Herd health is one area where most producers will agree: Profit-oriented herds have a top-notch herd health program. Although elements of herd health programs will differ, it helps to understand the role vaccines play when you're developing an effective program.

Vaccines are used for prevention of disease. Ideally, you start with a healthy animal and administer the vaccine with the goal of keeping diseases at bay so animals remain healthy, productive and efficient.

Producers' other animal health tools - pharmaceuticals - are generally designed to treat disease after it occurs. Any improvement in the afflicted animal's health is considered a success when using pharmaceuticals.

Even if an animal is treated with pharmaceuticals successfully, however, losses due to reduced weight gain or other disease symptoms generally occur. Thus, preventing disease is considered the most economical option.

Antigens
To help prevent the possibility of disease, vaccines contain components known as antigens that are designed to elicit an immune response. Some vaccines have one antigen - and are called monovalent vaccines - while other vaccines provide broader coverage as they contain several different antigens - combination products.

How well the antigens in the vaccine work depend on the quality of the vaccine and the health of the animal vaccinated. Any given animal, at any given time, will have a specific level of immunity or resistance to a specific antigen based on the animal's environment, current health status, nutritional state, stress level and its degree of historical exposure. As these factors change, so does the animal's immunity or resistance change.

Herd immunity is based on the individual immunity of the herd members. Herd immunity will be less than the most resistant member and more than the least resistant member of the herd. As with individual animals, herd immunity or resistance changes over time as the resistance of the individuals that comprise the herd change.

When Disease Strikes
Disease occurs when an infectious agent comes in contact with, and is able to establish itself, in a given animal. The two main factors influencing whether an animal is susceptible to disease are the animal's resistance to the invading disease agent at the time of exposure and the relative amount and virulence of the invading agent at a given time.

At different points in an animal's life, the level of disease challenge will be higher than the level of resistance to a particular organism, or vice versa. Historical exposure to disease develops as animals age and their body repeatedly fends off disease organisms. Many diseases are more likely to occur during certain points in an animal's life due to higher levels of disease-causing agents - for example the high challenge state of salebarns during the fall cattle run - and/or lower levels of immunity to specific diseases - for example the defenseless state of newborn calves.

Intestinal, or enteric, diseases are more likely to occur during the first year of life. Respiratory diseases, on the other hand, are more likely to occur at or within the first two years while reproductive diseases usually occur when an animal is 18 months to four years of age.

Vaccinating for disease helps protect your investment. To develop an effective herd health program, contact your veterinarian and ask for assistance in designing a program specific for your animals, environment and management level.

Dr. Moore is a veterinarian with Novartis Animal Vaccines, Inc. He is based in Superior, Neb.