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.
Authors: Gröndahl Gittan, Righetti Francesco, Aspán Anna, Bjerketorp Joakim, Frosth Sara, Frykberg Lars, Jacobsson Karin, Guss Bengt, Paillot Romain, Flock Jan-Ingmar, Henriques-Normark Birgitta, Waller Andrew S
Journal: Equine veterinary journal
Summary
Swedish researchers investigated the real-world protective efficacy of Strangvac, a novel recombinant fusion protein vaccine incorporating DIVA (Differentiating Infected from Vaccinated Animals) technology, by following 17 healthy horses vaccinated during a strangles outbreak on a single farm, with serology measured at days 0, 28 and 489 to track antibody responses and infection exposure. All vaccinated horses mounted a robust immune response to the vaccine antigens (CCE, Eq85 and IdeE), with antibody titres increasing significantly from 2.50±0.28 to 3.63±0.31 between day 0 and day 28 (p<0.0001), whilst serological evidence confirmed that approximately half of the vaccinated cohort had been exposed to Streptococcus equi subspecies equi during the outbreak. Crucially, none of the vaccinated horses developed clinical strangles disease despite documented exposure, whereas all three unvaccinated clinical cases showed definitive infection markers on day 28, and the DIVA-compatible serology allowed researchers to distinguish vaccine-induced antibodies from infection-induced responses—a key advantage for disease surveillance and trade purposes. The persistence of antibody responses, with 5/7 vaccinated horses remaining seropositive 377 days after their third dose, indicates durable immunity. This field trial demonstrates that Strangvac offers meaningful clinical protection during outbreak conditions and merits further investigation in larger, controlled vaccination studies to establish efficacy rates and optimal dosing schedules for endemic farm management.
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Practical Takeaways
- •Strangvac vaccination appears to prevent clinical strangles even when vaccinated horses are exposed to S. equi during outbreak situations on the same farm
- •DIVA-compatible design allows differentiation between vaccinated and naturally infected animals, useful for outbreak management and disease monitoring
- •Consider vaccinating healthy horses early in a strangles outbreak as vaccination during exposure may confer protection without preventing subclinical infection/immunity development
Key Findings
- •All 17 vaccinated horses showed increased antibody titres to vaccine components (CCE, Eq85, IdeE) from day 0 to day 28 (p<0.0001)
- •Zero clinical cases of strangles developed in vaccinated horses despite serological evidence of exposure in 8/17 on day 0 and 9/16 on day 28
- •Seropositivity in antigen A/C ELISA (infection marker) was detected in 5/7 vaccinated horses at day 489, indicating prior exposure without clinical disease
- •All three unvaccinated clinical cases tested positive in antigen A/C ELISA on day 28, contrasting with protection in vaccinated cohort