Strangles is a bacterial disease affecting horses and other equids (including mules, donkeys, zebras) greater than 1 month of age and including all sexes and breeds. It is caused by the bacterium Streptococcus equi subspecies equi (Streptococcus equi or S. equi). Typically, horses with Strangles have swelling of the lymph nodes under and around the throat which can be severe enough to “strangle” the horse, affecting breathing.

Strangles occurs throughout the world and has been found in horses all over Ontario. Horses are moved between different areas of the province, country and continent for a variety of reasons including auction and competitions. Exposure to horses shedding the S. equi bacterium is a major reason for spread of this disease between regions.

To assist the Ministry of Agriculture, Food and Rural Affairs (OMAFRA) and the equine industry in understanding the risk of Strangles in various areas of the province, Strangles was designated an immediately notifiable disease under the Animal Health Act, 2009 in February 2023. This means that veterinary diagnostic laboratories in Ontario, and veterinarians using diagnostic labs outside the province, are required to report positive S. equi test results to OMAFRA.

Learn more about recent Strangles outbreaks in Ontario, Canada and the United States.


A horse becomes infected with S. equi when it touches noses with an infected equid or touches an object or surface that has been contaminated with S. equi including a person’s hands and clothing.

S. equi can survive in the soil for 1 to 2 days if exposed to ultraviolet light (the sun) or 4 to 6 weeks in water. It is imperative, therefore, when dealing with a horse diagnosed with Strangles, or with horses of unknown health status such as occurs when horses are commingled at auctions or competitions, that shared water sources such as buckets and troughs are not used.

Clinical signs

S. equi enters the nose or mouth of the horse and travels to the back of the throat where it enters the local tissue and travels to the lymph nodes of the head. This can take anywhere from 3 to 14 days.

After S. equi enters the tissue, the horse may develop a fever (which can be as high as 39.5°C or 103.1°F), look depressed and not want to eat. The fever may last a few days, after which time the horse will often develop a thick white nasal discharge and, most commonly, enlarged lymph nodes of the head. The lymph nodes under the jaw bone, called the submandibular lymph nodes, are the lymph nodes most commonly affected but other nodes, such as those in the throat latch area, down the neck and in other areas of the body, may become affected.

Lymph nodes infected with S. equi become large, painful and will eventually break open and drain pus. The pus draining from lymph nodes or present in nasal discharge contains large numbers of S. equi that can easily infect other horses. Once the lymph nodes open and drain, the horse usually recovers quickly. In severe cases, the lymph nodes around the head may become so large that they obstruct the airway and the horse needs to extend its head and neck to breathe. This is a situation that needs to be addressed immediately by your veterinarian.

Some horses only develop nasal discharge without the enlarged and draining lymph nodes. This may be because their immune system is able to control the S. equi infection from either previous infection or vaccination. Because nasal discharge can be a sign of other airway diseases such as viral infections, if the horse is not tested, a diagnosis of Strangles may be missed and S. equi may spread throughout the barn.

Strangles is rarely fatal. However, death can happen due to airway obstruction or the development and rupture of internal abscesses, leading to septic shock.

Less common clinical signs

Some horses develop disease associated with S. equi infection in other parts of the body, including:

  • disease associated with an abscess in the chest or abdominal cavity, called a metastatic abscess. Signs will depend on the cavity where the abscess is located but can include:
    • weight loss
    • intermittent fever
    • colic
    • abnormal breathing
    • abnormal heart rhythm
  • immune-mediated inflammation of the blood vessels visibly affecting the skin and mucous membranes, called purpura hemorrhagica.
  • immune-mediated inflammation of the muscle, called S. equi myositis. This occurs most commonly in Quarter Horses and will cause significant muscle wasting as well as increased muscle enzymes in the blood.


A Strangles diagnosis is confirmed through laboratory testing. Veterinarians will take a:

  • swab of nasal discharge or discharge from an abscess
  • swab from the back of the throat
  • sample obtained from collecting fluid used to “wash” the back of the throat and nasal area (nasopharyngeal lavage)
  • sample taken directly from both guttural pouches (guttural pouch lavage). There are 2 guttural pouches, one on either side of the head behind the jawbone and below the ear. They contain vital nerves and blood vessels and provide a warm, moist environment for S. equi to grow when infected.

The most practical method used for screening and diagnosis of Strangles in the early stages of infection is the nasopharyngeal lavage. A horse should not be sampled too early, however, such as at the fever stage, to avoid a false negative result.

The guttural pouch lavage is the most sensitive test for determining persistence of S. equi infection after the clinical signs are no longer present. To perform the procedure, the veterinarian may pass a catheter into the guttural pouch by feel. However, most veterinarians are more comfortable obtaining a guttural pouch sample using an endoscope, which is a piece of equipment that allows them to visualize the inside of the pouch for the presence of pus or chondroids (hardened pus). The veterinarian may then take a sample from the pouch directly through the endoscope or use the endoscope to ensure proper placement of a catheter to obtain a sample.

  • Regardless of the type of sample obtained, it is submitted to a veterinary diagnostic laboratory for testing. A veterinarian may request: a polymerase chain reaction (PCR) to detect the DNA of the bacterium
  • a bacterial culture

The advantage of the PCR test is that it can detect small numbers of bacteria and is, therefore, more likely than a culture, to catch subclinical or “carrier” horses. It is also much faster with same day results possible in exceptional circumstances. The drawback of the PCR test is that it only detects DNA and the bacteria may be alive or dead.

A bacterial culture is useful for identifying S. equi as the cause of the abscess or discharge as well as providing the ability to determine the strain as well as the sensitivity to different antibiotics if needed. The drawbacks of culture include a prolonged time until a result is obtained, and it will miss low numbers of bacteria.

In Ontario, a blood test, called SeM ELISA, is available to determine the presence of S. equi antibodies. It may indicate infection for up to 6 months or more, thus an increase in titre may not represent recent exposure. Antibodies may also increase after vaccination. This test is used predominantly to identify those horses who may be at risk for developing purpura hemorrhagica if vaccinated for Strangles. This test is also used to support a diagnosis of metastatic abscesses or purpura hemorrhagica.


Most horses with Strangles are not treated. The lymph nodes are allowed to mature and rupture. Sometimes the process may be quickened by hot packing the nodes or using a drawing agent recommended by your veterinarian.

Antibiotic use is reserved for those horses that have difficulty breathing or for foals that may develop complications. Horses that have difficulty breathing due to Strangles, should be seen immediately by your veterinarian.

Horses diagnosed with metastatic strangles, purpura hemorrhagica or S. equi myositis may also be treated with long term antibiotics.

In certain challenging outbreak situations, antibiotics may also be used to slow the outbreak down. It is important to remember that in situations where antibiotics are used for outbreak management, infection control procedures within the facility must be strictly enforced as those horses on antibiotics do not develop a strong immune response to S. equi and will become re-infected when exposed to a contaminated environment.

Outbreak management

Infected horses

When a horse is diagnosed with Strangles, it should be isolated immediately to prevent further spread of the bacteria. Ideally the horse should be placed in an empty barn or paddock away from other horses. Fly control is important if the abscesses are open and draining as flies can be a vector for disease spread to nearby horses.

Only barn staff and equipment dedicated to the management of that horse should be used. This means staff and equipment must not be going back and forth between healthy and diseased animals. Staff working with infected horses should wear protective clothing (such as coveralls), boot/shoe coverings (or use a foot bath) and gloves. There should be no shared water sources between infected and healthy animals.

Infected horses should remain isolated for a minimum of 21 to 30 days after the clinical signs have resolved. At this point, the horse should be tested to ensure it has not become an asymptomatic carrier. Testing can be completed by performing a minimum of three nasopharyngeal washes one week apart or a single guttural pouch lavage (where both pouches are sampled). If the horse continues to test positive with either sample technique, the horse’s guttural pouches should be evaluated by endoscopy and treated appropriately.

Other horses on the farm

After the horse with Strangles has been diagnosed, the farm/facility manager should stop all movement of horses on and off the farm. Human movement should also be limited. The attending veterinarian, along with the farm/facility owner will determine which other horses on the farm will have been exposed to the infected horse. Horses can be exposed directly through nose-to-nose contact with the infected horse or indirectly by exposure to people or equipment that have touched the infected horse.

Veterinarians may suggest using coloured electrical tape wrapped on the halter or placed on the stall to indicate the exposure level of the horse.

Red tape indicates that the horses have been infected. These horses should be isolated from other horses and monitored daily for any change in clinical signs. The day the clinical signs are no longer visible should be recorded for the veterinarian.

Yellow tape indicates that the horses have been exposed, while green tape indicates that the horses have not been exposed. Either way, these horses’ rectal temperatures should be determined twice daily and recorded for the veterinarian. Any horse that develops a fever should be placed in the red group as the fever precedes the shedding of bacteria. Isolating a horse at the fever stage is a good way to prevent further spread on the facility. Yellow and green horses are visually monitored for any abnormal health signs and the veterinarian informed if any occur.

Dedicated staff and equipment should be used to look after each of the above groups. If utilizing dedicated staff is not possible, staff should work from the least affected group to the most affected group (green to yellow to red), changing clothing and cleaning and disinfecting hands and footwear between groups. When possible, equipment should not move between groups and should be identified using the appropriate colour tape according to the group of horses for which they are being used.

Horses in these groups are monitored as above for 21 to 30 days. If no horses develop clinical signs, then the movement restrictions may be removed. The horses with or recovering from Strangles in the isolation barn/area will still remain in isolation until released by the attending veterinarian. Due to the variation in isolation and infection control procedures at different farms and facilities, the veterinarian may, at their discretion, keep all movement restrictions in place until affected horses have been released from isolation.

It is recommended that horses be tested before leaving isolation to ensure they have cleared the infection. They should be tested no sooner than 21 days after the resolution of clinical signs and should be sampled using 3 nasopharyngeal lavages 1 week apart or a single guttural pouch lavage. All samples should be negative on S. equi PCR.

S. equi carrier horses

About 10% of horses that recover from Strangles become carriers of S. equi and are thought to be the source of most infections on a farm or facility. The bacteria live in the guttural pouches and may exist there for months to years. Although these horses appear healthy, they intermittently shed the bacteria from the nose and can be a risk to other horses.

Infection control

For disinfectants to work effectively, they need to be in contact with the surface of interest for a period of time specified by the manufacturer. For this reason, foot baths or disinfectant mats are no longer recommended for infection control as people tend to quickly immerse their boot in the bath or walk quickly over the mat, disregarding contact time. Using disposable boot or shoe covers or dedicated footwear such as rubber boots are preferred ways to control disease spread by foot traffic. If foot baths must be used due to specific management decisions, they should be placed at the entrance and exit to the barns housing infected and exposed horses. Foot baths should contain products that are active in the presence of organic material (such as manure) and include Virkon and Prevail. Bleach is not an appropriate agent unless people are willing to clean all organic debris from their footwear prior to dipping in the bath. Foot baths should be changed when dirty and at least daily.

Gloves, dedicated boots or boot covers and protective clothing (such as disposable gowns, coveralls) should be used when working with infected horses. Gloves should be removed after handling the horses or hand sanitizer used. Protective clothing and boots should remain in the isolation area so as not to contaminate other areas of the facility.

If the exposed horses are turned out in paddocks in different groups, it is important to treat each group as a cohort. That means hands should be sanitized (or gloves changed) between handling each group, protective clothing changed and boots cleaned.


Horse owners can reduce the risk of Strangles by vaccinating their horse against S. equi infection. Anecdotally, the intranasal vaccine (the only type available in Canada) either protects horses from disease or drastically reduces the clinical signs of disease. In some situations the vaccine may cause mild signs of Strangles since it contains a modified live bacterium. Therefore, when possible, vaccinated horses should be kept separate from non-vaccinated horses for 48 hours after vaccination. No other veterinary procedures should be done on the same day as the vaccination is given.

Through testing, veterinary diagnostic laboratories can determine if S. equi cultured from a horse is due to the vaccine strain or the bacterium.

Generally, horses should not be vaccinated during an outbreak.

It is recommended that horses with unknown exposure to S. equi be tested for their antibody titre to S. equi (SeM ELISA) prior to vaccination. Horses with high titres (>1:1600) are at a higher risk of developing purpura hemorrhagica or immune mediated myositis if vaccinated.

Farm and facility owners can reduce the risk of a Strangles outbreak by quarantining new horses or horses returning from another facility, such as a training stable or competition grounds, where they have been commingling with horses of unknown health status for more than 2 weeks. These horses should also be tested for S. equi shedding. New horses should be quarantined for at least 14 days and monitored twice daily for fever and other signs of infection.

Obtaining a sample from a nasopharyngeal lavage or guttural pouch lavage and testing with PCR should also be performed to determine if the horse is an S. equi carrier. Carrier horses will not show any clinical signs. It should be noted that a single nasopharyngeal test is imperfect as many carriers are intermittent shedders and a single negative test only indicates the horse was negative at the time of testing. This can be useful, however, in detecting recently infected or carrier horses particularly if testing occurs towards the end of the 14-day quarantine period.

Public health

S. equi is not generally considered a human health concern. However, there has been the rare report of horse to human transmission of this bacteria. If your immune system is compromised due to illness and/or medication, it is recommended that you discuss the risks of exposure to S. equi with your physician.