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Vaccinating Understanding the Fundamentals

There are several ways that we as horse owners and veterinarians work at maintaining our horses' health. A major way of protecting our horses' health is by way of vaccinating them against deadly and costly diseases. When we vaccinate our horses, we are attempting to stimulate that horse to protect itself (by its own immune system) against those diseases that we have chosen to vaccinate against.

The immune system is a very complex subject, but one needs a basic understanding of it to understand vaccines and how they can help your horse. The body has several barriers that keep microorganisms out, but when these are broken through or invaded, the body's white blood cells attack. If these microorganisms are in enough numbers or are powerful enough, they can cause disease. Every microorganism has unique proteins on their outer body called antigens that identify them to the body's immune system. The body produces antibodies that are specific to the antigen that when used, neutralizes that organism so it can be eliminated from the body. In the future when the body encounters a microorganism with this kind of antigen, the body is more resilient to the organisms and not be as ill as the previous time or ill at all. The basis of vaccinations is to trick the body into mounting a response to the chosen microorganism without inducing the disease that we hope to prevent. We do this by exposing the horse to either the killed organism, the live but modified organism (modified to prevent it from causing the illness) or pieces of the microorganism. All of these methods produce some sort of immune response, some better than others, and some more dangerous than others.

There are about a dozen equine diseases that we can vaccinate against in the United States. Some have specific geographic areas where that disease is prevalent. Others are as widespread as the horse population. The important vaccines in our area are Tetanus, Encephalomyelitis (Eastern and Western Equine Encephalomyelitis), Influenza, Rhinopneumonitis (Equine Herpes, Streptococcus Equi (Strangles), and West Nile Virus. Tetanus, Encephalomyelitis (Eastern and Western Equine Encephalomyelitis), Rhinopneumonitis (Equine Herpes), and West Nile Virus are fatal or often are fatal. Whereas Influenza, some cases of Rhinopneumonitis (Equine Herpes), and Streptococcus Equi (Strangles), are rarely fatal but cause economic losses from time out of competition, labor and medication to treat sick horses. These losses can be up to 10 times the cost of a series of vaccines.

Only federally licensed products should be used and strict attention should be afforded the manufacturer's recommendations regarding storage, handling, and routes of administration of the vaccine to maximize efficacy and safety. However, results of research or clinical experience of the veterinarian may support alternate protocols for vaccination that will improve the efficacy of a vaccine without increasing adverse effects. Protection is not afforded the patient immediately after administration of a vaccine that is designed to induce active immunity. In most instances a series of multiple doses of an inactivated vaccine must be administered initially for that vaccine to induce protective active immunity. Two to three weeks are required to produce adequate concentrations of antibodies and before booster revaccinations can be protective. Foals with residual maternal antibodies generally produce a greater serologic response when an initial series of 3 doses is administered rather than the 2-dose series recommended by most manufacturers of vaccines for older horses without residual maternal antibodies.

It is important to vaccinate broodmares 4 to 6 weeks before foaling for their own protection, as well as to maximize concentrations of immunoglobulins in their colostrum to be passively transferred to their foals. Simply vaccinating the mare is not sufficient. The foal must receive adequate amounts of high quality colostrum and absorb adequate amounts of specific colostral immunoglobulins before absorption of macromolecules ceases (generally 24 to 48 hours). Specific colostral immunoglobulins provide protection against field infections for several months but also may interfere with vaccinal antigens and prevent the active immunologic response by the foal; a phenomenon termed "maternal antibody interference." Although protective concentrations of antibody decline with time, vaccination of a foal while these colostral antibodies are present - even at concentrations less than those considered to be protective - is of minimal to no value because of maternal antibody interference. Consequently, that foal may be susceptible to infection before the primary vaccinal series can be completed.

One must have realistic expectations and understand that vaccination serves to minimize the risk of infection but does not prevent disease in all circumstances; the primary series of vaccines and booster doses should be appropriately administered prior to likely exposure; each horse in a population is not protected to an equal degree nor for an equal duration following vaccination; and all horses in a herd should be appropriately vaccinated, and whenever possible, the same schedule should be followed. Another point is that it is impractical to recommend a "standard" vaccination program for all horses because each individual situation must be evaluated based on risk of disease (anticipated exposure, environmental factors, geographic factors, age, breed, use, and sex of the horse), potential for adverse reactions to a vaccine(s), anticipated efficacy of the selected product(s), and the cost. To maximize your horses health call Equine Medical Service to review your vaccination schedule or ask where you get your vaccines.