2/16/10 All Vaccines are Not Equal

Vet Tip of the Day: How Vaccines Vary
Key Words: Antibody, Cell Mediated, Immune System, Virus, Bacteria, West Nile, DNA

We've looked at the factors that help you decide whether or not to vaccinate your horse against a certain disease.  Now let's examine how vaccines work.

All vaccines work by stimulating the immune system in some fashion.  We think most simply of a vaccine as something that stimulates the production of antibodies.  These antibodies then prevent invasion by either a bacterial or viral disease causing organism.  In fact, there are many different classes of antibodies in the equine immune system, and certain classes of antibody are more effective than others in fighting particular diseases.  For instance, IgA antibodies are very effective in fighting off infections that begin in the upper respiratory tract, such as Strangles.  Therefore, the intranasal Strangles vaccine was designed to stimulate a strong protective IgA reponse.  It does this by physically depositing live, chemically altered Strep Equi bacteria onto the back of the horse's pharynx when the vaccine is injected through the nasal passages.  This live bacteria mimics the actual Strangles organism and causes the horse to produce IgA antibodies in the upper airway which then combat the actual Strangles organism when your horse is exposed to it.  The protection is not absolute, and the antibody response to the vaccine is variable.  In addition, because the vaccine contains live, chemically altered organisms, there is a small risk of inducing a vaccine strain case of Strangles in response to the vaccine itself.

In addition to antibody production, the body has is a second, non-antibody dependent arm of the immune system.  It is called cell mediated immunity and is very important in the body's ability to fight viral diseases.  You may have read about the new technologies being used in some of the equine West Nile vaccines.  Instead of using an altered form of the disease causing organism to directly stimulate antibody production, these vaccine use a variety of technologies involving the actual genetic material (DNA) of the disease causing organism.  DNA based vaccines are more effective in stimulating all aspects of the immune system, thereby providing a stronger, more broad based protection against viral organisms, such as West Nile
Virus.  The Intervet DNA chimera vaccine is a fascinating cutting edge product with excellent safety and efficacy against the West Nile Virus.  To learn more about it, Google: Prevenile, Intervet, West Nile.

Finally, while the ideal vaccine will stimulate all arms of the immune system and prime it to rapidly eliminate an invading pathogen, it can only work if the organism causing disease can be consistently identified by the body.  For instance, the West Nile Virus is a very stable, unchanging organism that looks exactly the same to the horse in Massachusetts and the horse in California.  Influenza, on the other hand, is a viral organism that is constantly changing its appearance.  In exactly the same way our human flu produces new strains all the time, equine influenza undergoes small changes in its viral structure that allow it to evade the defenses of horses vaccinated with products produced using older strains of the virus.  So in the case of equine influenza, not only are we dealing with a viral disease, which challenges both the antibody and cell mediated arms of the horse's immune system, but we also are dealing with a disease that frequently alters its appearance, thereby posing one of the greatest challenges to researchers whose goal is to develop highly effective vaccines.

The subject of vaccines, the immune system, and disease prevention is extremely complex.  Luckily, those diseases which our horses may contract which are most deadly, are also the diseases against which it is possible to develop excellent, safe, and effective vaccines.  The prime examples of these are Tetanus and West Nile Virus.  Therefore, these are diseases against which ALL horses should be vaccinated.

The decision whether or not to vaccinate against influenza, rhinopneumonitis, and Strangles, should be made based on your individual horses risk of disease, and the impact that disease is likely to have on your horse's performance and well-being.  Refer to this past Saturday's blog to review the information that will help you make this decision, and then consult with your veterinarian as well.