What is Strangles?
Streptococcus equi subspecies equi
(S. 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.
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.
S. equi and S. equi subspecies zooepidemicus are antigenically similar organisms. However, exposure to, or vaccination against, one does not confer reliable immunity to the other.
The injectable, inactivated S. equi. vaccine 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 vacinne
An intranasal, modified live bacterial vaccine 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. After administration of the modified live vaccine a small number of horses may experience noncontagious transitory upper respiratory signs including nasal discharge or lymphadenopathy, especially in animals less than 2 years of age.
Nasopharyngeal wash samples may be positive on PCR for up to 6
weeks after administration of the attenuated live vaccine strain. Culture of
nasopharyngeal wash samples may grow the vaccine strain for a few days
following IN vaccine administration.
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 (IM) or other injectables before the handling and
administration of the modified live intranasal vaccine against S. equi.
Adult horses previously vaccinated: Vaccinate annually based on risk assessment and manufacturers’ recommendations.
Adult horses unvaccinated or having unknown vaccinal history:
Broodmares previously vaccinated:
Broodmares previously unvaccinated or having unknown vaccinal history: