Strangles is an infectious, contagious disease of horses caused by Streptococcus equi equi bacteria. It is characterized by abscesses in the lymph tissue of the upper respiratory tract. Strangles is a highly contagious disease, but it has a low death rate in otherwise healthy horses. Transmission occurs through infected objects and direct contact with infectious secretions. Infected horses may become carriers that show no signs but harbor and spread the disease. Paddocks and barn facilities used by infected horses should be regarded as contaminated for about 2 months after resolution of an outbreak.
The incubation period of strangles is 3 to 14 days after exposure. The first sign of infection is usually fever. Within 24 to 48 hours of the initial fever spike, the horse will exhibit signs typical of strangles, including nasal discharge containing mucus and pus, depression, and swollen lymph nodes under the jaw. Horses with swollen lymph nodes at the back of the throat will have difficulty swallowing, noisy inhalation, and will extend the head and neck. Older animals with some immunity from a previous infection may develop a less typical form of the disease with nasal mucous discharge, cough, and mild fever. Metastatic strangles (sometimes called “bastard strangles”) is a condition in which lymph nodes in other parts of the body, such as the abdomen and chest, are affected.
The diagnosis of strangles is confirmed by bacterial culture of secretions from abscesses or nasal swab samples. Complicated cases may require endoscopic examination of the upper respiratory tract, ultrasonography of the throat, or x-ray examination of the skull to identify the location and extent of abscesses.
Although strangles is a bacterial infection, there are pros and cons to using antibiotics to treat it. Most researchers agree that antibiotic therapy will provide temporary improvement in fever and depression; however, it may also prolong the course of the disease by delaying maturation and drainage of the abscesses. Antibiotic therapy can also reduce the horse’s natural buildup of immunity, making it more susceptible to reinfection. Despite the disadvantages, antibiotic therapy is often necessary when the horse has labored breathing, difficulty swallowing, prolonged high fever, and severe lethargy or loss of appetite. Your veterinarian will weigh the pros and cons of antibiotic therapy based on your horse’s specific condition.
The veterinarian may recommend applying warm compresses to the sites of swollen lymph nodes to help abscesses drain more quickly. Ruptured abscesses may need to be flushed with dilute iodine or a similar treatment until drainage stops. Nonsteroidal anti-inflammatory drugs (NSAIDs) may be prescribed to reduce pain and fever and improve appetite in horses with rapidly developing disease.
The horse should be kept in an environment that is warm, dry, and dust-free. It should be isolated from any other horses as soon as strangles is suspected, and appropriate procedures to prevent spread of infection should be followed. Flies can transmit infection mechanically; therefore, efforts should be made to control the fly population during an outbreak. Individuals, such as trainers, who visit multiple horse facilities should wear protective clothing or change clothes prior to traveling to the next facility. Horses being newly introduced to a group should be carefully scrutinized for evidence of disease. Consult your veterinarian for advice on testing, vaccination, and quarantine procedures.
Most horses continue to shed the infectious bacteria for about 1 month following recovery from disease. Analysis of swabs from the nose and throat can be used by the veterinarian to assess whether it is safe to introduce (or reintroduce) a previously infected horse into a group.