STRANGLES (Streptococcus Equi Infection)

“Strangles” is a term used by most lay-people to describe the clinical syndrome produced by the bacterium Streptococcus equi.  This disease has been recognized as a problem of horses for 400 years.

Although the symptoms of strangles vary from case to case, the classic description includes swelling and infection of the throat region.  There is usually a fever of 102° – 106° F in the beginning.  The horse may be depressed and unwilling to eat.  There is often a nasal discharge, which may be serous or mucoid initially but quite purulent as the disease advances.  Affected animals usually cough.  Many affected animals may resist palpation of the submandibular and retropharyngeal area (this is between and just behind the jaw bones).  In most individuals the organism spreads to cause abscesses in the retropharyngeal or mandibular lymph nodes – in this area, 10 – 14 days after the initial onset of signs.

The disease is most commonly transmitted through infective secretions.  Once the disease establishes itself on a farm or ranch, it often becomes a persistent, recurrent problem even though there may be long periods when no resident horses have clinical signs of the disease.  Once the organism establishes itself in a susceptible population, the percentage of animals affected is generally quite high and often approaches 100%.  However, mortality in most uncomplicated cases is low.  Listlessness and rapid spread are also common.  Most uncomplicated cases are confined to the upper respiratory tract, but involvement of the lower respiratory tract can cause pneumonia.

In some horses the organism localizes in the guttural pouch or paranasal sinuses and is a constant source of purulent discharge from the nostrils.  Persistent drainage from the submandibular or pharyngeal areas occurs in some cases.  Cranial nerve dysfunction may occur in cases of retropharyngeal abscesses affecting the recurrent laryngeal nerve.  In horses with the most severe form of strangles, any part of the body or viscera may be affected, including the heart.

Streptococcus equi is quite susceptible to penicillin; therefore, it is often the treatment of choice.  Some say the use of penicillin only retards the growth of the bacteria and makes the infection last longer.  Some say penicillin may even cause the infection to spread internally where it is much more difficult to assess and treat.

Penicillin is unwarranted if the horse is alert, and has a reasonably good appetite.  Good nursing care and minimization of stress are important in all cases.  A palatable diet and dry, comfortable surroundings should be provided.  If warranted and practical, application of hot packs or poultices helps promote maturation of abscesses.  Mature abscesses should be lanced ventrally for good drainage.  The name “strangles” comes from the effect that can come about when the throat abscesses grow so large that they cut off the trachea (wind pipe) and actually strangle the horse.

Severely affected horses may require intensive supportive therapy, including intravenous fluids, feeding by nasogastric tube, and tracheotomy (cutting a breathing hole in the wind pipe.  The use of penicillin is indicated if the temperature remains elevated for a long period, the horse is severely depressed and lethargic, and the pharyngeal and retropharyngeal areas are severely affected.  Although some veterinarians argue against the use of penicillin before abscesses have matured, administration of penicillin at the proper dosage for an adequate period is unquestionably beneficial.  The recommended dosage in such cases is 10,000-20,000 IU/lb. daily for a period to include 57 days after the last abscess has drained.  Administration of penicillin in small doses or intermittently should be avoided.

The decision to administer penicillin should be closely evaluated if young animals or many animals on a farm are affected.  In some cases the stress of rounding up and restraining affected animals to administer penicillin outweighs the benefits of therapy.  Other antibiotics should only be used if indicated by culture and sensitivity tests.

If recognized early, an outbreak of strangles can be contained by isolation of affected horses and strict hygiene by handlers.  Although vaccines have been developed, they are not nearly as effective as other types of vaccines.  Difficulties in vaccine development stem from the relatively low immunogenicity of the bacterium and a lack of serologic tests to measure the immune response to the organism.  The horse owner should take the advice of the veterinarian about vaccination.  He has knowledge and experience with the disease and vaccinations in the area.  The vaccine may be more effective in one area than in another.

If a persistent severe problem with Strangles exists on a farm, the use of the vaccine may be beneficial.  The owner and clinician must be aware of the potential failures and adverse reactions if use of the vaccine is elected.  Local reactions and abscesses at the injection site can occur from use of vaccines presently available.  In most younger animals, especially those intended for racing, this is probably of little consequence.  However, an injection reaction in a young horse being groomed for show purposes can be unsightly and can cause the animal to be removed from competition for several weeks.