Reining in strangles: absence of disease in horses vaccinated with a DIVA compatible recombinant fusion protein vaccine, Strangvac, following natural exposure to Streptococcus equi subspecies equi

Background: Strangles, caused by Streptococcus equi subspecies equi ( S. equi), is a prevalent infectious disease of horses. This is the first report on protection conferred by a new vaccine, Strangvac, against natural infection. Objectives: To measure the effects of Strangvac vaccination during an outbreak of strangles at a Swedish farm. Study Design: Longitudinal cohort study. Methods: Healthy horses ( n=17) were vaccinated on day 0, which occurred 23 days after strangles was first confirmed in three unvaccinated horses on the same farm. Blood serum samples were collected on day 0, day 28 and day 489. A combined iELISA was used to measure total antibody titres towards the components of the vaccine. Specific antibody levels in blood sera to S. equi were also quantified using the antigen A/C iELISAs, which can differentiate infected from vaccinated animals. Clinical signs were monitored. Results: All vaccinated horses had increased total antibody titres to CCE, Eq85 and/or IdeE from day 0 (2.50±0.28) to day 28 (3.63±0.31; p<0.0001). Seropositivity in the antigen A/C iELISA was noted in 8/17 healthy horses on day 0 (prior to vaccination), in 9/16 vaccinates on day 28, and in 3/3 unvaccinated clinical cases on day 28. None of the vaccinated horses developed strangles, but 5/7 of the original vaccinated horses tested seropositive in the A/C iELISA on day 489, which was 377 days after their third vaccination. Main limitations: Aside from the three unvaccinated clinical cases, no unvaccinated control group was available. Conclusions: Despite adherence to recommended biosecurity measures, serological evidence of exposure to S. equi was demonstrated in half of the vaccinated horses. Notably, all horses vaccinated during the outbreak responded to the vaccine components and remained healthy, supporting the protective effects of vaccination of healthy horses with Strangvac in outbreak situations.