Strangles: Understanding Equine Distemper and Purpura Haemorrhagica

Grey horse with grass sticking out of its mouth in front of forest

One of the most alarming of infectious diseases in the equine industry is Strangles, which is noted for the characteristic large swelling of lymph nodes under the jaw or in the throat area. Sometimes the node enlargement progresses to the point of interfering with airway or swallowing functions creating a concern that the patient might strangle.

Strangles, also known as equine distemper, is caused by a bacterial infection of the highly infectious Streptococcus equi (Strep equi). It most commonly affects young horses, generally two years of age or less. Although the disease is potentially fatal, the mortality rate is generally less than 10 percent. The morbidity rate, however, is quite high due to the infectious nature of the germ and its ability to survive once infected horses contaminate the environment. The disease has an incubation period ranging from a few days to two weeks. Therefore, minimum isolation time of two weeks is recommended with horses that have been exposed, or horses having an unknown history for biosecurity purposes.

“We see it so commonly in young horses when the germ is found in endemic areas; the younger animals often lack adequate immune protection” said Dr. Glennon Mays, clinical assistant professor at the Texas A&M College of Veterinary Medicine & Biomedical Science (CVM). “That means that once a location is contaminated with Strangles, we often see it reappear in the horse population because the bacterium is located in that environment. When horses are born or brought to that location, if they don’t have protective immunity, they become infected. “

The Strep equi germ can survive in contaminated soil, water troughs, feed buckets, and tack. The germ can even be transferred by people from one location to another. Direct transmission between horses occurs through contaminated mucus excretions of the infected horse to the naïve horse. Contaminated horses may shed the germ for two to three weeks.

Infected horses can exhibit several signs, such as general depression and dull behavior, runny nose or eyes and fever. The classic symptom of the infection is swollen lymph nodes beneath the jaw or throat areas or in other external and/or internal body locations. Lymph nodes will usually swell two to three days after infection, and horses will be able to spread disease for approximately two to three weeks after clinical signs appear.

“These lymph nodes swell in reaction to the infection, and often develop into large pus formations,” Mays said. “After swelling, when the nodes soften, it is often therapeutic to drain the nodes by surgical incision. But this requires great caution since large blood vessels are often in close proximity. Care to contain the recovered pus is necessary because it can be very contaminating wherever it collects. The lymph nodes really serve as an area of collection of the bacteria and a concentration point of the infection.”

“You don’t want that fluid getting into the soil, or on objects that will come in contact with uninfected horses,” Mays said. “Remember that the person handling the infected horse or the infected material coming from the horse can become a vehicle for spreading the disease.”

In addition to relieving the swelling of glands, veterinarians will typically administer supportive care, such as anti-inflammatory medications, to horses that retain an adequate appetite, hydration status, and don’t display difficulty breathing.

Some patients may experience high fever spikes (103 F or more) and require more aggressive therapy. In these instances, administration of antibiotics, electrolyte fluids, and anti-inflammatories become necessary. Additional products believed to stimulate the immune system are sometimes incorporated into the therapeutic plan, however, many equine practitioners choose against initiating antimicrobial therapy unless the patient is fevered, depressed, and listless to the point of not eating and drinking adequately.

Mays explained that in some cases equine Strangles can develop into a clinical presentation called Purpura Haemorrhagica. Purpura causes vasculitis in the extremities resulting in painful swelling of the legs due to acute inflammation of peripheral blood vessels. The legs become tender and sore because circulation is impaired. Horses that develop this condition are either infected with Strangles and progress to the purpura stage, or were previously exposed to the germ without developing an infection.

“The reason behind this development is poorly understood, “Mays said. “But horses that develop purpura are extremely depressed and actually show more clinical signs of illness – essentially sicker than with Strangles alone.”

The vasculitis in the legs can significantly prolong recovery, and can be severe enough that the swelling permanently damages musculoskeletal structures. Laminitis is not an unusual resulting chronic condition. Complications from purpura can produce career-ending results for performance horses. Purpura can also become fatal if not treated in a timely or effective manner.

“My biggest concern is purpura creating a systemic effect resulting in a generalized, body-wide infection producing toxins that becomes a life-threatening condition, “Mays said.

The best defense against equine Strangles is to create a barrier through biosecurity.  Avoid transporting young horses into contaminated or endemic areas. Isolate horses of unknown exposure history for a minimum of two weeks to prevent possible exposure of disease to other animals. Do not bring horses displaying signs of illness into contact with healthy horses. Remember, the infection can be transferred on clothing, tack, vehicle tires, or hands. Due to Strangles’ extremely contagious character, seek veterinary care immediately if suspected. . Often, the veterinarians prefer to examine the animal away from their clinic to avoid contamination.

Vaccines have been available for decades, but remain a contested topic for recommendation. Mays explained that many veterinarians approach vaccination recommendation with caution. “Strep vaccines historically have more potential for reactive properties than other equine biological products. Many practitioners express concern for an increased frequency of post-vaccination complications with Strangles vaccine,” Mays said.

Pet Talk is a service of the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University. Stories can be viewed on the Web at Suggestions for future topics may be directed to

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