Equine Strangles

Strangles is caused by bacterial infection with Streptococcus equi_subspecies equi (referred to as S. equi). The gram-positive bacteria typically infect the upper airway and lymph nodes of the head and neck. The disease has been in the equine population for centuries and was 1st reported in 1251. The infection is highly contagious in horse populations, particularly affecting young horses, and can recur on farms with previous outbreaks of the disease.  Approximately 30% of equine infections are caused by Streptococcus equi.  Morbidity may approach 100%, but the mortality rate ranges from 3% to 20%.
Though strangles is highly contagious and can affect many horses on a farm, most horses with infection recover without complication.  Complications from the infection include spread of the infection to lymph nodes other than the head and neck (also known as metastatic infection or bastard strangles), immune mediated disease (such as purpura hemorrhagica), muscle disease and pain, colic, diarrhea, pleuropneumonia, and lack of milk production. Horses that develop complicated infections typically require antibiotic and additional therapies based on veterinary examination.  These symptoms may develop several weeks after infection.
The hallmark clinical signs of infection are fever (temperature higher than 101.5 F [38.6 C]), nasal discharge, and enlarged submandibular lymph nodes (in the space between the lower jaw bones), which ultimately abscess. The retropharyngeal, parotid, and cervical lymph nodes may also be affected.  Purulent nasal discharge is typically present, although it may initially be clear, and it may be intermittent. The retropharyngeal lymph nodes (located deep behind the throatlatch) may also become enlarged and abscess. These will sometimes drain into the guttural pouches, which are air-filled spaces within the head that are an expansion of the Eustachian tubes. They may occasionally rupture to the outside in the lateral laryngeal region.  Guttural pouch infection and pus accumulation (empyema) are often the result of retropharyngeal lymph nodes that abscess and rupture into the guttural pouches. Guttural pouch infection may also occur from bacterial entrance through the pharynx (throat). Anorexia, depression, and difficulty swallowing may also accompany signs of infection.
Nasal discharge, which may persist for 2-4 weeks, from the infected horse is the largest source of contamination. Sources of infection can be nose to nose contact with an infected horse, sharing contaminated water buckets, feed tubs, twitches, tack, and clothing and equipment of handlers who work with infected horses.  The bacterium may persist in the environment for several weeks or months, depending upon the conditions.  It will survive better in purulent nasal secretions smeared on the walls or wood fences.  It will also survive in water buckets.  It is not likely to survive long in hot, dry conditions.
Carrier horses may shed the bacteria continuously or intermittently for months to several years.  They are the major reservoir for the bacterium, and account for outbreaks when new horses are introduced to the farm.  The bacterium may be carried in the guttural pouches or the paranasal sinus cavities.
Once the horse comes into contact with a potential source of infection, it may take 3-14 days after exposure before the horse will show the 1st clinical sign of strangles (fever). Based on this information, a minimum isolation period for introduction of new horses to the farm should be 14 days.
Clinical signs of strangles are highly suggestive of the diagnosis.
There are 3 methods to confirm the diagnosis of strangles:
– Culture of the bacteria from the nasal discharge or abscess;
– Polymerase chain reaction (PCR) that detects DNA of the S. equi bacteria;
– Blood test (serology) that measures a titer to a specific protein (SeM) of S. equi_
Antibiotic therapy remains controversial for the treatment of strangles. Complicated cases and those requiring tracheostomy for management of respiratory distress generally do require antibiotic and other supportive therapies. There is some evidence that treatment with antibiotics (such as penicillin) at the 1st sign of fever and in horses with no lymph node enlargement may prevent infection; however, early antibiotic treatment will also prevent these cases from developing immunity to the infection, and subsequently makes them susceptible to re-infection sooner.  It may also prolong the course of disease; resolution may be much faster without antibiotics for simple cases of equine strangles.
Penicillin is the drug of choice when antibiotics are required.  Potentiated sulfonamides (SMZ) should not be used as there is much bacterial resistance due to high levels of folic acid in the abscesses.  Intramuscular penicillin may not be tolerated well, and can get into the blood stream, ultimately killing the horse.  It can be used safely in many animals, but I recommend IV penicillin four times daily instead. 
Vaccination is one method for prevention and control of infection with S. equi. However, vaccination cannot guarantee disease prevention.  With strangles, vaccination will likely reduce the severity of disease in the majority of horses that are infected. Available vaccines can be administered by intramuscular and intranasal routes. Improper administration of the vaccination can result in poor protection against infection and/or complications at the site of injection; therefore, administration by your veterinarian is recommended. The intranasal vaccination results in the best local immunity.  IgA is secreted by respiratory mucosa to protect against invasion, and the vaccine stimulates this immunity.  IgG is stimulated by the intranasal vaccine, but at lower levels, so protection against metastasis may not be complete. 
When an outbreak or potential outbreak occurs, it is important to work with your veterinarian. Strangles is a reportable disease in some states. Isolation of the affected animal or animals is important. Other measures to take include taking the temperature of all horses on the farm at least twice daily. Normal rectal temperature is 99-102 F [37-38.8 C]. Monitoring the rectal temperature and isolating horses at the 1st sign of fever is one of the most effective ways to stop the spread of infection. Infected horses can transmit the bacteria to healthy horses 1-2 days after they develop a fever. 
Unexposed horses should be kept in a “clean” area and ideally should have separate caretakers, cleaning equipment, grooming equipment, water troughs, and pasture. People and equipment can transfer the infection from horse to horse. Extreme care, hand washing, and disinfection of supplies must be observed by everyone involved. If different individuals cannot care for infected and healthy horses, then healthy horses should always be dealt with first.
All movement on and off of the farm should be stopped for eight weeks after apparent resolution of the last case.  This can result in quarantine for several months.  Horses may be screened to identify carrier horses by testing blood and sampling the guttural pouches or pharynx.  Most horses that are affected will have good immunity for five years following infection.
Fox Run Equine Center