Gerald Hauer, DVM
Chief Provincial Veterinarian
Alberta Agriculture, and Forestry
Reprinted from The Tracker, volume 3, issue 1
Vaccines are an important part of disease prevention. In order to work, vaccines must be administered properly and given at the right time. This article discusses some of the theory of administering vaccines at the right time to achieve their maximum benefit
In a recent discussion with a bison producer the topic of vaccines came up. He knew of a lot of bison farmers that were vaccinating their animals but were not following up the initial dose with a booster. People may think that their animals are protected, but if the vaccine has not been properly administered the animals may not be protected at all. This article will discuss the importance of vaccinating properly which includes giving timely boosters to ensure that your animals are properly protected.
Most people are well aware of what vaccines are and why they are given to animals. In short, we are exposing the animal's immune system to a selected antigen (agent that stimulates the immune system) to provide protection against the disease causing virus or bacteria. Because the animal has been pre-exposed, the immune system can build up some immunity and can respond to an invading infectious organism and fight off the disease before it can become established. Our objective should be to use vaccines properly so that we get the maximum protection which gives our animals a better chance of avoiding disease. To be effective vaccines must be given at the correct dose and by the correct route of administration ( intramuscularly or subcutaneously). The best source of this information is the instructions on the package. Even though there are no vaccines licenced for use in elk and bison in this country, cattle vaccines are generally used with reasonable efficacy. Also vaccines must be handled properly to ensure that they maintain their potency. Keeping them refrigerated until use and using them all up once they are opened are also good practices.
To be effective some vaccines require a that a booster be given within a certain amount of time after the initial dose of vaccine. Let's examine why this is the case by discussing the difference between the killed virus vaccines and modified live virus bovine vaccines. When a killed virus vaccine is made, the virus is produced and then deactivated by the manufacturer so that there is no chance of it causing the original disease. When the vaccine is injected into the animal, the immune system reacts to a limited number of virus particles. This stimulates a small response that lasts for a short while as seen in figure 1 where we compare the immune system's response to elapsed time.
This protection is good but only for a short time period. What is more important is that it has primed the immune system to respond to the same antigen if it is exposed again within a certain time. A second exposure (booster) to the antigen while the immune system is primed stimulates a much larger response that lasts for a longer time. This can be best visualized in Figure 2.
If the booster occurs after the priming of the immune system has worn off then the system has to be primed again. The chart in figure 3 shows that the late booster actually acts as another primer.
A modified live virus vaccine has a little bit different curve. Because the virus is still alive when it is injected, it has the ability to reproduce and grow in the animal's tissue. Since there are a lot more virus particles for the immune system to destroy it stimulates a much larger immune response from the initial dose of vaccine and therefore doesn't generally require a booster. It's vaccine curve is like the one in figure 4.
As you can see from the figures, all vaccines are not the same as some require boosters to be effective for a long period of time while others don't. Follow the instructions on the package because the manufacturer has made recommendations based on extensive research of the product. It is important not only to boost the vaccine if recommended, but also to boost it at the correct time. The diagrams also show that the immunity from a vaccine doesn't last forever and animals need to have a booster from time to time to keep their immunity strong. Most vaccines require a booster every 12 months. Again consult the package or talk to your veterinarian for further advice.