Bulletin #1009, Equine Facts: Facts on Strangles (Streptococcus equi) Infections in Horses

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Reviewed and approved by: Dr. Robert Causey, Ph.D., D.V.M., Department of Animal and Veterinary Science, The University of Maine, Orono, Maine; Dr. Jeff Fay, Ph.D., D.V.M., Annabaessacook Veterinary Hospital, Monmouth, Maine, Board Member, Maine Equine Industry Association; Dr. Chip Ridky, D.V.M., Maine State Veterinarian, Augusta, Maine; James A. Weber, Ph.D., D.V.M., Department of Animal and Veterinary Science, The University of Maine, Orono, Maine.

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  • Caused by bacteria — Streptococcus equi — that only causes disease in equine species (horses, donkeys, and mules). Normally not a danger to humans or other domestic species.
  • Reportable disease in Maine, although there is currently no legally enforceable quarantine period.
  • Highly contagious and often associated with facilities that experience a high turnover of horses (fairs, horse sales, competitive events, boarding stables).
  • The incubation period (time from exposure to first signs of disease) is 3 to 21 days.
  • Disease signs are usually visible for three to seven days but may last for up to two weeks.
  • Most animals recover from strangles with no long-term aftereffects.


  • Decreased appetite and depression.
  • Fever (greater than 102 degrees F).
  • Cloudy discharge from both nostrils.
  • Superficial lymph nodes become inflamed, enlarged and tender to the touch. These lymph nodes are located under the jaw, in the throat latch area of the upper neck, and rarely, over the eye sockets or in the chest or abdominal cavity. The inflammation is due to the accumulation of purulent fluid (pus) within the lymph node.
  • Affected horses may stand with neck outstretched to relieve pressure in the upper neck. Later in the disease, lymph nodes often burst and drain pus from openings in overlying skin.
  • Complications of disease are seen in 10 to 20 percent of cases (these should be considered emergency situations requiring immediate veterinary care).
  • Difficulty in breathing and signs of distress due to compression of trachea by enlarged lymph nodes in the neck. This can lead to death by asphyxiation (source of the name “strangles”) and may require emergency lymph node drainage or insertion of a breathing tube through the trachea.
  • Life-threatening infections caused by an internal release of pus from deeper lymph nodes into the chest or abdominal cavity (internal or “bastard” strangles).
  • An allergic reaction to S. equi can cause blood vessel inflammation with swelling and edema of the legs (purpura hemorrhagica).
  • Strangles is often fatal in young foals (less than six months old) that did not receive sufficient colostral immunity as newborns.
  • Strangles is sometimes considered to be a disease of foals and young adult horses. Older horses are often immune to strangles due to prior exposure during their youth, or they may display only lethargy and nasal discharge without obvious lymph node swelling (however, these animals can still spread the disease).

Transmission of Strangles

  • Strangles is caused by oral exposure of a horse to S. equi bacteria. Once within the oral cavity, the bacteria invade the tonsils and subsequently colonize the lymph nodes.
  • Bacteria can be transmitted through contact with pus or nasal discharges from an infected horse, or from contaminated bedding or barn equipment (water troughs, buckets, etc.).
  • Flies may also act as vectors, spreading the bacteria from horse to horse.
  • Under the right conditions, S. equi can survive in the environment for weeks or months.
  • Exposure of a horse to S. equi does not necessarily mean that it will come down with strangles. Factors that influence the risk of disease include
    • a dose of bacteria (poor sanitation and direct contact with nasal secretions and pus increase the chance of disease);
    • immune status of the horse. Previously exposed horses are often immune to the disease or do not get as sick as unexposed horses. During the first three to six months of life, foals are often protected by maternal antibodies. Vaccination can also increase resistance to the disease;
    • stress (poor nutrition, overcrowding, lengthy transportation or pre-existing diseases increase the risk of strangles).
  • Strangles may be transmitted by “silent shedders” who do not display signs of disease. These horses commonly carry the strangles organism in the guttural pouch, an air sac at the back of the horse’s throat. Detection of these animals requires guttural pouch endoscopy (passing an endoscope via the horse’s nose into the guttual pouch). Strangles is most commonly transmitted by acutely ill or recovering horses that are still shedding bacteria in their nasal secretions. Bacterial culture results have a turnaround time of 2 to 3 days. The DNA test known as Polymerase Chain Reaction (PCR) takes less than a day. However, it may take an additional 1 to 2 days to send samples to the laboratory.

Treatment of Strangles

  • Due to the possibility of serious complications, a veterinarian should always be consulted when strangles is suspected. Since strangles is a reportable disease in Maine, your veterinarian is obligated to report any confirmed cases to the state veterinarian.
  • Although strangles can be diagnosed based on symptoms alone, it is confirmed by identification of the causative organism in fluids from the throat, nasal passages or abscesses. Bacterial culture results have a turnaround time of two to three days. A new DNA-based test has recently been described that could confirm a diagnosis in less than one day.
  • Regularly observe sick animals for signs of shock, high fever (above 103 degrees F), and difficulty in breathing or abnormal swelling of the legs or ventral abdomen.
  • Swollen lymph nodes may be softened by the application of hot compresses. This may hasten abscess drainage. Large abscesses that interfere with breathing may be lanced under veterinary supervision.
  • If the horse is cooperative, draining abscesses may be cleaned with dilute antiseptic washes to promote skin healing and avoid secondary infections.
  • Although S. equi is usually sensitive to antibiotics such as Penicillin, antibiotic treatment is generally discouraged if a horse is already exhibiting signs of disease such as fever and nasal discharge. Many veterinarians believe that antibiotic treatment at this time may increase the risk of complications such as internalized abscesses.
  • Antibiotic treatment of strangles-exposed horses before they display signs of the disease may reduce the rate of morbidity (percentage of sick animals). This is still a controversial subject and the final decision should be left to your veterinarian.
  • In contrast, complicated cases of strangles may require intensive antibiotic and supportive veterinary therapy to save the horse’s life.

Prevention of Strangles

  • Several S. equi vaccines are commercially available. These can reduce the rate of disease in the face of an outbreak, but are only about 50 percent effective under the best conditions. Vaccination of healthy animals on a farm during an outbreak is usually not effective since at least two weeks are required for the immune system to respond to the vaccine. Current vaccines produce only short-term (six months to one year) immunity and require regular boosters to maintain adequate protection.
  • Both intramuscular and intranasal vaccines are available. In rare cases, vaccine reactions may occur. Vaccinating a horse that is recently recovered from strangles, or has a high vaccination titre, may lead to a serious condition known as purpura hemorrhagica. Always discuss with your veterinarian the pros and cons of vaccinating for strangles. Vaccination with the intranasal vaccine during an outbreak may give false positives on the PCR test.
  • Following an outbreak at least 3 samples from nasal wash or swab should be taken at approximately weekly intervals from all recovered cases and their contacts and tested for S. equi by culture and PCR. Outwardly healthy appearing horses should be examined by guttural pouch endoscopy.
  • Although it is the most difficult measure to follow, quarantining all new animals on your facility (and resident horses exposed to “outside” horses) is the most effective way to prevent a strangles outbreak. Quarantine means no direct contact of a new horse with other animals or equipment used by other horses.
  • Horses that are disease-free after two to three weeks of quarantine pose almost no risk of disease transmission unless they are a “silent shedder.”
  • As soon as you suspect that one of your horses has strangles, isolate it from all other animals in your facility. Paddocks used by an infected animal should be considered contaminated for at least one month.
  • To decrease the risk of your horse contracting strangles during travel to a “foreign” facility, take care to minimize direct contact with other horses, disinfect all food and water containers before use, and make sure that box stalls are thoroughly cleaned and disinfected between horses. Minimizing travel-related stress on your horse will also reduce its risk of contracting strangles if it is exposed to a contagious horse.
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Reprinted with permission from the Maine Equine Industry Association

Published with funding provided by the Maine Farm Bureau Horse Industry Council

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