Advances in preventing Strangles

Great article written by Stacey Oke DVM MSc, first published here

If you live in an area where equine strangles runs rampant, then you’re probably aware that vaccines against this infectious disease are less than perfect. This leaves many horses unprotected from the disease and/or at risk for developing vaccine reactions, including abscesses or purpura hemorrhagica (damaged, leaky blood vessels resulting in swelling).

There are currently two strangles vaccines available in the United States: one injectable and one intranasal. Unfortunately, despite aggressive efforts, neither can protect a horse against the disease 100% of the time. This leaves even vaccinated horses at risk, which can be problematic as strangles, caused by the bacteriumStreptococcus equi subspecies equi, is one of the most frequently diagnosed equine infectious diseases worldwide.

“There is an urgent need to develop new preventative vaccines against strangles and, to date, our success in doing so has been limited,” said Andrew Waller, BSc, PhD, from the Animal Health Trust, in Newmarket, U.K.

Waller and colleagues set out to develop a strangles vaccine veterinarians could administer intramuscularly without abscesses developing at the injection site (a common problem).

“The other goal was to produce a vaccine that, after administration, allows veterinarians to differentiate between vaccinated horses and those that are actually infected,” Waller said. “This would facilitate movement of vaccinated horses between premises and permit identification of vaccinated horses that were later exposed to S. equi without developing clinical signs of the disease.”

Well-aware that S. equi has an arsenal of tricks up its microscopic sleeve to evade and “confound” a horse’s immune system, Waller and colleagues modified a strain of S. equi by deleting six genes from the bacterium’s DNA to create a new strain. They anticipated that this novel strain of S. equi (used in a live vaccine) would prime a vaccinated horse’s immune response so that if the horse was subsequently exposed to natural strains of S. equi, the immune system would be ready and better able to fight off infection.

Waller and colleagues administered two intramuscular doses of their new vaccine several weeks apart to nine ponies and found that:

  • After the first injection, four ponies developed slow-growing abscesses at the injection site; two of those four were removed from the study after the first vaccine due to humane reasons related to the abscesses;
  • One pony developed a slow-growing abscess after the second vaccine administration;
  • All of the vaccinated ponies, but none of the control ponies they were housed with, developed immune responses against S. equi (meaning the vaccinated ponies did not shed S. equi); and
  • Eleven days after challenge, 100% of the vaccinated ponies remained disease-free whereas six of the nine unvaccinated ponies developed clinical signs of strangles.

“This information will help us continue to build better vaccines against strangles, by directing improvements to the safety of new live vaccines and the effectiveness of recombinant subunit vaccines that have fewer S. equi targets, but benefit from an excellent safety profile,” Waller concluded. “Even with better vaccines, the requirements of good biosecurity in the prevention and management of strangles outbreaks cannot be emphasized enough.”

The study, “Vaccination with a live multi-gene deletion strain protects horses against virulent challenge with Streptococcus equi,” was published in Vaccine.

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