Strangles is a highly contagious bacterium that can spread quickly. Horses that travel the show circuit and horses that are newly introduced to a barn can infect other healthy horses. Understanding how it is transmitted and treated can help you control the spread of strangles (Streptococcus equi).

How It’s Transmitted

While all horses are susceptible, strangles is a disease most commonly contracted by groups of weanlings, yearlings, and horses less than five years of age. Transmitted directly as well as indirectly, it is a particularly difficult condition to control, especially in a herd situation. If vulnerable, a horse that comes into contact with another horse in the incubation stage, has just recovered from an episode, or is a long-term carrier, is at high risk for infection. Indirect contact can be equally as insidious. Buckets, feed, walls, doors, grass, fences, and water troughs can all become a catalyst by which horses sharing stable or pasture environments are quickly infected.

The Symptoms

The incubation period can be anywhere from four days to two weeks after exposure, depending on climate conditions or the level of stress to which a horse is exposed. Factors include prolonged travel time, crowded conditions, poor sanitation, inadequate nutrition, unsuitable housing, and introduction to a new environment.

Watch for these signs:

  • Lethargy or depression
  • Loss of appetite
  • Enlarged lymph nodes under the jaw
  • Thick, yellow nasal discharge
  • Fever—102° F to 103° F
  • Slight cough
  • Trouble swallowing
  • Abscesses which often burst and drain thick, yellow, highly contagious pus
  • Standing with neck extended due to pain in the throat area


The general consensus is to establish a regular vaccination program, with boosters administered prior to periods of increased stress. Mares, stallions, and performance horses before the breeding and show seasons respectively, and horses who are about to be transported over long distances, are the most likely candidates for immunization.

There are two types of vaccines available: killed and live. In either case, your veterinarian should be consulted to determine or administer what is right for your situation.

The more recently-introduced live vaccine is administered intranasally twice at intervals of one to two weeks. Because it produces the local antibodies necessary to provide immunity, it is more appealing than its traditional counterpart, but, because it is a low-virulence organism, the risk of inadvertent contamination makes careful handling extremely important. It is strongly recommended not to administer other medications or injections at the same time as administering a live vaccine.


The conventional treatment has been penicillin administered through a series of intramuscular injections followed by an annual booster shot. Even though there is the possibility of local swelling or even an abscess forming at the injection site, the reaction is not usually serious and should subside within a few days. Although the killed vaccine does not provide complete protection, there is a significant reduction in the severity of an outbreak, with fewer horses being affected, showing milder symptoms, and shorter recovery times for those who have been laid up. 

By Toby Raymond