Active immunization

Active immunization involves administration of antigen(s) derived from an infectious agent so that an animal mounts an acquired immune response and achieves resistance to that agent. When properly used, vaccines are highly effective in controlling infectious diseases. Several criteria determine whether a vaccine can or should be used. First, the actual cause of the disease must be determined. Although this appears self-evident, it has not always been followed in practice. For example, in the bovine respiratory disease complex, although Mannheimia (Pasteurella) haemolytica can be isolated consistently, these bacteria are not the sole cause of this syndrome. In some viral diseases (eg, equine infectious anemia, feline infectious peritonitis, Aleutian disease in mink) antibodies contribute to the disease process, and vaccination may make it more severe. Therefore, vaccination must be governed by the principle of informed consent. The risks of vaccination must not exceed those caused by the disease itself.

When vaccines are used to control disease in a population rather than in individuals, the concept of herd immunity must be considered. Herd immunity refers to increased resistance of a group because of the presence of some immune animals within the group, which reduces the probability of a susceptible animal encountering an infected one. As a result, spread of infectious disease is slowed or blocked.

An ideal vaccine for active immunization should confer prolonged, strong immunity in the vaccinated animal. It should not cause adverse side effects and should be inexpensive, stable, and adaptable to mass vaccination. It should also stimulate an immune response distinguishable from that due to natural infection so that vaccination and eradication may proceed simultaneously.