At the threshold of the paddock, I paused, in complete awe of what I was witnessing. A beautiful, dun Morgan mare lay flat on the ground, her devoted owners hovering over her holding an umbrella to shield her from the July baking sun. The mare would try to sit sternal with great effort. With a lumbering heave, she would hoist herself up on her front legs just enough to enable her owners to help her flip over onto the other side. Her limp hind legs were useless and completely paralyzed. Despite not understanding what was happening to her, she had enough survival instinct to try to “roll” over to relieve weight on her lungs and huge haunch muscles.
The diagnosis was obvious — this horse was afflicted with West Nile Virus (WNV) during the time nearly 25 years ago when it had recently emerged in the United States.
At this current time, there are no less than four licensed vaccines for horses to protect against WNV. This mare did not have the advantage of such a simple injection to keep her safe. Let’s look at some whys and the science behind the use of routine immunizations for our equine partners.
Why are Vaccine Programs Important?
Infectious diseases remain a major cause of concern in both human and veterinary medicine. As the Covid-19 pandemic has so dramatically demonstrated, the introduction of an infectious agent into a susceptible population can have devastating consequences. The purpose of a vaccine is to stimulate a protective immune response that either prevents infection or limits seriousness of disease. Horses are susceptible to a number of viral and bacterial diseases for which the equine industry has produced effective and safe vaccinations for prevention.
Vaccines really come into their own if enough individuals in a population are vaccinated to achieve herd immunity. Herd immunity relies on the principle that resistance to the spread of an infectious disease within a population is based on pre-existing immunity in a high proportion of individuals as a result of previous infection or vaccination. The more horses within a herd that are immunized, the fewer unimmunized animals will get sick – herd immunity is optimized if the number of immunized exceeds 75%, and preferably ranges from 83 – 94%. In these circumstances, outbreaks of disease can be…