The gold standard for judging the effectiveness of a vaccine is the called the ”challenge test.” In challenge studies, vaccinated animals as well as unvaccinated controls are deliberately exposed to the infection to see if the vaccine protects them from disease.
With West Nile virus, challenge studies to date have been inconclusive with respect to prevention of disease since researchers were unable to make either vaccinated or unvaccinated horses sick simply by exposing them to mosquitoes carrying the virus.
Merial, the manufacturer of the recombinant canarypox-vector WNV vaccine, has taken the studies one step further and injected live virus directly into the spinal fluid of vaccinated and unvaccinated horses. This is a much-more severe challenge than any horse would have to fight with natural exposure. The results were published recently in the journal Veterinary Therapeutics.
The findings were that eight out of 10 unvaccinated horses developed symptoms of encephalitis and none of the vaccinated, although one had transient muscle fasiculations/twitching. Nine of 10 unvaccinated developed a fever, only one of 10 vaccinated. None of the vaccinated horses, and all of the unvaccinated showed virus circulating in their blood (viremia). Microscopic examination of brain tissues confirmed the symptoms observed were related to WN encephalitis.
A previous study published two years ago in the American Journal of VeterinaryResearch had shown that even a single dose of the Recombitek vaccine was able to protect eight out of nine vaccinated horses from developing circulating virus after exposure to infected mosquitoes. In contrast, eight out of 10 of the unvaccinated horses had virus circulating, indicating successful infection. All exposed horses developed antibodies, but the horses that had received even one dose of the two-dose vaccination program produced antibodies quicker.
Prevention of infections requires more than just a vaccine. If the immune system of a vaccinated horse is compromised by a genetic weakness, stress, illness or suboptimal diet (inadequate levels of protein, vitamins, minerals), the vaccine may fail. The role of vaccines is to give the immune system a ”heads-up” call on the infection, prime the pump so that antibodies and cellular immune responses are on alert to jump into action at the first appearance of the infection.
The best vaccines are those that are able to protect the horse from any symptoms of the infection, assuming the immune system is up to par. Merial has gone the extra mile, spending the research dollars, to prove that the their Recombitek vaccine will get the job done.
Editor’s Note: We are at work on a recommended vaccines protocol article, which will appear in an upcoming spring issue.