Successful vaccination programs mix the art of administering pharmaceutical products with the science of immunology.
Disease protection is shaped to a cowherd's environment and molded around factors like nutrition and genetics. While you don't need a degree in microbiology to make a vaccine work, a basic understanding of passive and acquired disease-fighting systems can help.
“Natural mechanisms are an animal's first line of defense,” points out Lawrence Rice, of San Antonio, TX, a retired Oklahoma State University professor of veterinary medicine. “Tears, saliva, enzymes, stomach acids, white blood cells — all these may act to kill or inhibit bacteria or viruses.”
Passive immunity, however, most commonly occurs when disease antibodies are transferred to newborns through the mother's colostrum. These immunities have finite lives.
“In cattle, colostral blackleg antibodies are gone by about two months of age,” says Rice. “Bovine viral diarrhea (BVD) antibodies may last six months or longer.”
An animal can't develop permanent or acquired immunity through vaccination if passive antibodies are present. Once passive immunity fades and natural mechanisms can't withstand the disease challenge, vaccines can stimulate a rise in the resistance level.
“To properly immunize cattle against diseases, it's important to know how vaccines are formulated, the route of administration and how the body responds to the vaccine in the presence or absence of maternal antibodies,” adds Ed Richey, DVM, beef cattle specialist with the University of Florida.
B-Cell & T-Cell Immunity
There are two main arms of the immune system — B-cell and T-cell immunity, says Ron Torell of Elko, NV.
A University of Nevada-Reno Extension beef specialist, Torell says proteins called antibodies are produced by “B” lymphocytes (white blood cells). These antibodies bind to and inactivate disease microbes and toxins that exist in such body fluids as blood or mucus. This is called humoral protection.
Microbes, such as viruses, inside cells are protected from antibodies, so a second arm of defense is needed. This cell-mediated immunity is produced by “T” cell lymphocytes. These destroy the entire infected cell.
Inactivated or “killed” vaccines elicit production of antibodies but not T cells. These vaccines are generally effective against antigens that exist and replicate outside the cells. Their duration is questionable unless a boostering protocol is followed.
Cell-mediated immunity is best achieved with modified-live vaccines (MLVs), which cause a transient, low-grade infection that mimics natural infection. MLVs can produce both antibodies and T cells.
Cell-mediated immunity is important in control of respiratory diseases. But, cell-mediated immunity is not as easily produced as humoral immunity, explains Torell.
“The immune system has an infinite capacity to recognize antigens,” Torell says. “Some ranchers worry they'll overwhelm the immune system by giving too many antigens at once. In non-stressed livestock, however, this is probably not cause for concern.”
Four Vaccine Forms
Richey separates vaccines into four forms:
Replicating MLV. These vaccines must replicate (reproduce) in the animal before an increase in resistance is stimulated. Usually only one dose will stimulate long-lasting resistance in an animal.
Some examples include MLVs for infectious bovine rhinotracheitis (IBR), bovine viral diarrhea (BVD), parainfluenza-3 (PI3) and strain 19-brucellosis.
Non-replicating MLV. Some vaccines are live but won't replicate in the body. Therefore, these MLV vaccines require at least two doses to stimulate adequate resistance. Because they don't replicate, the first dose usually only triggers the memory mechanism in the body. A second dose, no sooner than 21 days, will stimulate high levels of resistance.
Examples include chemically-altered IBR/PI3 and MLV-BRSV (bovine respiratory syncytial virus).
Inactivated non-replicating. These vaccines have been inactivated or “killed” when manufactured. Thus, they can't replicate in the body. As with the non-replicating MLVs, the animal will require at least two doses to stimulate adequate resistance.
Examples include killed virus vaccines and bacterins — Leptospira, Clostridia, Haemophilus somnus, Vibriosis and pinkeye.
Intranasal. These vaccines usually replicate only in surface cells of the upper respiratory tract and stimulate localized resistance. Generally, these types of vaccines provide a quick, short-lived rise in resistance and trigger the “memory” cells in the body. Examples include nasal IBR and nasal PI3.
“When repeating or boostering vaccines, remember it's the type of vaccine that's important (i.e., IBR, BVD, Lepto), not necessarily the form of the vaccine,” explains Richey. “For example, the first dose of IBR and PI3 could be in the intranasal form, and the second dose given in the non-replicating form.”
Risk Assessments Are Critical
It usually takes one to two weeks following infection or vaccination for the immune system to produce the antibodies necessary for protection.
Remember that stressful events for cattle can produce hormonal or chemical imbalances that suppress the immune system and its response to vaccines. And, vaccination in the face of a disease outbreak will only produce disappointing and costly results, Richey says.
“Obviously, not all vaccines will produce a 100% immune response, and not all immune responses are protective against disease,” says Rice. “Therefore, when selecting a vaccination program, it's critical to do a risk assessment, and balance the art of application with the science of immune response.”
Vaccines Require TLC
A good vaccination program can be ruined by careless handling or storage of the vaccine, says Ryan Rueter. He's a livestock specialist for the Noble Foundation in Ardmore, OK.
“Remember, a freezer is not a refrigerator, and the dashboard of a pickup is seldom room temperature,” Rueter says. He likes to use a small ice chest at the chute to keep vaccines cool and out of the light.
“If you use vaccines that are shipped freeze-dried,” he adds, “reconstitute only what you can use in 30 minutes. They become inactive soon after they're mixed.”