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Streptococcus equi subspecies equi (S. equi var. equi) is the bacterium which causes the highly contagious disease strangles (also known as “distemper”).  Strangles commonly affects young horses (weanlings and yearlings), but horses of any age can be infected. Vaccination against S. equi is recommended on premises where strangles is a persistent endemic problem or for horses that are expected to be at high risk of exposure. Following natural infection, a carrier state of variable duration may develop and intermittent shedding may occur. The influence of vaccination on intermittent shedding of S. equi has not been adequately studied.

The organism is transmitted by direct contact with infected horses or sub-clinical shedders, or indirectly by contact with water troughs, hoses, feed bunks, pastures, stalls, trailers, tack, grooming equipment, nose wipe cloths or sponges, attendants’ hands and clothing, or insects contaminated with nasal discharge or pus draining from lymph nodes of infected horses. Streptococcus equi has demonstrated environmental survivability particularly in water sources and when protected from exposure to direct sunlight and disinfectants, and can be a source of infection for new additions to the herd.

Infection by S. equi induces a profound inflammatory response. Clinical signs may include fever (102-106o F); dysphagia or anorexia; stridor; lymphadenopathy (+/- abscessation); and copious mucopurulent nasal discharge.

S. equi and S. zooepidemicus are antigenically similar organisms. However, exposure to, or vaccination against, one does not confer reliable immunity to the other.

Following natural or vaccinal exposure to streptococcal antigens, certain individuals may unpredictably develop purpura hemorrhagica, an acute, non-contagious syndrome caused by immune-mediated, generalized vasculitis. Clinical signs develop within 2 to 4 weeks following natural or vaccinal exposure to streptococcal antigens. Clinical signs may include urticaria with pitting edema of the limbs, ventral abdomen and head; subcutaneous and petechial hemorrhage; and sloughing of involved tissues. Severe edema of the head may compromise breathing. Immediate medical attention should be sought for individual horses suspected of having purpura hemorrhagica.


Vaccination in the face of an outbreak should be carefully considered, as there is significantly increased risk of adverse reactions in exposed horses.  Purpura hemmorrhagica can be associated with vaccine administration. In a recent retrospective study of 53 horses with purpura hemorrhagica, 5 cases were vaccinated with a S. equi M protein vaccine. Outbreak mitigation and the prevention of spread of S. equi infection are centered on management of horses, personnel, and facilities.

(View AAEP Infectious Disease Control Guidelines—S. equi.; view ACVIM Strep equi consensus statement) 

Killed vaccines:

Killed vaccines are an adjunct to the prevention of strangles. Vaccination with these products should not be expected to prevent disease. However, appropriate pre-exposure vaccination with these products appears to attenuate the severity of clinical signs in affected horses, should disease occur, and has been shown to reduce the incidence of disease by as much as 50% during outbreaks.

All injectable, inactivated S. equi. vaccines, can be associated with an increased rate of injection site reactions as compared to other equine vaccines. Due to the limited variability between commercially available vaccinal bacteria and field isolates, autogenous bacterins are not advocated.

Modified live vaccine: 

An intranasal product has been shown to stimulate a high level of immunity against experimental challenge. The inductive sites are the pharyngeal and lingual tonsils. Vaccinal organisms must reach these sites in sufficient numbers to trigger protective responses; therefore, accurate vaccine delivery is critical to vaccine efficacy. In a small percentage of cases, residual vaccinal organism virulence may result in formation of slowly developing mandibular or retropharyngeal abscesses. The risk of vaccine-associated adverse events is increased when the product is administered to young foals.

Maternal antibody interference with respect to the development of mucosal immunity needs to be studied further.

In order to avoid inadvertent contamination of other vaccines, syringes and needles, it is advisable and considered a good practice to administer all parenteral vaccines or other injectables before the handling and administration of the intranasal vaccine against S. equi.

Vaccination Schedules:

Adult horses previously vaccinated:  Vaccinate every 6 to 12 months based on risk assessment and manufacturers’ recommendations.

Adult horses unvaccinated or having unknown vaccinal history

Killed vaccine:

Manufacturers' recommendations are for primary vaccination with a series of 2 or 3 doses administered at intervals of 2 to 4 weeks, depending on the product used, followed by annual revaccination. Revaccinate at 6- month intervals, regardless of the injectable product used.

Modified live vaccine:

Administer intranasally a 2-dose primary series with a 3-week interval between doses. Semiannual (6-month intervals) or annual revaccination is recommended.

Broodmares previously vaccinated 

Killed vaccine:

Vaccinate 4 to 6 weeks pre-partum with approved products that contain inactivated M-protein. Maternal antibody interference is not known to occur when injectable, M-protein vaccines are administered.

Broodmares previously unvaccinated or having unknown vaccinal history

Administer primary series of killed vaccine containing M-protein (see above, Adult horses unvaccinated) with final dose to be administered 4 to 6 weeks pre-partum.


Killed vaccine:

For foals at high risk for exposure to strangles, administer a 3-dose primary series of an M-protein product beginning at 4 to 6 months of age. An interval of 4 to 6 weeks between doses is recommended.

Modified live vaccine

Administer intranasally at 6 to 9 months of age a 2-dose primary series with a 3-week interval between doses. This vaccine has been safely administered to foals as young as 6 weeks of age when there is a high risk of infection, such as occurs during an outbreak, but the efficacy of its use in very young foals has not been adequately studied. If administered to young foals in this manner, a third dose of the modified live vaccine should be administered 2 to 4 weeks before the foal is weaned to optimize protection during that time of high risk of infection. The risk of vaccine-associated adverse events is increased when the product is administered to young foals.

Horses having been naturally infected and recovered:  Following recovery from strangles, most horses develop a durable immunity, persisting in over 75% of animals for 5 years or longer. This indicates that stimulation of a high level of immunity is biologically feasible given appropriate presentation of protective immunogens. Currently, a diagnostic test is available and may be used to assess the level of immunity conferred by natural exposure or vaccination. Since natural exposure or vaccination can provide variable levels of immunity, use of this test may provide a guideline in determining the need for current or future vaccination. Additional testing information is available from; ACVIM Strep equi consensus statement.