Why We Vaccinate - Strangles

Most horse people have heard of Strangles, and many have had the unfortunate experience of dealing with an outbreak.  The disease, sometimes referred to as “distemper,” is not new—it was first reported in 1251.  The disease is highly contagious.  Young animals (weanlings, yearlings, and other young stock) are particularly susceptible, however any age of horse can be affected.

What is Strangles?

Strangles is a bacterial infection caused by Streptococcus equi subspecies equi (commonly shortened to Strep. equi.).  The first sign of disease is sudden onset of high fever, during which the horse typically shows becomes depressed, listless, and goes off feed.  After this, usually the horse develops a thick yellowish nasal discharge that does not have a foul odour, and then abscessation of lymph nodes beginning approximately 1 week after the onset of fever.  Typically the lymph nodes that are affected are the ones in the head, the submandibular lymph nodes between the lower jaw and the retropharyngeal lymph nodes deep in the back of the head.  In severe cases, the obstruction of the airways by these enlarged, abscessed lymph nodes can suffocate the animal and is what led to the common name for the disease, “Strangles.”  Coughing is not usually a feature of Strangles, but can occur in some cases.

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Typically, the abscesses break and drain and the horse recovers within a matter of weeks.  Abscesses are not necessarily confined to the lymph nodes of the head, however, and can be found in other external locations (especially on the head) or deep within the body.  When abscesses caused by Strep equi are found in other parts of the body, including the liver, spleen, lung, kidney, or brain, it is colourfully called “bastard strangles.” Another potential complication of Strep equi infections is purpura hemorrhagica.  Purpura is a painful swelling of the legs, underside of the belly, or even the head, caused by inflammation of the peripheral blood vessels in the limbs.  Signs typically develop 2-4 weeks after Strep equi infection, however the infection does not even have to cause clinical symptoms of strangles to cause purpura.  Although the condition is not fully understood, these horses are typically very sick and depressed, much more so than horses with an uncomplicated case of strangles.  Purpura can be fatal if not treated promptly.

How do horses contract strangles?

The bacterium causing strangles is extremely contagious from horse to horse, directly or indirectly.  Nasal discharge from horses with active or even inactive infections are a very important source of infection.  Indirect transmission can occur via contact with contaminated buckets, troughs, stalls, tie rails, trailers, pastures, tack, or even the contaminated hands or clothing of handlers.  It is also becoming increasingly recognized that horses that have apparently recovered from a clinical case of strangles can shed the Strep equi bacterium from their nose intermittently for years, even though they outwardly appear healthy.  This may explain many unexpected outbreaks when there has been no known exposure to a sick horse.  Also, the bacterium can survive in the environment for months under ideal circumstances, so environmental contamination can explain some cases of strangles that develop in new additions to a herd.

How is strangles prevented?

The best approach to preventing strangles is diligent biosecurity protocols. Avoid transporting horses to an area with a known outbreak, and isolate any new additions to a farm or barn for a minimum of 2-3 weeks upon arrival.  When travelling with your horse to new locations or shows, do not allow your horse to touch noses with other horses, or with communal structures that other horses may have rubbed their noses on, such as hitching rails.  Bring your own bucket and feed tubs, and avoid common water troughs, etc.  Be aware that the infection can be transferred by you—on your hands, clothing, etc.  If an outbreak does occur, prompt and strict quarantine is necessary to minimize transmission of the disease.

If your horse has been exposed to other horses with the disease, or has been in contact with horses of unknown disease status, the disease can be minimized if it is detected early.  Because the first sign of disease is reliably a high fever, and because nasal shedding of the organism does not occur for several days after the fever occurs, further spread of the disease can be limited if the fever is detected early.  After potential exposure, monitoring your horse’s temperature twice daily can be very helpful.  At the first sign of a fever (temperature > 38.5C), contact your veterinarian as quarantine and antibiotic treatment at this stage can prevent transmission of the disease and in many instances can limit or prevent further progression of the disease.

Vaccination can help decrease transmission of the disease, but does not provide complete protection.  Because the site of entry of the bacterium is the nasal passageways, an intranasal vaccine can stimulate an immune response directly where the organism is first encountered.  However, proper delivery of the vaccine into the nose is critical to ensure a proper response to the vaccine.  The intranasal vaccine contains a modified, attenuated strain of the bacterium, so extreme care must be taken to ensure none of the vaccine is inadvertently injected, for example through contamination of an injection site by a horse snorting after they received the intranasal vaccine.

This video shows an endoscopic examination of pharynx of a horse with Strangles, demonstrating how close the affected lymph nodes can be to the gutteral pouches in the back of the horse's throat.  The gutteral pouches can be a site where the bacteria that cause Strangles can "hide out," providing an inapparent source of infection for other horses even after the affected horse has seemed to clear the disease.

Strangles is persistent in our area, with outbreaks popping up frequently.  Although vaccination can help limit the disease and lessen the clinical severity of many cases, the cornerstone of prevention and containment of outbreaks is thoughtful and diligent biosecurity measures.