaging!!navigator!!

  • GP/pharyngeal endoscopy—determine the severity of upper airway obstruction and the presence of GP empyema/chondroids in both active cases and asymptomatic carriers
  • GP radiographs—presence of chondroids
  • Abdominal ultrasonography or per rectum—detection of intra-abdominal abscessation

Pathologic Findings!!navigator!!

Hyperplastic lymph nodes—increased numbers of neutrophils, monocytes, and macrophages with Gram-positive cocci. Nasal lesions—edematous, hyperemic, and occasionally ulcerated mucosa with a variable amount of creamy yellow exudate. Complicated strangles—the pathologic findings are variable, depending on the organ system involved.

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

To control transmission of S. equi and to eliminate infection while providing future, effective immunity to the disease.

Appropriate Health Care!!navigator!!

  • Acute phase of the fever and depression—supportive care. Hot packing or topical treatment with 20% ichthammol to encourage maturation of the abscess and drainage, followed by flushing with 3–5% povidone–iodine solution once opened. Judicious use of nonsteroidal drugs can decrease swelling and promote eating. Treatment with antibiotics may prevent the formation of abscess. This practice is considered controversial
  • Horses with complications benefit from systemic antibiotic therapy (22 000–44 000 IU/kg IM every 12 h of procaine penicillin or IV every 6 h of aqueous potassium penicillin) for 7–10 days
  • If nasal discharge persists >2 weeks, GP examination is indicated to identify horses that may have empyema and require additional treatment
  • Horses with GP empyema/chondroids require copious lavage with or without 20% acetylcysteine solution and systemic antibiotics. Local infusion of antibiotics once gross contamination is removed

Nursing Care!!navigator!!

Minimal unless respiratory obstruction or complications occur.

Activity!!navigator!!

Horses and stables should be quarantined until there are no clinical signs and cases and in-contacts have been tested for carrier status.

Diet!!navigator!!

Soft, moist, palatable food.

Client Education!!navigator!!

Segregation and preventing cross-contamination. Stables that housed infected animals should be rested for 2 weeks after cleaning and disinfecting.

Surgical Considerations!!navigator!!

Tracheostomy for horses in severe respiratory distress. Surgical removal of chondroids from the GP is rarely necessary.

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

See Appropriate Health Care.

Alternative Drugs!!navigator!!

Chloramphenicol; ceftiofur; treatment failures have been observed with trimethoprim–sulfa combinations.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Clinical and in-contact horses should be tested for carrier status no sooner than 3 weeks after resolution of clinical signs or potential exposure via GP endoscopy and PCR
  • Wait at least 3 weeks from cessation of antibiotic treatment prior to testing
  • Endoscopically guided GP lavage PCR to screen for carriers in cases and their contacts provides increased efficiency and sensitivity over 3 nasopharyngeal washes. Disinfect equipment between horses
  • Continual positive tests despite endoscopically normal GP should be considered infections. Consider treatment with systemic antibiotics and sinus radiography

Prevention/Avoidance!!navigator!!

  • Isolation of new horses for 3 weeks, with close observation for signs of strangles or any disease. Monitor temperature twice daily
  • Persistent carriers of S. equi with the GP are a potential source of new infections on a farm. Negative GP PCR prior to entering the resident population
  • If not treated with antibiotics, approximately 75% of horses develop a waning convalescent immunity to strangles as a result of individual immune response as well as natural exposure to disease over time contributing to reboosting and herd immunity

During an Outbreak

  • Affected or suspect horses should be quarantined immediately
  • Make 3 groups—affected, in-contacts, and healthy with designated equipment and caretakers
  • Observe all horses closely for signs of disease and monitor temperatures twice daily. Move febrile horses to affected group as soon as identified
  • Clean and disinfect—appropriate disinfectants include phenols, iodophors, and chlorhexidine compounds
  • Exposure to air is essential for disinfection

Vaccination

Does not guarantee prevention. Currently, the following systemic vaccines are available:

  • Strepvax II (concentrated M-protein extract of S. equi)—IM vaccine; adverse reactions include soreness or abscesses at injection sites and occasional cases of PH
  • Pinnacle IN—intranasal vaccine contains an attenuated live strain of S. equi that is antigenic but has low pathogenicity. Live vaccine should be administered only to healthy animals >1 year of age with no known exposure to disease
  • Equilis StrepE—intermittently available in Europe for administration submucosally on the inside of the upper lip. Immunity to experimental challenge persists for about 3 months. Painful reaction at injection site can occur and veterinarians have accidentally injected themselves
  • Strangvac—a multicomponent vaccine in development in Sweden which has differentiation of infected from vaccinated animals capability

Possible Complications!!navigator!!

  • Reported in about 20% of cases
  • Bastard strangles—S. equi metastasize to other lymph nodes or body systems (lungs, mesentery, liver, spleen, kidney, and brain); low occurrence 2–10%
  • Upper respiratory tract obstruction from retropharyngeal lymph node abscessation, suppurative necrotic bronchopneumonia
  • Myocarditis and myositis
  • PH—aseptic vasculitis reported in mature horses after second natural exposure to infection or vaccination of animals that previously had strangles. Clinical signs—mild to life-threatening. Typical signs—pitting edema of dependent areas of the head, trunk, and extremities; petechiation and ecchymoses of mucous membranes. Therapy—antimicrobials, corticosteroids, and supportive care
  • Septicemia and the development of infectious arthritis, pneumonia, and encephalitis

Expected Course and Prognosis!!navigator!!

Prognosis is good for full recovery in cases of uncomplicated strangles. The course of the disease depends on the phase of the infection.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Age-Related Factors!!navigator!!

Horses between the ages of 1 and 5 years are immunologically naive are most prone to developing the disease. Older horses may develop a mild form of strangles owing to previous exposure.

Zoonotic Potential!!navigator!!

Cases in debilitated humans and a dog have been reported.

Pregnancy/Fertility/Breeding!!navigator!!

Avoid infection in a pregnant mare. Suckling foals benefit from the protective effects of IgGb and IgA in milk from mares that recovered from strangles or were vaccinated IM.

Synonyms!!navigator!!

  • Distemper
  • Strangles

Abbreviations!!navigator!!

  • EHV = equine herpesvirus
  • ELISA = enzyme-linked immunosorbent assay
  • GP = guttural pouch
  • Ig = immunoglobulin
  • PCR = polymerase chain reaction
  • PH = purpura haemorrhagica
  • SeM = S. equi M protein

Suggested Reading ⬆ ⬇

Boyle AG. Strangles and its complications. Equine Vet Educ 2016;29:149–157.

Boyle AG, Stefanovski D, Rankin SC. Determining optimal sampling site for Streptococcus equi subsp equi carriers using loop-mediated isothermal amplification. BMC Vet Res 2017;13:75.

Boyle AG, Timoney JF, Newton JR, et al. Streptococcus equi Infections in Horses: Guidelines for Treatment, Control and Prevention of Strangles-Revised Consensus Statement. J Vet Intern Med. 2018;32:633–647.

Waller AS. New perspectives for the diagnosis, control, treatment, and prevention of strangles in horses. Vet Clin North Am Equine Pract 2014;30:591–607.

Durham AE, Hall YS, Kulp L, et al. A study of the environmental survival of Streptococcus equi subspecies equi. 2018;50:861–864.

Author(s) ⬆

Author: Ashley G. Boyle

Consulting Editor: Ashley G. Boyle