Expert  Q & A

New Vaccine for Strangles in Development

By Adam Marcus

Strangvac is a recombinant vaccine built on 8 fragments of the genome of Streptococcus equi, the organism responsible for the highly transmissible strangles infection. Initially approved in Europe in 2021, the vaccine is now reportedly entering trials in the United States as a step toward regulatory review.

In a recent post-marketing study, researchers in Sweden reported Strangvac—from Intervacc and Dechra Veterinary Products—proved highly effective in a real-world outbreak of strangles, with no immunized animals developing the disease after exposure to the bacteria.

The Modern Equine Vet spoke with Gittan Gröndahl, DVM, PhD, of the Department of Animal Health and Microbial Strategies at the Swedish Veterinary Agency in Uppsala, who led the study.

MEV: Five of the 7 horses in the study remained seropositive for S. equi 377 days after their third vaccination. Did the Strangvac protocol induce sterilizing immunity, or did it only prevent clinical disease while allowing subclinical infection? Could the observed protection against clinical disease have been due to the vaccine mitigating subclinical infections, regardless of when the exposure occurred?

Dr. Gröndahl: This is a valid question; however, this field study does not allow conclusions regarding whether Strangvac induced sterilizing immunity. Vaccination is expected to reduce morbidity and mortality, and the implementation of appropriate biosecurity measures remains essential during outbreaks to further mitigate transmission risk.

The approved indication for Strangvac in the European SPC [Supplementary Protection Certificate] is the reduction of clinical signs, which is supported by the findings of this study, as no vaccinated horses developed clinical disease despite evidence of exposure. This outcome is relevant for individual animal welfare.

Vaccinated horses may still become infected with and shed S. equi according to the SPC, indicating that the immunity conferred is not sterilizing. However, experimental studies have demonstrated a marked suppression of bacterial shedding in vaccinated horses, both in magnitude and duration. This reduction is epidemiologically relevant, as it may decrease transmission, lower the effective reproduction number during outbreaks, and reduce the risk of prolonged movement restrictions and associated economic losses at affected premises.

Persistent carriage cannot be excluded in the absence of post-outbreak guttural pouch endoscopic examination and lavage. Seropositivity on A/C-iELISA indicates exposure to S. equi but does not allow determination of the timing, frequency, magnitude or persistence of infection. Prolonged seropositivity following exposure to S. equi is well recognized. Blood samples from newly introduced horses at the time of arrival were not available, precluding the determination of whether exposure occurred before or after arrival.

These data therefore neither confirm nor exclude the presence of persistently infected horses on the studied farm, or the possibility that the newly introduced horses exposed the resident horses to S. equi again.

Do you think the vaccine is equally effective when used for primary prevention (seronegative horses) as it is when used as a rapid booster during an active outbreak (seropositive horses)?

Dr. Gröndahl: Yes. No vaccinated horses developed clinical signs, and all mounted robust vaccine-induced antibody responses, regardless of whether they were seronegative or seropositive due to prior natural exposure. Thus, within the limitations of this study, there was no evidence to suggest reduced vaccine effectiveness in either group. This is further supported by field observations, where good clinical protection has been observed following vaccination in horses not directly associated with an active outbreak, but where strangles subsequently occurred on the premises.

While our results show that vaccination of healthy horses in outbreak situations can be beneficial, the burden of infection in some exposed healthy horses may progress beyond the point where it is possible for vaccination to prevent the onset of clinical disease. Therefore, the optimal strategy to minimize the likelihood that a horse succumbs to infection is to use the vaccine well in advance of exposure to S. equi.

This product has not yet been approved for use in the United States. In the meantime, does your study offer any broad takeaways for equine veterinarians in this country about how to mitigate outbreaks of strangles?

Dr. Gröndahl: This study reinforces that strangles remains a highly contagious, potentially serious, and sometimes fatal disease in horses. Early suspicion, prompt diagnosis, and strict biosecurity are essential to limiting transmission during outbreaks. While vaccination strategies vary by region and regulatory approval, the data support the concept that achieving herd immunity with a safe and effective vaccine could become an important additional tool in strangles control.

Importantly, the findings suggest that this new vaccine, unlike earlier strangles vaccines, may be used as part of outbreak management to reduce clinical disease. Available data support a favorable safety profile, even when horses have already been exposed to S. equi. MeV

FOR MORE INFORMATION
Gröndahl G, Righetti F, Aspan A, et al. 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. 2025 Nov 23.doi: 10.1111/evj.70125. Online ahead of print.
https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.70125