With pressure increasing to lessen antibiotic use in livestock, the focus is shifting from treatment of disease to disease prevention. With this emphasis on prevention, virus vaccines and bacterins are in the spotlight.

"Virus vaccines and bacterins are tools used to trick an animal's immune system to create immunity to a specific disease," states Dr. Bruce Wren, veterinary consultant from Overland Park, Kan.

Although many people use the term "vaccine" when they talk about biologicals used at vaccinating time, Dr. Wren points out that bacterins contain strictly bacterial antigens while a vaccine contains strictly viral antigens. The correct term for a true vaccine is "virus vaccine."

Virus vaccines fall into two general groups: modified live virus vaccines (MLV) and killed virus vaccines.

MLVs
MLV vaccines contain live, infectious viruses isolated from the field and brought into the laboratory. Special techniques allow these viruses to be modified so they can be injected into the animal where they multiply and grow. Rather than causing the disease, the modified live viruses give the animal an immune response to the disease.

Because viruses in MLV vaccines only need to live and grow in the animal for a short period of time to produce the desired immune response, most MLV vaccines do not have adjuvants (formulations added to killed virus vaccines and bacterins to enhance the immune response)

Dr. Wren says MLV products have several key advantages:

1) Usually involve one injection, with no booster needed

2) Smaller dose - more doses per syringe, increased comfort for the animal and convenience for the person vaccinating.

3) Less chance for severe allergic response.

4) Quicker and better immune response against viral diseases.

One disadvantage of an MLV vaccine is that it is less stable than a killed product. Once the freeze-dried virus in an MLV vaccine is reconstituted by a diluent, the virus is extremely susceptible to heat and sun.

Unlike killed product, MLV vaccines should not be administered to pregnant females. Why? Because some specific viruses in the MLV vaccine can sometimes work their way into the uterus of a naive female or female without any specific viral immunity and affect the fetus, particularly in the first three months of pregnancy. The result could be an aborted fetus.

MLV vaccines also should not be administered to calves nursing pregnant females. The "why not" is because the virus administered to calves may be shed from the nose or eyes for a short time as the virus grows and multiplies. The calf can then pass the MLV-virus to its naive mother when it nurses causing abortion.

Another disadvantage of an MLV vaccine is that the modified live viruses in MLV vaccines may be inactivated by maternal antibodies that pass to the calf from the female via the colostrum. Initially, the calf will be protected due to the maternal antibodies, but as the maternal antibodies wane with time, the calf can be left unprotected because the modified live viruses may have been inactivated.

Killed Vaccines, Bacterins
Killed virus vaccines or bacterins contain isolated viruses or bacteria that are grown to large numbers artificially and inactivated or killed during manufacturing. An adjuvant is then added, and it is the adjuvant that slowly releases the killed bacteria or viruses over a period of time in the animal, giving the animal a strong immune response.

Dr. Bob Parker, technical services veterinarian for Grand Laboratories, Larchwood, Iowa, says the one main reason many producers choose a killed or inactivated product over an MLV vaccine is that killed or inactivated products can be used in pregnant cows and in calves nursing pregnant cows.

Two other reasons for selecting a killed or inactivated product are that it is more stable than MLVs, plus properly adjuvanted inactivated products can overcome maternal antibodies.

"Longer lasting immunity is another reason. If the killed product you're using has the right adjuvant and contains an optimal viral load, you get longer lasting immunity," Dr. Parker adds. "Why settle for 3, 4 or 6 months of protection for IBR or BVD when a killed product can deliver 10 months and 12 months protection respectively?"

On the downside, killed products require a larger dose than MLVs, need a booster to maintain the immune response and contain an adjuvant which can sometimes irritate and lead to an injection site blemish reaction if not administered in the correct location or by the correct techniques.

All in all, there is a time and place for an MLV product and a time and place for a killed or inactivated product. It's not MLV vs. killed product. It's a case of "What type of product would work best in this situation?" ©