Shedding Light on Strangles in Horses


Researchers are working to develop a safe and effective vaccine for this highly contagious disease..

One morning you find your horse with his head in a stall corner, feed still in his bucket and discharge coming from his nose. You check his temperature and find it's elevated. When your veterinarian examines him, she says he might have strangles.

This highly contagious equine disease is caused by a bacterium (Streptococcus equi) that gains access to the body through the nose or throat. Some affected horses suffer breathing obstruction due to enlarged lymph nodes that narrow the air passages--hence, the name strangles.

For these reasons strangles causes considerable concern to horse owners and veterinarians, especially given the difficulty in developing an effective and safe vaccine. Containing the disease requires diligent biosecurity measures.

"Although horses can recover from and then have immunity to this disease, some horses do not respond to typical treatments or have a more serious infection and complications," says Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, professor of equine medicine at Colorado State University.

The Disease

Clinical signs of strangles include abrupt onset of fever, upper respiratory tract discharges, and acute swelling and abscess formation in lymph nodes in the head and throat/neck areas. The bacteria target the tonsillar regions (located in the back of the throat and on the tongue) if the horse lacks adequate immunity. If a horse is very susceptible or exposed to large doses of bacteria, the infection might attack the lymph nodes--and other parts of the body.

The most serious form of the disease--and greatest chance of resulting fatality--occurs when bacteria get into lymph nodes of the abdomen or chest or into the brain or spinal cord. "Even in cases in which the disease is not severe or life-threatening, the horse feels quite bad and may be sicker longer than with some of the viral infections like influenza," says Traub-Dargatz.

"Horses with strangles always pose a risk to other horses," she adds. "Even after they recover, a segment of recovered horses will continue to shed bacteria well beyond when they look normal again," she says. These horses present risk for a new outbreak if they move into populations of horses with inadequate immunity.


A common perception about the strangles bacterium is that once it is on a farm it is always there; but this is not true. That horses on a certain farm might later develop the disease is more likely due to the presence of shedders. According to John Timoney, MVB, PhD, DSc, MRCVS, professor and Keeneland Chair of Infectious Diseases at University of Kentucky's Gluck Equine Research Center, S. equi bacteria survive in soil for about three days and live only four to six weeks in water.

"With the exception of being able to live in water for a while, this pathogen does not survive away from the horse," says Timoney. Strangles could be eliminated if people understand it and work toward making sure susceptible horses are never exposed.

"It has disappeared on some farms that used to have problems and has been gone for long periods of time in Japan and in Ireland--only reintroduced when brought in by a horse from somewhere else," he says. "If we can recognize an infected horse and either isolate it until it stops shedding bacteria or effectively treat it, this disease could be eradicated. The ideal situation would be to develop a successful vaccine, in combination with better diagnosis."

Chronic Shedding

"The challenge for horse owners is that chronically shedding individuals (asymptomatic carriers) look perfectly fine," says Traub-Dargatz. "They don't have fever or nasal discharge or abscesses."

It is relatively expensive, however, to test horses for chronic shedding of the bacteria. To do so, you need a fairly large sample that includes material from the back of the throat and both nasal passages, not just from a nasal swab. "The sample is obtained from a bath of sterile fluid that contacts the back of the throat and comes out the nostril," Traub-Dargatz says, a sampling method called a nasopharyngeal wash.

The PCR (polymerase chain reaction) test for checking this sample detects DNA of S. equi and is more sensitive than a bacteriologic culture from a nasal swab. But only a few laboratories conduct this test. You can also run a PCR test on or culture an abscess sample.

"We have two options for when we test to see if a horse is a chronic shedder," says Traub-Dargatz. "One is at the point where the horse is recovering--post-outbreak. We can test those horses before we move them. This is very targeted because we're only testing horses that had the disease or were exposed during the outbreak."

The other option is necessary when farms require a negative test for incoming horses as part of their biosecurity program. "In these instances we apply the test to all horses. But if horses were not tested when leaving the site where they had the disease, you don't know if they are still shedding," she explains.

"In the 30 days right after the disease (outbreak), the percent of horses shedding is moderately high," Traub-Dargatz explains. "Past 30 days, the percent still shedding goes down dramatically. The percent that continue to be chronic shedders (for years, rather than weeks or months) is very low, but there is nothing we can see from a clinical aspect to predict which ones might be shedding."

A horse might shed bacteria if he has a long-standing infection in the guttural pouch (a structure in his head that opens/drains into the nasal cavity). "In most of these horses, however, the organism tends to be trapped there," says Timoney. "You can find it if you put an endoscope in the guttural pouch and culture a sample, but most of those horses do not succeed in spreading the infection to other horses."

The guttural pouch is an extension of the eustachian tube, which drains into the nasopharynx. "If the horse puts its head down, and swallows at the same time, any pus the guttural pouch can be squeezed out and could drain down through the nose," he says. "You might see pus come down on the affected side. This is a clue the horse has a guttural pouch infection. Many of these horses are affected only on one side." In this instance the infection can be spread between horses if they come in contact, for example, through a shared water trough.

Traub-Dargatz says some chronic shedders have an occasional runny or dirty nose on one side, which the owner might think is due to dusty conditions.

"Another clue would be an unexplained cough," Timoney notes. "Some horses with this problem will cough on and off for no reason. But less than 50% of horses with a persisting infection will cough."

Over time (on average, up to 4½ months after acute infection), these horses cease to be shedders. "The pus dries up in the guttural pouch and opportunity for escape diminishes," he says. "There must be a flow of pus out through the nostril. This is why the disease may disappear on a farm for a while. Even though carriers may exist on some farms, they are not recognized and do not successfully spread the infection."

Because most horses shed bacteria for a while after having strangles, it is important to wait long enough for a horse to stop shedding before you test whether he's a chronic shedder. "Many will be shedding during the first 30 days, so I don't advise testing during that period," says Traub-Dargatz. "Instead, keep the horse(s) isolated and don't move them until after 30 days from the time they recovered. Apply the test after you've given horses a chance to clear the bacteria themselves. If you then find some chronic shedders, keep them away from other horses--or leave them with horses that had the disease at the same time and would potentially have some immunity--until you find that the horse is no longer shedding bacteria. Another option is to detect where the shedding is coming from, looking into the guttural pouch to see if there is material that needs to be removed. This can often be done by flushing and breaking up the material (via endoscope), then treating the pouch to inactivate the bacteria. Then later we'd retest that horse to see if that was sufficient to clear it up."

Prevention and Control

As with many other infectious diseases, prevention is key. When introducing a new horse to a farm, keep him separate from resident animals for three weeks, and watch for signs of disease, says Traub-Dargatz. "This includes taking temperature each day. If the horse develops a fever, have your veterinarian examine him to determine the cause," she says. Any caretakers (and any equipment used on that horse) should not come into contact with other horses unless measures are taken for removal of bacteria from hands, clothing, or equipment.

"If a person takes precautions from the beginning--isolating the horse (no nose-to-nose contact or shared water sources) and cleaning hands after handling the horse--it's possible to limit further spread of the disease." Success of these efforts will depend on how much contact the horse had with other horses during the time leading up to development of strangles and how much human traffic has been on the farm, with people touching multiple horses.


The first strangles vaccines early in the 20th century had questionable efficacy and carried risk of serious reactions and side effects. Horses exposed to strangles or known to have had strangles within the previous year were at risk of developing infection if vaccinated. Newer vaccines are safer, but not as safe or efficient as veterinarians would like them to be.

Timoney says there currently are two extract vaccines (intramuscular injections) for strangles available in North America. "Production of these involves extraction of the bacteria with either a hot acid or an enzyme to remove surface proteins, which are then (purified and) mixed with an adjuvant to create the vaccine," he explains.

Many owners, however, prefer the intranasal vaccine. "The intranasal vaccine uses an attenuated strain of a virulent S. equi that lacks the outer capsule that provides resistance to phagocytosis (being killed and ingested by white blood cells)," he says. "This strain has a second mutation in a gene involved in metabolism." These mutations make this organism less virulent.

"All the vaccines carry some risk," notes Timoney. "There are also concerns about efficacy. The efficacy of the extract vaccines does not reach the level provided by natural infection. Horses recovered from strangles have a high level of resistance to reinfection. A live organism provides a better protective immunity, which can last from two to five years in the majority of horses." How long this immunity (or vaccination immunity) will protect a horse depends partly on the level of exposure the horse encounters. There is always value in trying to minimize exposure to this disease because we can't totally rely on natural immunity or vaccination immunity.

A number of labs are working to improve strangles vaccines. "Most of the current work centers around proteins that have been predicted from the genomic sequence of S. equi, which was obtained a few years ago," says Timoney. Researchers have been looking for the proteins that might be useful in a vaccine.

Other strangles treatment methods include antibiotics and non-steroidal anti-inflammatory medications, but timing the ad-ministration of these is crucial when dealing with S. equi, so consult your veterinarian about the best approach.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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