The purpose of the diary biosecurity program is to protect the health of the herd and thereby increase the production and profits of the producer and the quality and safety of the milk/meat for the consumer.

No other biosecurity tool for protecting the health of the herd is better recognized than the use of vaccines. While vaccine usage is familiar to most dairy producers, the when, what, how, and why of vaccines and vaccination programs is inconsistent and often confusing at best. Since the use of vaccines constitutes part of the critical foundation for our herd health program, we need to understand what we can and cannot expect from vaccines and how to best utilize this biosecurity management tool.

Raising Herd Resistance
The desired effect of vaccinations is to raise the resistance of the herd to levels which will provide protection and relief from a specific disease challenge situation (figure 1). Vaccinations accomplish this by tricking the immune system into thinking that a real infection has occurred. The better the vaccine is at imitating a real infection the higher the resistance level moves and the more complete the protection is. Herein lies the challenge in creating vaccines: to present to the body a situation that induces the machinery of the immune system to provide protection without making the animal sick. Vaccines should serve as a guide or 'blueprint' so the immune system can build protection and raise resistance.

Bacterial vs. Viral Infections
Most of our disease situations on the dairy fall into one of two categories -- bacterial or viral. Bacterial infections create illnesses primarily by producing toxins or poisons. Viruses, on the other hand, create illness by destroying the cells in the body that they have invaded. Bacteria are complete living cells which usually live outside the body's own cells and reproduce by enlarging and them splitting in half. Viruses are extremely small particles which reside inside the cells of the body. Viruses cannot reproduce by themselves, but rather invade the 'mind' of a cell and reprogram it in such a way that the virus infected cell now is a slave to the virus. It then begins to create millions of new virus particles which will be used to infect other cells of the body. Viruses, by residing inside the host animal's cells, can 'hide' from the immune system. 

Figure 1: Effect of Vaccination on Resistance Levels

Figure 2: Increase in Resistance Level Following Booster Dose

Table 1: Partial Listing of Bacterial and Viral Agents
Bacteria vs. Viruses
Bacteria	Viruses
Lepto	IBR
Pasteurella	BVD
E. Coli	P13
Clostridium	BRSV
Salmonella	Rotavirus
Moraxella	Coronavirus

Viral Vaccinations
Viral vaccinations can be roughly divided into two types: inactivated virus vaccines (killed) or attenuated virus vaccines (modified live virus or MLV). The inactivated virus vaccines may contain the entire killed virus organisms or it may contain pieces or fragments of the killed virus organisms. In either case the killed virus vaccine will not be able to invade, infect, and reproduce in the body's own cells. This inability to invade, infect, and reproduce makes inactivated viral vaccines very safe for the animal being inoculated. This inactivation, however, is also what creates the weakness in killed vaccines -- that being a reduced or decreased ability to fully stimulate the immune system compared to a live virus vaccine. Since the killed virus does not infect the body's cells, the immune system gets an incomplete picture of exactly how the virus works and what the viral infected cells look like. Most killed viral vaccines need frequent booster vaccinations to maintain the maximal protections possible for that particular vaccine product.

Attenuated virus vaccines, commonly known as modified live vaccines, do contain live virus organisms and they do invade, infect, and reproduce in the cells of the vaccinated animal's body. The virus in these vaccines has been 'neutered' in the lab to the point that the virus does not cause disease. A properly designed modified live vaccine will imitate the real disease-causing viral infection and therefore give the immune system a complete look at how it works. Modified live viral vaccines are generally considered to produce a better total immune response which lasts longer than the killed viral vaccines. Modified live vaccines may cause a slight elevation in body temperature following administration and may not be safe or approved for pregnant animals. Read all vaccine labels thoroughly before administration.

Bacterial Vaccines
Bacterial vaccines which contain bacterial organisms are known as 'bacterins'. Most bacterins commonly used in the animal health industry are inactivated or killed bacterins. One notable exception to this is the brucellosis or bangs vaccine which is a live bacterin. Lepto is an example of a killed bacterin, and like the killed viral vaccines, booster vaccinations are critical to keep the resistance levels adequately high enough to protect.

Pictured here, Dr. Gregory Quakenbush, D.V.M,.Technical Services, Pfizer Animal Health

Toxoids are vaccines which contain the inactivated bacterial toxin or poison which ultimately causes the illness and damage of a bacterial infection. Many bacterial vaccines are a combination of both bacterins and toxoids. The most recognizable of these would be the clostridial vaccines (blackleg) as well as the better pasteurella pneumonia vaccines. Bacterin/toxoid vaccines induce the immune system to provide protection against both the bacteria and the toxins produced and therefore provide a wide spectrum than either vaccine type might by itself. 

Antitoxins or antiserum are not really vaccines, but rather the antibodies against specific toxins which have been harvested from the immune serum from another animal. Antitoxins are used to reduce the level and damage caused by the release of the bacterial toxins, but they are temporary and do not stimulate the immune system to create protection. Antitoxins are usually used as part of the treatment in certain bacterial infections and are used in combination with antibiotics. Tetanus antitoxin is an example of an antiserum.

Vaccine Strengths and Weaknesses
Each vaccine type has its own strengths and weaknesses, and one particular type of vaccine may not fit all situations. Dairy producers would be well advised to take it upon themselves to understand the limits of the various types of vaccine products so that they are not presented with a disease outbreak as a result of not paying attention. Table 3 provides an example of this by listing some of the advantages and disadvantages between killed and modified live viral vaccines.

Table 3: Modified Live vs. Inactivated Vaccines
Modified Live Vaccine		Inactivated Vaccine
Advantages	Disadvantages	Advantages	Disadvantages
more rapid protection	reversion to virulence*	safer than modified	increased adverse reactions
longer lasting immunity	immunosuppressive*	stable in handling and storage	requires adjuvants
one dose usually	-contaminated*		more expensive
no adjuvant	-cause abortions		usually two doses
more economical	-must be handled carefully		shorter protection
better able to stimulate the immune system			poorer stimulation of the immune system
*depends on manufacturer

Pictured here, Dr. Victor Cortese, D.V.M,.Technical Services, Pfizer Animal Health

Vaccine Failure
Vaccines are not infinite in their ability to protect. Too often a disease outbreak is blamed on the vaccine when the real culprit is a breach in management or biosecurity procedures. The greatest misuse of vaccines is to rely on them solely as your only defense against disease. A properly constructed biosecurity program should be your primary defense and should actually reduce dependency upon vaccinations by reducing the level of exposure to infectious agents.

Vaccines cannot and ultimately will not replace good, aggressive management. Vaccinations are a tool which is best utilized with other management tools such as proper nutrition, cow comfort, isolation/quarantine and testing of new arrivals, proper sanitation, stress management, etc. 

The number one reason for a vaccine to fail to produce the desired immune response is due to the animal's immune system itself. A completely functional immune system is an absolute requirement for the vaccine to work. Vaccines do not by themselves create or provide any protection, but rather provide the blueprints for the immune system to follow. If for any reason the animal's immune system is compromised and not functioning 100%, one cannot reasonably expect the vaccine to work 100%. This is why on any given day the best herd response to vaccination is probably only 95%. If the animals are sick, stressed, nutritionally deficient, etc., the herd response to vaccination will be much lower. 

Other reasons for vaccine failure are not quite so clear-cut and obvious as above. Additional causes for decreased vaccine response would include: 

• Failure to provide a booster dose or improper timing of the booster dose.
• Improperly handled vaccine (e.g. modified live vaccine that got too hot sitting in the sun). 
• Vaccine was given too late (animal was already coming down with the illness).
• Wrong strain of vaccine administered or the disease-causing virus or bacterial is outside the limits of the vaccine.
• Age of the animal.
• Inadequate dose or wrong type of vaccine administered.
• Poor quality vaccine. 

Vaccination Programs and Schedule
A vaccination program should be tailor-made for each dairy and each management situation. While some basic dairy vaccination programs and strategies hold true for many dairies, most programs may need to be "tweaked" to meet the needs of a particular dairy's current or anticipated disease exposure experience and management style. Utilize the expertise of your veterinarian and resources from your vaccine manufacturer to ensure that the selection of the correct vaccine type and the timing of their administration results in the best possible protection for your herd.

This article reproduced with permission from Midwest Dairy Business.