Strangles is an equine disease almost universally recognized and feared within the horse industry.  It is an upper respiratory infection of horses, donkeys, and mules, resulting in characteristic lymph node enlargement and the development of abscesses. 

Strangles is found worldwide.  Our awareness of the disease dates far back in recorded history, back to the time of the Roman Empire.  The first documented description of strangles is credited to the stable master Jordanu Ruffus in 1251, who, under the urging of King Fredrick II, wrote his book, Horse-Healing.  By the 1600’s, it became evident that contaminated water buckets played a role in transmission of the disease.  It wasn’t until 1888, however, that the causative organism was identified from the pus of a strangles abscess.

The Culprit

Strangles is a bacterial disease.  The cause is the bacterium Streptococcus equi.  This organism is highly adapted to infecting horses and is unlikely to infect other animals and humans.  Respiratory secretions and pus from an infected horse are highly contagious.  It is this material that spreads the disease.  This infectious material can be spread in a variety of ways:

• Direct nose to nose contact
• Aerosolized droplets when an infected horse coughs or sneezes
• Shared water buckets – this is a particularly common and potent mechanism for spreading the disease
• Shared barn implements
• Shared grooming supplies
• Shared tack
• People (owners, barn hands, etc.) can literally carry infectious material in the form of mucus or pus on clothing, footwear, or skin.
• Animals, conceivably even insects, can also carry infectious material on their bodies

Streptococcus equi requires the horse for its ultimate survival.  The bacteria can survive in the environment for variable amounts of time, depending on the conditions.  Water is essential for its survival.  Here is what the data shows:

• Direct sunlight and drying destroys the bacteria in 24 hours
• The bacteria survives for
• 3 days in dirt
• 7 days on a fence
• 30 days in water
• 2 months on a wood surface

Clinical Signs of Infection

In all cases, Streptococcus equi enters the horse body through the mouth or the nose.  The organism quickly moves through the lining of the upper airway and into the lymphatic vessels; it rapidly makes its way to the regional lymph nodes.  The incubation period for any infectious disease is the time between initial infection and the appearance of symptoms. For strangles, the incubation period is 3 to 14 days.  The symptoms of strangles vary tremendously depending on the amount of exposure, the age of the horse, and the previous exposure history of the individual.  Horses with previous exposure generally have milder symptoms. Young, previously unexposed horses are typically the most severe cases.  Stress factors, such as overcrowding, poor nutrition, heavy parasite infestation, extreme weather conditions, and transport will cause more severe cases. 

The first symptom usually is a fever, generally around 103ºF.  A sudden two degree rise in temperature should also be viewed as suspicious.  Affected horses often become listless and depressed.  There may be a mild cough.  Initially, there is a watery nasal discharge that later becomes mucoid and purulent (with pus).  Pharyngitis and swollen lymph nodes in the throat cause a sore throat, visible as difficulty with swallowing and an extended head and neck posture.

If deeper lymph nodes of the throat or neck are affected, they can result in pressure on the trachea as they enlarge and become abscessed.  Difficulty breathing and even suffocation can result.  This problem is obviously where the disease derived its name.  Affected lymph nodes eventually develop into abscesses that rupture in 7 to 14 days.  Submandibular (i.e. below the jaw) lymph node abscesses rupture through the skin.  Deeper lymph nodes of the throat rupture into the pharynx and pus drains out the nostrils.  Some deeper lymph node abscesses rupture into the guttural pouches (large air-filled cavities behind the throat).

Transmission

Most infected horses begin shedding (spreading the bacteria) 2 or 3 days after the onset of fever.  Shedding usually continues for 2 to 3 weeks.  As testing methods become more sophisticated (see below), we are learning that some cases continue to shed the strangles bacteria for up to six weeks or longer after the symptoms of the disease have cleared up.  Furthermore, up to 10% of the cases become chronic shedders that continue to spread the disease for months or even years, yet they so no outward signs of strangles.  Rupture of deeper abscesses into the guttural pouches and/or the paranasal sinuses are the prime sources of chronic shedders.  It seems infection into these anatomical sites creates a sanctuary for the organism to persist while not causing outward strangles symptoms. 

Complications from Strangles

Most horses develop good immunity to strangles following an infection.  Antibodies generally peak at about 5 weeks after an infection and remain high for at least 6 months.  In a large percentage of cases, strong immunity lasts well over a year--up to five years in many cases.

Unfortunately, approximately 20% of strangles cases develop serious, life-threatening complications.

• Disseminated strangles: this complication is more commonly referred to as “bastard strangles,” and is life- threatening.  This occurs when Strep. equi escapes the upper respiratory tract and spreads to other parts of the body, such vital areas as the lungs, liver, spleen, kidneys or brain.
• Suffocation: already described above, this life-threatening circumstance requires emergency tracheotomy. 
• Pupura hemorrhagica: when infected with strangles, all horses will develop antibodies to combat the infection.  Some horses, however, develop a hyper reactive antibody response.  This eventually leads to inflammation in the walls of the blood vessels, resulting in reduced blood flow.  The head, legs, and trunk of the horse become severely swollen and serum can literally ooze through the skin.  Large patches of skin may slough off.  This impeded blood flow can occur internally as well, compromising blood flow to internal organs and muscle tissue.  Without proper blood flow, tissues are starved of oxygen and damaged by accumulating waste products and toxins, creating a life-threatening situation.

Diagnostics

New diagnostic tools have enhanced our ability to diagnose strangles, identify chronic carriers, and assess immune status when making decisions on whether to vaccinate.

·         Culture: a culture is a sample of material that is placed on nutrient agar in a Petri dish, and then incubated to grow and identify any bacteria present.  Although not new, this is still considered an effective and essential tool in strangles evaluation.  There are three basic samples that can be used for strangles culture.  One is a sample of pus either aspirated from an abscess into a syringe or sampled from a draining abscess.  Another sample is collected by swabbing the nasal cavity with a cotton swab.  Finally, a nasal wash is performed by instilling sterile saline into the nasal cavity and collecting the liquid that flows out of the nostrils.  By sampling a much larger surface area of the airway, the nasal wash is more likely to “find” the bacteria if it’s present.

·         PCR: PCR stands for “polymerase chain reaction” and is a test that detects DNA from the strangles bacteria.  It is reportedly 3 times more likely to “find” the bacteria than a culture would.  Either a swab or a nasal wash can be used to collect the sample, but the wash, as mentioned above, samples a much larger surface area.

·         Endoscopy: an endoscope is flexible fiber optic instrument used to visualize internal spaces, body cavities, etc. (e.g., colonoscopy).  When a sub-clinical strangles carrier has been identified through culture and PCR testing, the guttural pouches can and should be examined.  Samples can be collected from here for laboratory analysis.  Affected guttural pouches likely will contain pus or “chondroids,” which are dried, coalesced accumulations of pus.  Treatment can successfully cure the chronic carrier condition.

·         Serology: serology is a blood sample that measures the antibody level for a particular disease.  Strangles serology can provide useful information about whether an individual horse should or should not be vaccinated.  If the titer is low, vaccination is indicated and will help protect against infection.  If the titer is very high, for instance after a recent known or unknown exposure, vaccination can be harmful and is contraindicated.    

To summarize, equine strangles is a highly contagious bacterial disease of the upper respiratory tract of horses. Clinical signs are usually limited to the upper respiratory tract.  Less commonly, severe life-threatening complications develop.  Today’s diagnostic tools are increasing our ability in diagnosing, controlling, and preventing this disease.