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.
Abstract: Strangles, caused by Streptococcus equi subspecies equi (S. equi), is a prevalent infectious disease of horses. This is the first report on the use of a new vaccine, Strangvac, in a natural outbreak of strangles. Objective: To measure the effects of Strangvac vaccination during an outbreak of strangles at a Swedish farm. Methods: 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. Conclusions: 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, suggesting a protective effect of vaccination of healthy horses with Strangvac in outbreak situations.
© 2025 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
Publication Date: 2025-11-23 PubMed ID: 41276995PubMed Central: PMC12892393DOI: 10.1111/evj.70125Google Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
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Overview
- This study evaluates the effectiveness of a new vaccine called Strangvac in protecting horses from strangles, a common infectious disease caused by Streptococcus equi subspecies equi, during a natural outbreak on a Swedish farm.
- Results show that vaccinated horses developed antibodies to the vaccine and did not get sick despite exposure to the bacteria, suggesting Strangvac offers protection during outbreaks.
Background
- Strangles is a contagious disease in horses caused by the bacterium Streptococcus equi subspecies equi (S. equi).
- It leads to abscesses and respiratory illness, causing economic and welfare issues in horse populations.
- Vaccination is a key strategy to control strangles; however, existing vaccines have limitations, and distinguishing vaccinated from infected animals (DIVA – Differentiating Infected from Vaccinated Animals) is important for disease management.
- Strangvac is a new recombinant fusion protein vaccine designed to be DIVA-compatible, allowing differentiation via specific antibody tests.
Study Objective
- To assess the immune response, safety, and protective effect of Strangvac in healthy horses exposed naturally during a strangles outbreak.
- To monitor clinical signs of disease and antibody levels over time in vaccinated horses compared to unvaccinated ones.
Methods
- Study Design: Longitudinal cohort on a Swedish horse farm experiencing a natural strangles outbreak.
- Participants: 17 healthy horses vaccinated with Strangvac on day 0, approximately 3 weeks after initial cases appeared; 3 unvaccinated horses exhibited clinical strangles.
- Sampling Timeline: Blood serum collected at three key points – day 0 (before vaccination), day 28 (post-vaccination), and day 489 (long-term follow-up).
- Antibody Testing:
- Combined indirect ELISA (iELISA) measured total antibody titers against vaccine components (CCE, Eq85, IdeE proteins).
- Antigen A/C iELISA distinguished antibodies induced by infection from those induced by vaccination, enabling monitoring of natural exposure versus vaccine response.
- Clinical Monitoring: Horses were observed for symptoms of strangles throughout the study.
Results
- Vaccinated Horses:
- Significant increase in antibody titers to vaccine components occurred after vaccination (mean titer rose from 2.50 to 3.63 at day 28; statistically significant with p < 0.0001).
- None of the vaccinated horses developed clinical strangles during the outbreak, despite exposure.
- At day 489, 5 of 7 vaccinated horses remained seropositive in the infection-specific A/C iELISA, indicating prior exposure or long-term immune response.
- Half of the vaccinated horses showed serological evidence of having been exposed to S. equi based on antigen A/C iELISA, despite no clinical disease.
- Unvaccinated Horses:
- Three horses not vaccinated developed clinical strangles.
- Seropositivity in antigen A/C iELISA confirmed active infection in these clinical cases.
Conclusions
- Strangvac vaccination during a natural strangles outbreak was associated with:
- Robust immune response to vaccine components.
- Prevention of clinical strangles disease in vaccinated horses.
- Ability to track exposure vs. vaccination using DIVA-compatible antibody tests.
- Despite the presence of the disease on the farm and serological evidence of exposure, vaccinated horses remained healthy.
- Limitations:
- No unvaccinated control group other than the few clinical cases, limiting direct comparison.
- Study was observational on a single farm during a specific outbreak.
- Implications:
- Strangvac shows promise as a protective vaccine for use during strangles outbreaks in healthy horses.
- DIVA-compatibility allows for distinguishing natural infection from vaccination, aiding disease control efforts.
- Supports vaccination as a strategy to control spreading of strangles in equine populations.
Cite This Article
APA
Gröndahl G, Righetti F, Aspán A, Bjerketorp J, Frosth S, Frykberg L, Jacobsson K, Guss B, Paillot R, Flock JI, Henriques-Normark B, Waller AS.
(2025).
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.
Equine Vet J, 58(2), 476-485.
https://doi.org/10.1111/evj.70125 Publication
Researcher Affiliations
- Department of Animal Health and Microbial Strategies, Swedish Veterinary Agency, Uppsala, Sweden.
- Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Solna, Sweden.
- Department of Animal Health and Microbial Strategies, Swedish Veterinary Agency, Uppsala, Sweden.
- Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
- Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
- Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
- Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
- Department of Animal Biosciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.
- Intervacc AB, Stockholm, Sweden.
- Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Solna, Sweden.
- Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Solna, Sweden.
- Clinical Microbiology, Karolinska University Hospital Solna, Stockholm, Sweden.
- Intervacc AB, Stockholm, Sweden.
MeSH Terms
- Animals
- Horses
- Horse Diseases / prevention & control
- Horse Diseases / microbiology
- Horse Diseases / epidemiology
- Streptococcal Infections / veterinary
- Streptococcal Infections / prevention & control
- Streptococcal Infections / microbiology
- Streptococcal Infections / epidemiology
- Streptococcus equi / immunology
- Longitudinal Studies
- Antibodies, Bacterial / blood
- Bacterial Vaccines / immunology
- Streptococcal Vaccines / immunology
- Female
- Sweden / epidemiology
- Male
- Vaccination / veterinary
- Disease Outbreaks / veterinary
- Disease Outbreaks / prevention & control
- Streptococcus
Grant Funding
- Intervacc AB
Conflict of Interest Statement
Andrew S. Waller and Romain Paillot are employed by Intervacc AB. Bengt Guss and Jan‐Ingmar Flock are founders and former board members of Intervacc AB. Francesco Righetti, Sara Frosth, Joakim Bjerketorp, Lars Frykberg, Karin Jacobsson, Bengt Guss and Birgitta Henriques‐Normark have received research grants from Intervacc AB.
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Citations
This article has been cited 1 times.- Rask E, Righetti F, Ruiz A, Bjerketorp J, Frosth S, Frykberg L, Jacobsson K, Guss B, Flock JI, Henriques-Normark B, Hartman E, Gustafsson A, Paillot R, Waller AS. Closing the Stable Door on Strangles: Serological Responses of Vaccinated Horses on a Farm Following the Arrival of a New Horse. Animals (Basel) 2025 Dec 13;15(24).
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