Courtesy of Melissa McKee, McKee-Pownall Equine Services
There are a couple of reasons why we tend to vaccinate horses more frequently than humans. Take tetanus, a horrible and fatal disease in horses, for example. Humans generally receive a tetanus booster shot every 5 years. This is because we have established, through research and vast numbers of vaccines administered over many years, that the vaccine induces a strong immune response that protects an individual for at least five years from developing the disease if they have a wound inoculated with Clostridium tetanii bacteria. You will still usually have a booster shot if you go to emergency with a dirty wound, just for safety?s sake.
The potential strength of an individual?s immune response can be determined with a blood test called a ?titre?, which essentially de?tects the amount of antibodies in your circulation, and data has been analyzed to determine what the minimum protective titre is to prevent clinical illness in humans and to some degree in our small animal pets. When I was a veterinary student we had to receive rabies vaccinations in our pre-vet year. Every year after that we were required to have a blood test to check our rabies titre, and those that were considered low received a booster shot. To this day I still have mine checked regularly since I could easily come into contact with a rabid animal in my line of work. Southern Ontario still has many active rabies cases each year and horses are just as susceptible to it as our dogs and cats.
Another reason horses are vaccinated more frequently is due to greater sensitivity to a particular disease (we don?t vaccinate small pets for tetanus because they are fairly resistant, but horses are exquisitely sensitive to the bacteria), high likelihood of repeated exposure, or the immune response after vaccination is weak and/or short lived. Most of the vaccines for respiratory illness in horses have a short duration of effectiveness, because the pathogen is continually changing and it is difficult to stimulate a lasting response. This is the same reason that we are encouraged to get a flu shot for ourselves every year while our childhood polio vaccination should provide decades of protection.
Finally, although we can measure blood titres to determine antibody levels, there is still little equine data to determine the line between protective and ineffective levels. Vaccine companies are aware of this and are working towards vaccines that allow a longer treatment interval, which is already evident in small animals where several vaccines are now on an every three-year interval.
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