Strangles

by Lorie A. Valentino, DVM, MS, Diplomate American College of Veterinary Internal Medicine

New York Equine Hospital

Strangles is a disease of horses that is caused by infection with Streptococcus equi bacteria.  The disease was first described in the veterinary literature in 1251.  It was given the name strangles because sometimes horses were suffocated by enlarged lymph nodes that obstructed the airway. 

Strangles is characterized by an abrupt onset of fever (103ºF or higher) followed by nasal discharge and swelling with subsequent abscess formation in the sub-mandibular and retropharyngeal lymph nodes.  The severity of clinical signs depends on the immune status of the animal affected.  Younger horses are more likely to develop severe disease while older horses often have a milder form.  Horses with strangles are often depressed and listless.  Inflammation of the throat can cause difficulty swallowing.  Horses may become reluctant to eat and often stand with the neck extended. 

Nasal discharge is initially watery but rapidly progresses to a thick, profuse mucus consistency.  Accumulation of mucus in the upper airway can cause a rattling noise.  Submandibular and retropharyngeal lymph nodes become swollen and painful about one week after infection.  Serum may ooze from the overlying skin for several days as the lymph node abscesses mature before rupturing to drain creamy pus.  Retropharyngeal lymph nodes may open and drain into the guttural pouch.  Other lymph nodes of the neck can be involved and abscessate.  Lymph node abscessation may result in obstruction of the upper respiratory tract.  Infection with Streptococcus equi is diagnosed by culturing nasal swabs, nasal washes, or pus from abscesses. 

Although strangles usually involves the upper airway, involvement of other locations occasionally occurs.  The bacteria may spread through the bloodstream or lymphatics to form abscesses in the lymph nodes and other organs of the chest and abdomen or even the brain.  This form of the disease is known as “bastard strangles”.  The diagnosis and treatment of strangles infections that have spread are potentially more complicated than in cases of uncomplicated strangles.  The specific means of diagnosis vary depending on the site of infection and whether there are concurrent signs of classic strangles.

In rare cases, horses can develop an immune-mediated disease called purpura hemorrhagica following infection with or vaccination from strangles.  Purpura hemorrhagica is an immune-mediated vasculitis caused by deposition of immune complexes in the blood vessel walls.  This causes edema and hemorrhage of the head, limbs, and/or trunk.  The severity of clinical signs varies from a mild, transient reaction to a severe, fatal disease.  Severe edema may result in oozing from the skin surfaces and sloughing of the skin may occur.  Corticosteroids are the primary treatment for purpura.

Nasal discharge from horses with active disease or recovering from strangles is an important source of new strangles infections in susceptible horses.  Nasal shedding of Streptococcus equi bacteria begins 2 to 3 days after the onset of fever and lasts for 2 to 3 weeks in most horses.  Strangles may originate from outwardly healthy horses that are incubating the disease and go on to develop the disease.  Some recovering horses may shed the bacteria for a long period of time, even years, if the infection persists in the gutteral pouch.  Direct transfer can occur from horse-to-horse contact or indirect transfer can occur from sharing contaminated housing, water sources, tack, or from the clothing of people. 

Approximately 75% of horses develop an enduring immunity (5 years or longer) to strangles after recovery from the disease.  Injectable vaccination for strangles is marginally effective in protecting against disease (only about 50% a few weeks after the final booster) and adverse reactions such as soreness, abscesses at the injection site, and occasionally immune mediated reactions can occur.  Intranasal vaccination with a modified live form of the bacteria stimulates a high level of immunity but also has safety issues.  Because the vaccine contains modified live bacteria, abscesses, nasal discharge, and immune mediated problems can develop.         

The majority of strangles cases require no treatment other than proper rest and a dry, warm stall with soft, moist, and palatable food of good quality while letting the disease run it’s course.  During an outbreak, immediate antibiotic therapy of new cases with fever and depression may prevent lymph node abscessation.  However, antibiotic treatment will inhibit the development of immunity to strangles so horses will be highly susceptible to reinfection once treatment is discontinued.  Once lymph nodes become enlarged in otherwise healthy horses, antibiotic therapy will only prolong the eventual enlargement and rupture of lymph abscesses.  Therapy should be aimed at enhancing the maturation and draining of abscesses.  Anti-inflammatory medications such as phenylbutazone and flunixin meglumine may be used to reduce fever, pain, and inflammation.