Closing the Stable Door on Strangles: Serological Responses of Vaccinated Horses on a Farm Following the Arrival of a New Horse

Infection of susceptible horses with <i>Streptococcus equi</i> subspecies <i>equi</i> (<i>S. equi</i>), the causative agent of strangles, is associated with commingling. Exposure may occur among horses at equestrian events, sales, or horses moved among different equine stabling environments. Strangles can affect all horses on a farm, leading to the death of up to 10% of cases depending on their immunity status at the time of infection, the development of complications, the success of biosecurity measures, and the use of vaccination. The current retrospective study used ELISAs to measure the exposure of horses to <i>S. equi</i> at a farm that experienced an outbreak of strangles shortly after the introduction of a new horse on the same day that the majority of the 17 resident horses were vaccinated with Strangvac for the first time. One vaccinated horse, which subsequently tested positive for <i>S. equi</i> and EHV-4, developed a cough, elevated body temperature, and nasal discharge 11 days after the first vaccination. Two other horses developed fever for one day at 22 days post-first vaccination, but only one had serological evidence of exposure to <i>S. equi</i>. All vaccinated horses had high antibody titres to vaccine components, whilst 7 of the 17 resident horses, and the new arrival, tested seropositive for exposure to <i>S. equi</i>. Although 3 out of the 17 vaccinated horses developed mild signs of disease before second vaccination, serological data support the effectiveness of vaccination in resident populations of horses to minimise the risk of strangles following the introduction of a new horse.