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Livestock Vaccines:  How They Work and How to Ensure They Do Their Job

Dr Russ Daly and Dr Ann Price

Vaccines are suspensions of microorganisms administered with the goal of stimulating the body's immune system against these same organisms, thereby preventing a clinical disease or reducing its sever­ity. The agents included in vaccines are inactivated or modified such that, once introduced into the body, they do not cause clinical illness.




Inactivated ("killed") vaccines contain bacteria or viruses that have been inactivated by heat or chem­icals. Whichever method is used needs to completely render the microbe incapable of growing and repro­ducing, while maintaining the microbe's structure enough to be recognized by the body.


Because inactivated organisms do not multiply inside the body, substances called adjuvants are included in such vaccine suspensions. Adjuvants are compounds (such as aluminum salts or oil-in-water emulsions) that non-specifically stimulate the immune system to respond to--and slow down the body's elimination of--the injected inactivated microbes.


Subunit vaccines, which contain only parts of the bacteria or virus of interest, and toxoids, vaccines created from inactivated toxins released from certain disease-causing bacteria, are also examples of inacti­vated vaccines. Antitoxins, preparations of antibod­ies against bacterial toxins, are not technically vac­cines: they passively tie up toxins instead of actively stimulating an immune response, as is the case with toxoids. However, like most components of vac­cines, antitoxins are protein molecules, and the issues regarding handling and care of those products are the same as for vaccines.


Modified-live virus (MLV) vaccines contain whole viruses that have been altered such that, while they are able to multiply within the body, their ability to cause disease has been taken away. Vaccine manu­facturers typically achieve this by making the microbe grow under prolonged or slightly abnormal growing conditions. Because the modified microbes actively grow within the host—stimulating the immune system in the process—fewer of them are needed in a vaccine dose compared to inactivated vaccines. MLV vaccines are typically packaged in two vials—one containing a freeze-dried cake that contains the modified microbes, and the other containing the diluent, which re-sus­pends the microbes.



The goal of vaccines is to produce "immunity" against specific organisms. There are two major branches of the immune system: 1) humoral immu­nity refers to stimulating the production of antibod­ies against the microbes in the vaccine, and 2) cell-mediated immunity directs the cells of the immune system to neutralize the invading microbes or destroy the cells the microbes have infected. Each type of vaccine stimulates these branches in slightly different manners.2


Inactivated vaccines depend on cells within the body called antigen-presenting cells to process the material in the vaccine. These cells are normally found in high numbers in skin, muscle, and subcuta­neous tissue, especially in the presence of the inflam­mation that normally follows injections. Antigen-pre­senting cells carry important parts of the inactivated microbes between the injection site and areas that are active in creating the immune response, such as the lymph nodes. With inactivated vaccines, this impor­tant interaction is aided by the adjuvant, along with the relatively high number of inactivated organisms present in the dose of vaccine. These vaccines are especially good at stimulating humoral immunity.


MLV vaccines depend on their ability to infect living cells within the body, in addition to interacting with antigen-presenting cells, to activate the immune system. Significantly fewer organisms are needed per dose of vaccine, but these organisms need to be viable in order to adequately stimulate the immune system. If the organisms cannot do this, such as when they are inactivated through improper storage or handling, the immune system will not effectively respond, due to the smaller number of organisms and the lack of adjuvant. The action of modified live vaccines is espe­cially good at stimulating cell-mediated immunity.




Proteins are the major components of the organ­isms that make up both killed and MLV vaccines. Proteins disintegrate according to two major factors: time and temperature.


As time passes, the proteins that make up the vaccine organisms disintegrate into smaller parts. Eventually, given enough time, there will no longer be enough intact organisms to effectively stimulate an immune response. This is especially important for re­hydrated modified-live organisms, which rapidly lose their viability over time periods as short as 1-2 hours, depending on environmental conditions.


Effects of age apply to both inactivated and MLV vaccines. Expiration dates are placed on all vaccines and reflect the latest date upon which a dose of vac­cine is considered to contain enough intact organisms to effectively stimulate the immune system. All expi­ration dates assume recommendations for storage and handling have been followed.

Storage temperatures higher than label recom­mendations will result in a quicker rate of disin­tegration and will render any vaccine, whether inactivated or MLV, impotent. Reconstituted MLV vaccines are particularly sensitive to rapid inacti­vation by high temperatures.


At the other extreme, freezing temperatures will also adversely affect vaccines. In the case of certain inactivated vaccines, freezing temperatures result in physical changes to the adjuvants included in the vaccine. These altered adjuvants may cause a reduced immune response or an increase in the number of adverse reactions to the vaccine. The microbes pres­ent in rehydrated MLV vaccines are very sensitive to freezing and may be rapidly inactivated by freezing temperatures as well.


Most common disinfectants will render modified-live organisms inactive. Traces of disinfectant left in syringes, tubing, or transfer needles following clean­ing will reduce the effectiveness of such vaccines. In addition, ultraviolet light has an adverse effect on the viability of modified-live organisms. Such vaccines should be stored in their original packaging away from light sources prior to use.


The success of any livestock vaccination program depends on the effectiveness of the vaccine used. Vac­cines--which in essence are suspensions of biological organisms--will become ineffective if proper stor­age and handling recommendations are not followed. Allowing these important tools for preventive health to become ineffective means a wasted investment by producers, and the potential for increased livestock illness and death loss due to diseases that might have been avoided.



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Article made possible through the contribution of Dr Russ Daly, Dr Ann Price, South Dakota State University, College of Agriculture & Biological Sciences and USDA

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