Feed Lot Magazine Online, Vaccinating Kicks Off Complex Process

Administering a vaccine doesn't guarantee that a protective immune response will occur and that disease will be prevented. Administering a vaccine is simply giving an injection that exposes an animal to a vaccine (antigen).

What is important is the process that is set into motion by administering a vaccine. And, speaking at Cow-Calf Conference IX, Dr. Gerald Stokka, DVM from Kansas State University, pointed out that this process is "immensely complex and completely misunderstood."

Dr. Stokka explained that the complex process starts with cells, called lymphocytes, which mediate the portion of the immune system you are attempting to stimulate. He said these lymphocytes must recognize the foreign substance, respond to the foreign substance by making soluble chemotactic factors, kill viruses infected and foreign cells and produce antibodies.

"The goal in herd (and feedlot) immunization is to raise the level of immunity in a sufficient number of animals to prevent epidemics and the catastrophic monetary losses associated with them," Dr. Stokka stated. "It is important to note that individual animals may still become ill if other factors that reduce their level of resistance are present.

"In a population of animals, disease transmission is reduced as disease resistance increases."

Immunity

Dr. Stokka said that every animal has four types of immune defense mechanisms, with each providing a unique type of protection.

The first line of defense: the animal's native defense mechanism. This includes enzymes in the saliva and tears, acids in the stomach, fatty acids in the skin, the normal flora at the mucosal surfaces, the complement system and the phagocytic white blood cells that are capable of killing some bacteria and viruses. Even if an animal is not vaccinated, this defense mechanism is functional immediately.

The second line of defense: humoral immunity, often referred to as serum immunity. Humoral immunity is due to the presence of antibodies in the bloodstream. When an animal is vaccinated, B-lymphocytes will respond to the vaccines and produce the IgG or IgM class of antibodies. These antibodies circulate the bloodstream and attach to the infectious agent when encountered in the blood or tissues.

The third line of defense: cell-mediated immunity. If organisms are resistant to humoral antibody immunity, they must be removed by cell-mediated immunity. When an animal is exposed to a vaccine (antigen) that induces cell-mediated immunity, the T-lymphocytes reproduce themselves through mitosis and a large number of T-lymphocytes will then recognize a natural infectious agent when exposed to it. The T-lymphocytes will attempt to destroy the organism by either directly attacking it or by protein molecules called lymphokines which direct other white blood cells to attack and destroy the infectious agent.

The fourth line of defense: secretory IgA. Secretory IgA is a special class of antibody that protects mucosal surfaces of an animal - such as the intestinal tract, respiratory tract and reproductive tract. Secretory IgA is secreted onto mucosal surfaces where it binds to mucus and remains in fairly high concentrates. Dr. Stokka stressed that secretory IgA is resistant to destructive proteolytic enzymes that reside on mucosal surfaces.

Vaccine Failure

Dr. Stokka noted that achieving a protective immune response in every animal within a population is nearly impossible. Why? Because vaccines may fail. Vaccine failure can usually be traced to four key factors: 1) The animal may have been incubating the disease when it was vaccinated, 2) Something may have happened to the vaccine to make it ineffective, 3) The physiologic status of the host may make it unresponsive or hyporesponsive to the vaccine, 4) The host may be exposed to an overwhelming challenge dose of infectious agent.

"The most likely cause of vaccine ineffectiveness is improper handling," Dr. Stokka said. He advised storing and administering vaccines as recommended by their manufacturers, adding that not doing so may reduce their efficacy. Other factors to consider: proper diluent, one syringe per vaccine, expiration date, proper storage temperature and keeping cattle as dust-free as possible after vaccination.

"Special care must also be taken with any modified live vaccine (MLV), either viral or bacterial, to prevent inactivation of the vaccine by exposure to temperature extremes, ultraviolet radiation (sunlight) and disinfectants," he stated. "Because vaccines do lose their viability over time, even when stored properly, they should not be used if past the expiration date."

If an animal is incubating a disease or is exposed to the disease-causing organism soon following vaccination, Dr. Stokka warned that sickness may result -- and the vaccine will appear ineffective. He said, in this situation, disease symptoms will appear shortly after vaccination and may be mistakenly attributed to vaccine causing the disease.

"It is important to remember that it takes several days for an animal's immune system to respond and for the animal to be protected," he continued.

Dr. Stokka noted that the host factor most likely to attribute to vaccine failure is "the inability to respond to the vaccine properly."

In young animals, he said, the presence of maternal antibodies can prevent an adequate response to vaccination.

Another host factor is immunosuppression, which can stem from a variety of causes, including stress, malnutrition, concurrent infection and immaturity of the immune system.

In summary, Dr. Stokka advised producers to prevent important infectious disease by selecting the correct vaccines, or antigens, that 1) produce a good humoral, cellular and local immune response similar to natural infection, 2) produce protection against clinical disease and reinfection, 3) give protection over a number of years, preferably a lifetime, 4) result in minimal side reactions (weight loss, abortions, congenital abnormalities, reduced milk production), 5) can be administered in a form that is safe and acceptable to producer and veterinarian.

He wrapped up his presentation by stating "the cost and benefits should clearly outweigh the cost and natural risk of infection." Plus, he suggested producers discuss their vaccination programs with their veterinarian.