A series of clinical trials for Strangvac have been performed at the Animal Health Trust in Newmarket, England. The vaccinations on Welsh Mountain Ponies were well tolerated throughout the studies and no serious adverse events were observed. A specific safety and tolerability trial has also been carried out on Swedish trotters at Menhammar, Ekerö. The clinical trials aimed to measure the safety, immune response, efficacy and DIVA capability. DIVA (Differentiation between Infected and Vaccinated Animals) capability is an invaluable feature during epidemics when you have to be able to distinguish vaccinated animals from asymptomatic carriers. A vaccine based on an attenuated microorganism gives rise to the same serological antibody response as the disease itself. Such a vaccine, without DIVA capability, can thus be difficult to use in circumstances where you want to identify and limit the spread of infection by using serological testing.
In the clinical trials conducted at the Animal Health Trust, the vaccinations were well tolerated and a smaller proportion of ponies displayed mild, transient local reactions lasting between one to five days. No unexpected side effects of Strangvac® or placebo injections occurred. At the special safety study, standardbred trotters tolerated the vaccines very well and no side effects were observed.
Intramuscular administration of Strangvac® induced antibodies against the vaccine components in all vaccinated animals throughout the studies. A booster response was demonstrated both following a single re-vaccination at three months as well as at six months after basic vaccination. Thus, only a single injection was required for a full and rapid antibody response reaching the same level as after the primary vaccination.
Efficacy – Immunity
In one trial, the test animals were given an artificial form of strangles two weeks after they received their primary vaccination (two doses) of Strangvac®. The trial showed that vaccination significantly delayed and reduced the clinical symptoms.
Efficacy – Duration of immunity
A similar trial as above but with a challenge two months (instead of two weeks) after the ponies got their basic vaccination was performed. In this case, a majority of the animals were protected from contracting the disease.
Efficacy – immunity after revaccination
Yet another clinical trial was conducted in which ponies received a basic vaccination of Strangvac® but also a re-vaccination at 3 months after the basic vaccination. After another two weeks, they were given an artificial form of strangles. This clinical trial showed excellent protection against strangles, only one pony out of the sixteen displayed fever and pathological changes, while all fifteen placebo animals fell ill with a typical course of the disease, see figure below.
In the clinical trials it was demonstrated that Strangvac® has DIVA capacity, which will be invaluable in horse populations where infection with S. equi is endemic.
The clinical trials provide strong evidence in support of the use of intramuscular vaccination with Strangvac for the prevention of Strangles in horses. Strangvac ® was safe, immunogenic and effective, particularly at two-weeks post-third vaccination. Furthermore, the vaccine has DIVA capability enabling it to be used alongside conventional disease prevention strategies. Strangvac® could play a significant role in the prevention of Strangles, in particular through the re-vaccination of horses following the identification of an index case or prior to the transportation of an animal to equine events where there is a risk of exposure to S. equi.