Due to its contagious nature, outbreaks of the bacterial disease strangles, which can cause serious complications, periodically occur. This can send owners scrambling to protect their horses.

Owners are justifiably concerned because strangles is transmitted easily, and can be a drawn-out and unpleasant illness. The good news is that most horses recover from it.

“Strangles is a fairly common disease for horses,” said Kathy Seino, DVM, PhD, a Washington State University assistant professor of equine medicine. “There seems to be a lot of misunderstanding and phobia about it, even though it is a disease that has been around for about 800 years. The first case was reported in 1251.

“It is important that owners know the majority of horses that get strangles become clinically affected, but recover without any veterinary attention,” she said. “People fear it though because once you get it in a barn full of horses, strangles spreads easily and causes a lot of clinical sickness. So it is a big pain to manage, and often people don’t know how to treat and control it.”

The disease is caused by bacteria called Streptococcus equi. It is spread through nasal secretions or pus from abscesses that can form to fight off the bacteria. Streptococcus pyogenes, or group A streptococcus, is what causes strep throat in humans.

Exposed horses might begin to show generalized clinical signs within three to 14 days. These might include fever, loss of appetite, cough, and clear nasal discharge. As the disease progresses, horses develop swollen lymph nodes around the throat and a thick, yellow nasal discharge.

“The bacteria often spread to a horse’s regional head lymph nodes under the jaw and the guttural pouches, which lie on each side of the back of the throat,” Seino said.

As the infection matures, the lymph nodes can become large and painful. A horse might keep its heads low and extend its neck outward to relieve some of the pressure and pain.

“Sometimes, the lymph nodes become so large that they block a horse’s airways,” she said. “This is how strangles got its name. Eventually, the lymph nodes burst like an abscess and drain.”

Draining pus can run out of a horse’s nose, or sometimes the abscesses rupture through the skin. This pus is highly contagious. Horses often spread it to other horses from nose to nose contact, or through coughing, sneezing, and snorting. People can also inadvertently spread it via contaminated buckets, bedding, feed, clothes, and hands. Caretakers should be sure to sanitize any area or items the sick horse has had contact with as the bacteria can potentially live by itself in the environment for several weeks.

In addition to hygiene measures, in order to control an outbreak of strangles, movement of all horses on and off the affected premises should be stopped. All horses with strangles and horses that have had contact with these cases should be segregated or quarantined from the unexposed group. Rectal temperatures of the healthy horses should be taken once a day, to detect and segregate any new cases.

“Many horses begin to recover without treatment within seven to 10 days after the first symptoms appear, but it is important to know that they can continue to shed the infection to other horses for another 21 to 28 days,” Seino said. “In fact, the main transmission source of strangles is infected horses with sub-clinical signs. So it is imperative to keep these horses quarantined for at least three to four weeks after their clinical signs resolve to prevent the spread of disease. Additionally, we recommend all horses (those recovering from strangles and horses that have had contact with cases) have three nasal swabs taken for culture or molecular testing which should test negative to S. equi before they are allowed out of quarantine. A veterinarian can also take a guttural swab to check for the infection.”

Complications

While the infection often runs its course within a couple of weeks, about 20% of horses develop complications from strangles and might need veterinary help.

Some horses might have difficulty breathing due to the enlarged lymph nodes. Veterinarians can help by lancing the abscess and draining the infection, and might use antibiotics to help kill the remaining bacteria. Antibiotics should only be used in the early stages of strangles or after any abscesses have burst as it might prevent forming abscesses from opening and draining, a key to recovery.

In some horses, the infection spreads beyond the lymph nodes in the throat to lymph nodes in other body cavities like the abdomen, lungs and brain. This complication, referred to as bastard strangles, often develops weeks after the initial infection seemed to resolve. Although rare, fatalities can occur in this instance.

“Another complication is called purpura haemorrhagica, in which horses develop fluid swelling or edema in their lower limbs from acute inflammation of the blood vessels,” Seino said. In severe cases, bloodflow to the skin and be compromised, causing the skin to slough off.

Another complication is muscle tissue inflammation called myositis.

While most horses will not develop complications and are able to recover without any treatment, it is helpful to involve a veterinarian to make a definitive diagnosis and instruct owners on how to quarantine, care for and clean up after a horse with strangles. He or she can also make a horse more comfortable during the most painful aspects of the illness, treat any major complications if or as they arise, and take cultures to test horses before they are released from quarantine.

Prevention

Any horse can become infected with strangles. Horses younger than five years old are most commonly affected. Foals less than four months of age are usually not a risk because they are still protected by passive immunity from their mother’s colostrum.

“For horses that have had strangles, it is thought that 75% develop a five-year immunity against it or even longer,” Seino said. “Unfortunately, approximately 10% of horses that have had the disease become silent carriers that can re-shed the bacteria for months or even years. The vast majority of these horses develop a chronic infection or empyema in the guttural pouches, sometimes with the purulent discharge becoming inspissated or thickened into masses known as chondroids. This potential outcome emphasizes the importance of testing all convalescing horses with a series of three nasal swabs. Any horse that tests positive for S. equi should then have endoscopy of their upper airway and guttural pouches. Treatment of these horses with chronic infection may require multiple flushings of the infected pouches, or if there are many chondroids, surgery may be needed to remove the material.”

Basic biosecurity measures can help prevent horses from getting strangles.

“If you have a new horse coming to your farm, implement a mini-quarantine by keeping it in a separate area from the rest of your horses for about 14 days and watch to see if the horse develops any illness,” Seino said. “Do this for your own horses as well if they leave the farm even for a short time to a show or breeding farm. If you have a horse recovering from strangles, isolate it and have it tested before it is let near other horses on the farm or travels to any shows.”

There are live and killed vaccines available for strangles, but neither provide complete protection. Their main use is to reduce the severity of the disease should a horse become infected.

For more information, contact the WSU Veterinary Teaching Hospital at 509/335-0711, or look online at the American College of Veterinary Internal Medicine’s Web site for the ACVIM Consensus Statement on Strangles (link directs to a PDF file).

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Reprinted from the WSU College of Veterinary Medicine Equine News Winter 2009 issue