INFECTIOUS DISEASES

Experimental S. equi Vaccine APPEARS EFFECTIVE

By Adam Marcus

Shutterstock/Grigorita Ko

An experimental vaccine against Streptococcus equi provides at least partial protection against strangles, particularly in animals that receive multiple injections over time, a new study shows.

Researchers in Sweden and the United Kingdom who tested the Strangvac vaccine, from Intervacc, said the intramuscular agent protected 31% of ponies that received 1 dose, and 94% of ponies that received 3 doses over roughly a 4-month period. The data from the study are part of the submission package for approval from the European Medicines Agency for marketing of the shot, according to the researchers, who published their findings in a recent issue of the journal Vaccine.

Strangles is a significant problem for horse owners and the most serious bacterial illness for equines. The infection, which is reportable to health agencies in the United States and some other countries, passes easily between animals and is known to lurk silently in the guttural pouches and sinuses of animals that have recovered from an acute bout—representing a reservoir of potential transmission. Younger horses are considered to be more vulnerable to acute illness than older animals, according to a 2018 consensus statement from the American College of Veterinary Internal Medicine (J Vet Intern Med 2018;32:633–647).

For the new study, Jan-Ingmar Flock, PhD, a professor in the Department of Microbiology, Tumour and Cellbiology at the Karolinska Institute, in Stockholm, and a co-founder of the pharmaceutical company Intervacc AB, and his colleagues evaluated 4 dosing schedules of the recombinant-fused-protein vaccine in 56 ponies, ranging in age from 6 months to 1 year. For comparison, the researchers also included groups of horses that received placebo injections.

All of the treated animals received 2 rounds of injections, 1 at Day 0 of the study and another at Day 28. Sub-groups of ponies received additional shots at Day 119, Day 210 and Day 392 of the trial, which marked the 52-week point after the second injection.

Optimal protection was observed in the group that received 3 doses of the vaccine over a 119-day period, followed by a challenge of S. equi 2 weeks later. In that group, 15 of 16 ponies (94%; P<0.0001) were protected from the infection, the researchers reported.

“These data provide evidence that the administration of a third vaccination at up to 52 weeks post second vaccination induced antibody responses in sera and nasal secretions that were equivalent to those obtained following the second vaccination. Therefore, immune memory cells—which were induced following the primary course of 2 vaccinations— persisted for at least a period of 1 year,” they wrote.

Although maximum protection was achieved with repeated doses of the vaccine over a 119-day period, Dr. Flock said that this should not be a problem for horse owners. “It is very common that a vaccine is given at several occasions to obtain a good protection. Another example is vaccination against equine influenza, where at least 3 doses should be given and a revaccination at least once per year,” he said.

There are at least 2 other vaccines for strangles available in the United States and Europe, but Dr. Flock said the new vaccine has at least 1 critical advantage over the available vaccines: It produces immune reactions that can be differentiated from those sparked by infection with S. equi.

It is important to be able to distinguish between natural infection and a horse that was vaccinated on a blood test, he told Modern Equine Vet. “This is particularly important during an outbreak. None of the strangles vaccines on the market have this possibility.” MeV

The study was funded by Intervacc.

For more information:

Robinson C, et al. Intramuscular vaccination with Strangvac is safe and induces protection against equine strangles caused by Streptococcus equi. Vaccine. 2020;38(31):4861-4868.

https://www.sciencedirect.com/science/article/pii/S0264410X20306915?via%3Dihub

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