Thanks to a truly quantum leap in the quality of premarket testing, including challenge studies, we have solid information on West Nile vaccines, and there are now five choices.
Challenge studies are when a control group of horses and a vaccinated group are deliberately exposed to a disease. it’s a
more accurate way to determine if a vaccine is effective than simply checking for antibodies after vaccination.
The USDA is specific with regard to the type of language that can be used in vaccine claims. The wording must reflect exactly what has been proven in challenge studies. Vaccine claims will begin with either ?prevent? or ?aid in the prevention.? Prevention, rather than aid in the prevention, is a stronger claim as it means that the vaccine either eliminates or reduces to an extremely low level.
With specific reference to West Nile vaccines, claims are in reference to viremia (measured virus appearing in the blood after a challenge), or to the appearance of disease and/or encephalitis. Encephalitis claims must be supported by post-mortem examination.
The absence of a claim, for example ?prevention of? or ?aid in the prevention of? disease, doesn’t necessarily mean the vaccine doesn’t do that. It means the vaccine hasn?t been proven to do that. In some instances this is because the type of challenge testing done didn’t cause signs of disease in the unvaccinated control horses either.
That was the case with the challenge studies done for the first Ft. Dodge Innovator, Innovator DNA and for the Recombitek. Because of that, they are only labeled for aid in the prevention of viremia.
However, since the original challenge tests were done there has been a repeat study comparing Ft. Dodge West Nile Innovator, Prevenile and Recombitek using an intrathecal challenge of vaccinated and control horses.
Intrathecal is the injection of virus directly into the cerebrospinal fluid surrounding the brain. Four things were considered in evaluating the effectiveness: Fever greater than 102.5, viremia, clinical signs and evidence of encephalitis on post-mortem microscopic evaluation (see chart ?Evaluating Effectiveness?).
Each test group included six horses (except Recombitek, as one horse was found to already have antibodies and had to be removed from the trial). Prevenile provided the strongest protection, followed closely by Recombitek. These results aren?t surprising as killed vaccines don’t provide as broad-based an immune system response.
The new vaccine, Vetera, has not been tested side-by-side with the others as yet, but did also use an intrathecal challenge. The Innovator DNA vaccine has not been evaluated by intrathecal challenge.
Bottom Line.Intrathecal challenge is the most severe test of a vaccine?s effectiveness. A vaccine proven effective when given by this route has the greatest chance of protecting your horse under real-life conditions (challenge studies don’t involve large numbers of horses).
Since all of these vaccines are similarly priced, it’s hard to go against the well-proven effectiveness of Prevenile, which requires only a single initial dose. It also has the most rapid onset of protection and doesn’t use an adjuvant (adjuvants may be responsible for many adverse vaccine reactions).
Article by Eleanor Kellon, VMD, Veterinary Editor.
