By Margaret Evans

What do you do when a horse at your boarding barn has been diagnosed with strangles? How is it treated and managed? How vulnerable is your own horse to getting strangles? And how do you know when the sick horse is truly disease-free? 

Strangles is a worldwide, highly infectious, upper respiratory bacterial disease caused by Streptococcus equi subspecies equi (S.equi). It is sometimes nicknamed the strep throat of horses. Strangles can occur in horses of any age, but younger horses seem to exhibit more severe clinical signs which include fever, lethargy, and enlarged lymphoid tissue followed by abscesses on the lymph nodes in the head and neck. The abscesses result in discharge through the nose and drainage from the throat. When lymph nodes rupture they produce large amounts of thick, creamy pus. Some complications include impaired eating and breathing, a cough, and a horse may hold its head in an abnormally low and extended position, trying to ease the pain in the throat and lymph nodes. 

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“It originally transmits through the nasal route and tonsils and from there it goes into the lymph nodes in the head and neck,” says Dr. Ashley Boyle, associate professor of medicine at the University of Pennsylvania’s School of Veterinary Medicine. “Lymph nodes under the jaw and around the pharynx become infected with the bacteria. The lymph nodes swell and sometimes burst out through the skin. But sometimes they will swell inside the horse’s body and cause them to asphyxiate, hence the name “strangles.”

Dr. Ashley Boyle of Penn Vet took the lead in writing a new consensus statement on treating, controlling, and preventing the equine infectious disease Strangles. Photo courtesy of University of Pennsylvania.

The abscesses can also rupture into the guttural pouch, an air-filled sac which is an out-pouching of the eustachian tube. But when the infection drains from the guttural pouch, it doesn’t always drain completely and strangles can hide, leaving the horse at risk of infecting others even after the initial inflammation is thought to have gone and the horse looks completely normal. As a carrier, the horse intermittently sheds the disease and passes it along to another unsuspecting animal.

Horses vulnerable to strangles are those at higher risk such as rescue populations. These animals may be malnourished, under stress, or with compromised immune systems. Horses are also at risk when they are co-mingling, travelling together, or contained in crowded paddocks.

From an evolutionary point of view, strangles has done well for itself. It is endemic in the equine population and was first recorded by Jordanus Ruffus in 1251. He was a horse farrier in the court of Emperor Federick ll and wrote of horse care treatises in De medicina equorum.

Boyle says that all infections are a balance of the load, the degree of exposure, and how each animal deals with it. Treatment will vary depending on the severity of the disease and are guided by the protocols set by the veterinarian. In many cases, providing a horse with an isolated area in a clean, dry stall and keeping it on a diet of soft, moist, palatable, good quality food and clean water will often suffice as the disease runs its course. Severe cases may see the lymph nodes become hard and painful, obstructing breathing. Lymph node abscesses that have pushed through the skin may burst or they can be lanced to allow drainage. To encourage rupturing and draining, abscesses can be poulticed.

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Any use of antibiotics will be at the discretion of the vet and in accordance with the extent/complication of the disease in each individual horse. However, Boyle says in her report that veterinary opinion on whether or not to use antibiotics is markedly divided. In many cases, its use is unnecessary. In fact, antibiotics may pose additional concerns and they should never be used as a preventative measure for animals that may have been exposed. The overuse of antibiotics can lead to resistance, a false sense of security against infection, and a delay in convalescent immune responses.

Above and below: Odin, a two-year-old Shetland pony, is about a week into his illness. His first symptom of strangles was a runny nose; then he developed a fever, and now has swelling on the right side of his face all the way down to his mouth.

Complications can arise when strangles spreads from the head and neck to other areas, and abscesses can develop in the chest, abdomen, or even the brain. The term “bastard strangles” is often used to describe these metastatic abscesses. Spread of the infection may occur over several routes and one is that of the guttural pouch when an abscess in the retropharyngeal lymph node enters the air sac.

Serious complications include purpura hemorrhagica, which can be fatal. It is an immune-mediated inflammation of the blood vessels leading to oozing on the skin surface and sloughing of the skin, as well as serious complications in other parts of the body.

The prognosis for horses infected with strangles is normally excellent and they can make a full recovery despite the fact it’s a pretty rough disease for the horse to have to go through. 

Excellent nursing care is vital, but all precautions must be taken at the biosecurity level to isolate and/or safely dispose of or thoroughly clean and disinfect contaminated materials, tools, and clothing. It is essential to prevent the spread of the disease to other areas of the barn not only while the infected horse is being treated but during the full quarantine period until such time when the animal has fully recovered, been tested for complete elimination of the disease, and is no longer a threat to the equine community.

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Animal handlers, caretakers, vets, pathologists and, in the event of loss, equine post mortem technicians should take special care to avoid any contamination from infected horses. Those working directly in the quarantine area must change their coveralls and boots before leaving and should thoroughly wash their arms and hands. Boots should be scrubbed. A walk-through wash is not adequate. Stable equipment – pitchfork, broom, shovel – must be dedicated to the quarantine area, cleaned, and never used in other stalls. Manure must be disposed of in a way that it is isolated, covered, and protected from flies.

The strangles bacterium likes a wet environment and water buckets can be a big issue during treatment and convalescence since it can sit in water for a long time. All buckets (feed and water) used by a sick horse must be scrubbed and dried before being refilled. They should never be shared with any other horse. Control flies and other pests that can spread the disease around the barn.

“Your barn has had a strangles outbreak and is doing everything to minimize the outbreak,” says Boyle. “It then comes down to making sure the animals are free of disease based on quarantine. It can take two to three weeks but in a natural disease process it can actually be six weeks. You have to be sure they clear that infection. Then there are the horses that are true carriers and they can keep it for two years or longer.”

One challenge is that, if the bacterium has entered the guttural pouch, it can hide. This ability makes future infections challenging because the source of infection is not obvious. The horse can shed the bacterium through nasal secretions and an unsuspecting horse may pick it up. That horse may continue to appear healthy even as the infection is incubating, which is how the disease spreads so successfully. In addition, convalescing horses may also look outwardly healthy yet continue to harbor the organism after clinical recovery. Boyle says that these horses may be thought of as long-term carriers and the source of new or recurrent disease even in well-managed herds. Therefore, even in convalescence, isolation and protective management should continue until final testing certifies that the horse is completely strangles-free. The current gold standard to determine that a horse is strangles-free is testing for the DNA of the bacteria that causes strangles in the guttural pouch.

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Managing and isolating strangles can be a real challenge for large barns where horses are coming and going. If there has been an outbreak of strangles in the area, it may be necessary to have horses tested before they are allowed on the property. 

Vaccinations are available for strangles and are particularly valuable for young horses since they can get a more severe form of the disease. But a veterinarian will need to test the horse’s level of antibodies before administering it. A horse may appear healthy but may have already been exposed to strangles which has yet to express itself. Horse owners, and especially owners of young animals, would be well advised to seek advice from their veterinarian and explore all vaccine measures.

Testing for strangles has advanced over the years. Traditionally, culture of throat swabs has been done but they need to be done three times to get a more accurate reading. Even then, Boyle says, you can still miss 10 to 40 percent of the cases. That is a large percentage of horses when the priority is to contain the disease.

Cultures provide slow results (one to two days) but are low cost and considered a widely available method for detection of strangles, especially when nasal discharge, fever, and depression are first noticed. It has been considered the gold standard for diagnosis. But the problem is that bacterial cultures may be unsuccessful early on when the bacterial count is low and a false negative given. If strangles is suspected but still not confirmed by cultures, the horse should be isolated and managed on the assumption the disease is present until further testing is done.

Another test is Polymerase Chain Reaction (PCR) through the use of nasal swabs and washes using DNA amplification. The value of this test is that it can be done the same day the samples arrive in the lab, providing a faster turnaround, and it can detect a small amount of the bacteria in the horse.

To find out if the bacterium is hiding in the guttural pouch as the horse continues in convalescence, Boyle recommends an endoscope exam of the air sac along with a PCR test.

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“On looking for carrier horses, we did a swab of the throat, then we did a lavage of the throat, and we did the endoscope exam of the guttural pouch, all on the same horse on the same day,” she says. “We were able to find, in the guttural pouch, it was 50 times more effective. I am confident saying that if you go in and look with the camera and make sure that it all looks normal and the PCR test is negative, then you are good to go.” 

She adds that one big advantage of the guttural pouch examination is that you can do it once with a scope and the sensitivity is high enough that it does not need to be done again. It is far more effective than doing three throat washes. 

“The guttural pouch sampling with an endoscope is what we are now calling the gold standard,” she says. “We want to use PCR as it is more sensitive than a culture but we want to examine the guttural pouch as it’s going to hide there.”

Strangles comes with its complexities and that ability to hide in the guttural pouch in a seemingly healthy horse poses many new questions. 

“[On examination] it may be an abnormal looking guttural pouch or it has pus in it,” says Boyle. “How can this not affect the animal? The horse looks normal. The infection still exists yet it’s not making the horse sick any more. Why? But it’s a source of shedding and infection to the naive population. [Understanding] that situation is not all worked out yet. It’s still a work in progress.”

Strangles is a challenging yet manageable disease, but it comes with stringent controls and management, and extraordinary biosecurity protocols. A sound understanding of the onset of signals can alert horse owners to immediately set in place mitigating strategies to nurse the sick horse and immediately contain the disease to protect others.

Dr. Boyle is the lead author of the paper Streptococcus equi Infections in Horses: Guidelines for Treatment, Control, and Prevention of Strangles – Revised Consensus Statement published recently in the Journal of Veterinary Internal Medicine. She coauthored the report with John Timoney of the Gluck Equine Research Center at the University of Kentucky, Richard Newton and Andrew Waller of Animal Health Trust in the United Kingdom, Melissa Hines of the University of Tennessee, and Ben Buchanan of Brazo Valley Equine Hospital in Texas.

For further reference, the report can be accessed here. 

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Photo: Shutterstock/Sharon Morris