Strangles in Horses

Caused by the bacterium Streptococcus equi, this highly contagious disease is most common and severe in young horses.

One beautiful spring morning, you are milling around  in the barn after the morning feeding trying to decide how to spend the rest of the day. Then you notice that one of your horses just isn't quite right today. The mare is not typically an aggressive eater, but today she really is not interested in her morning grain ration at all. On closer inspection, you notice that she feels warm. A quick check with a thermometer reveals a temperature of 103.5 degrees Fahrenheit--significantly above her normal 100.4 degrees Fahrenheit. You also note that there is a cloudy nasal discharge from both nostrils, and as you are stroking her head, you notice several small "lumps" in front of the throat latch area that she resents you touching.

After a visit and consultation with your veterinarian, your fears about "strangles" are confirmed, and within several days, two more horses in the barn are showing similar signs.This scenario is a typical textbook description of a strangles outbreak, a problem with which many horse owners are all too familiar."Strangles" is a disease caused by the bacterium Streptococcus equi, a Gram positive (stains purple with the special Gram stain) organism that is considered to be an "obligate" parasite of the equine species.

The nature of a species of bacteria that is considered to be an obligate parasite is that it does not survive well outside the horse's body. But, as we will discuss later in this article, it survives well enough and long enough to be transmitted from horse to horse via nasal secretions and pus from draining abscesses.

Strangles is ostensibly a disease of the upper respiratory tract, and it generally involves the lymph nodes of the head region. The inner lymph nodes occasionally can become so large that the horse has great difficulty breathing and can actually asphyxiate, hence the name strangles. More technically, the disease is a purulent (associated with pus) pharyngitis (inflammation of the throat) and lymphadenitis (inflammation of the lymph nodes).

The bacterium Streptococcus equi has been the known causative agent of the disease for the past century, but the observation of a strangulating disease condition associated with abscessing lymph nodes in horses dates back to the 13th Century. In Bongert's Handbook of Pathogenic Microorganisms, it is noted that the first description of the disease was by Jordanus Ruffus in 1251. It was observed in the 1800s that transmission of this disease was associated with pus and contaminated drinking buckets, and by 1888 the bacterium Streptococcus equi was determined to be the causative agent.

The organism has been the focus of a great deal of scientific research due to the morbidity (ability to cause disease) and several unique characteristics of the organism itself; the organism has some special characteristics that make it extremely adept at evading the immune system. A great deal of this research has been performed by John F. Timoney, MVB, PhD, Dsc, MRCVS, formerly from Cornell University's College of Veterinary Medicine and currently with the Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky. Timoney has published numerous scientific papers on Streptococcus equi, and a comprehensive review of the disease in an edition of the Veterinary Clinics of North America: Equine Practice, much of which will be referred to here. Timoney is considered by his colleagues to be the world's leading authority on Streptococcus equi.

Streptococcus equi is considered to be highly "host-adapted" to the equine species, meaning that it has evolved as a micro-organism that is very good at causing disease in horses, donkeys, and mules. The organism does not typically cause disease in other species of animals. As previously mentioned, in addition to being host-adapted, Streptococcus equi also is considered to be an "obligate parasite,"and therefore requires its "host" (the horse) for survival. The pus and nasal secretions appear to have some protective effect such that the organism has been reported to survive for weeks to months outside the horse's body in a contaminated environment.

The disease is highly contagious and can be transmitted by direct contact with nasal secretions or pus from a draining abscess. In addition to direct "nose-to-nose" contact, the organism can be transmitted by people (your dirty hands), flies, or "fomites." A fomite, by definition, is an inanimate object (i.e., a water bucket) that transmits a disease to a healthy animal via its contamination from a sick animal. Both of these methods of disease transmission are very significant and will be discussed later in this article.

Members of the equine species of all ages can be affected, but the disease is most common and most severe in young horses. Many foals have some degree of immunity to the disease that they acquire from their dams' colostrum. Of course, the level of immunity that is transferred is dependent upon both the quantity and quality of the colostrum (first milk) that the foal received at birth. If the mare has had the disease and developed immunity and/or has been vaccinated, then there will be high levels of protective blood proteins (antibodies) that can be transferred in the colostrum to the foal. This "passive immunity" acquired by the foal via the colostrum might afford protection against this disease (and many others) for the first three to six months of life. It is during that time that foals become more susceptible to disease as the "maternal antibodies" start to decline and their own immune systems begin to create protection either from exposure to disease or via stimulus from vaccinations.

Many horses will not develop sound immunity against the disease until after they have suffered from the disease. The foal's immune system is much more capable of doing this when it is a weanling/yearling, so the importance of adequate colostrum intake at birth again is highlighted.Other factors associated with the occurrence of strangles are conditions of crowding (too many horses in too small a place), poor housing and sanitation, inadequate nutrition, and the stress of lengthy transportation. Another important factor is the addition of a new animal.

Because the organism does not survive well outside of the horse's body, there often is a horse or exposure to a horse (sale, show, etc.) that is a source of the organism. In a few isolated and rare reports, horses have been identified as "long-term" carriers of the disease, but the typical situation is one where the "source" horse is still incubating the disease or is in the recovery phase and shedding the organism in nasal secretions. The situations are endless: mares coming and going from breeding farms; sale horses coming and going; horses-to-horse contact at shows, events, and races; just who was in this stall before my horse and how well was it cleaned/disinfected? More on that later.

How Does The Disease Occur?

Once the Streptococcus equi organism is transmitted to a horse via the mouth or nose, the organism attaches itself to the tonsil and adjacent lymph nodes. The lymph nodes are connected throughout the body by channels called the lymphatics, so once the organism has invaded one lymph node, it has a highway, so to speak, of lymphatics to spread around to other lymph nodes. Most cases of strangles are restricted to the lymph nodes of the head, but the infection occasionally can spread to lymph nodes throughout the body--a disease condition referred to as "bastard" strangles.The bacterium is especially well-suited to causing disease. Remember that one of the factors that make a pathogen a good disease producer is the ability for it to evade the host's immune system; Streptococcus equi has several properties that allow it to quite effectively evade the host's immune system. There is a special capsule and "M" protein structure surrounding the bacteria--these act as a shield and decrease the ability of white blood cells to attach to and kill the organism. In addition, Streptococcus equi> releases protein toxins that can actually "kill" the white blood cell before it can attack the bacteria.

The Onset

There is typically a three- to 14-day incubation period after exposure to the organism before the onset of clinical signs. The typical disease onset is characterized by mild lethargy, a low-grade fever (102-103 degrees Fahrenheit), a nasal discharge that is first clear and thick, then becomes white and contains pus. There is often a slight cough, loss of appetite, trouble swallowing, and a progressive swelling and tenderness of the lymph nodes around the head. Some horses might stand with their head and neck extended because of pain in the throat area.

The "external" lymph nodes in which we might observe swelling commonly are those under the lower jaw (in between the mandible), behind the back edge of the mandible (the high throat latch area), and above the eye. In most cases, these infected lymph nodes "mature," rupture, and drain for seven to 14 days after the onset of clinical signs. It is noted that not all affected horses develop lymph node swelling. Some older horses which might have established some degree of immunity can have lethargy, fever, and nasal discharge without further progression of the disease--those cases still shed the organism and act as a source of infection for healthy horses.

In addition to these external or superficial lymph nodes, there also are "internal" or deep lymph nodes in the head area. Significant swelling of the deep lymph nodes occurs in some horses and can cause complications. When a lymph node abscesses, it can become very large and firm before it ruptures and drains. And lymph nodes usually expand in the path of least resistance. For the deep cranial lymph nodes, the path of least resistance frequently is inward; the inward swelling of deep cranial lymph nodes can place a great amount of pressure on other structures within the head.

Difficulty in breathing can be a major complication related to internal swelling, or extremely large external swelling, of affected lymph nodes. This can be so severe in some cases that a tracheostomy is required in order to allow the horse to breath. In addition to compression of the airway, the abscessed lymph nodes can rupture inward and drain pus directly into the upper airway, which might predispose the horse to aspiration of the pus and the development of pneumonia.

Some horses can become more "systemically" ill and potentially spread the streptococcus organism throughout their bodies. Those horses might have a very high and persistent fever and severe depression. The term "bastard strangles," as mentioned before, is used to describe the condition when other lymph nodes within the body are affected. This condition can be difficult to accurately diagnose and treat and generally carries a poor prognosis.

The treatment of uncomplicated strangles cases typically involves careful monitoring of the horse's vital signs, temperature, heart rate, and respiratory rate. Significant increases in these can indicate a more advanced disease or respiratory distress from airway compression and warrant further veterinary evaluation. It has been reported that up to 20% of horses suffering from strangles can suffer from complications, making the careful monitoring of the affected horse important. In addition to monitoring, any external abscesses are encouraged to "mature" and drain by applying hot compresses and lancing them, if your veterinarian determines this is appropriate. Affected lymph nodes generally rupture and drain within seven to 14 days from the onset of clinical signs. And remember, the pus from draining lymph nodes can be a great source for spreading the Streptococcus equi organism and must be handled carefully to avoid the spread of disease to healthy horses. In addition, it is often the pus from abscesses or from the nasal passage that is cultured in order to confirm the disease.

The use of antibiotics in the treatment of strangles is somewhat controversial in the veterinary literature. For many cases, by the time the disease is noticed, it often is too late to arrest the progression. In fact, it is the opinion of many that the use of antibiotics at that time can prolong the progression of the disease and decrease the immunity that can develop post-infection. In the case of complicated strangles and more severe disease progression, antibiotics might be necessary, but this decision should be left to your veterinarian.It has been reported that approximately 75% of horses develop a solid and long-lasting immunity following recovery from the disease; after a second attack of the disease, there is usually a development of solid immunity. As previously mentioned, the foal is solely dependent on the passage  of immunity from the dam via colostrum. The importance of adequate colostrum intake in newborn foals is crucial to their health and can never be over-emphasized.


There are three commercially produced vaccines available, but none that afford complete protection. Because of Streptococcus equi's unique ability to evade the immune system, it is difficult to develop a vaccine that creates 100% immunity. It has been shown that vaccination can reduce the "attack rate" (the percentage of horses which will be "attacked" by the organism during an outbreak) by 50%. There have been adverse reactions associated with vaccination, which typically consisted of local abscess formation in the muscle at the injection site and muscle soreness.

In some horses, an immune-mediated disease called purpura hemorrhagica can occur following vaccination or natural infection. Purpura consists of an inflammation of the blood vessels, causing them to become "leaky" and subsequently allowing edema or swelling of the legs and underbelly. Should limb swelling occur following vaccination or natural infection, your veterinarian should be consulted.

The best method of dealing with strangles probably is reducing the chance of having to deal with it at all. Instituting and actually following/enforcing strict on-farm "quarantine" for new animals for 14-21 days before exposing them to the general population of horses can prevent an outbreak. The quarantined horses should be monitored daily for the presence of nasal discharge or fever. Anyone working with the "new" horses should practice adequate hygiene procedures to prevent human or fomite transmission of strangles or other diseases. When traveling with your horse, new stabling should be disinfected prior to use, and all feed/water buckets should be appropriately cleansed.

If a horse on the farm shows signs of strangles, it should immediately be isolated in a "quarantine" situation, and all horses having had contact with the affected horses should be monitored carefully for the development of fever. Strangles is a highly contagious disease and the isolation of affected horses is extremely important in limiting the spread throughout a herd.

The simple act of hand washing with a disinfectant soap cannot be over-emphasized; hand washing can significantly reduce the risk spreading contagious diseases. Rubber boots and coveralls should be worn when working with affected horses, then appropriately washed.

Stalls and equipment should be washed thoroughly with a detergent cleanser, rinsed thoroughly with water, then disinfected with an appropriate dilution of either chlorhexidine gluconate of povidone iodine. It is extremely important to both wash down the "crude" debris with detergent and rinse off the detergent as both the presence of "organic debris" and detergent will make the disinfection less effective.

There are concerns about the ability of flies to transmit the disease, so sprays and other measures should be taken to reduce the local fly population. It also has been suggested that bedding removed from the stalls of affected horses be isolated and covered by a plastic tarp to prevent access by flies to contaminated bedding. Paddocks grazed by affected horses should be considered contaminated and not be used for at least one month.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at or by calling 800/582-5604.

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