A standard recommendation for all cow-calf herds in North America is to vaccinate with a viral vaccine that provides immunity to two important reproductive diseases: infectious bovine rhinotracheitis and bovine viral diarrhea virus.
These two diseases have potential to cause serious reproductive losses due to abortions, if they infect the unprotected pregnant cow. In addition, BVD can infect the fetus during early gestation and, depending on the timing of the infection, will produce a variety of clinical syndromes, such as early embryonic death, congenital defects or persistent infections.
It is essential that cows and heifers be given vaccines that contain IBR virus and BVD virus at least 30 days before breeding. BVD has a variety of genetic variations and is often classed into at least two different biotypes known as BVD type 1 and BVD type 2. Both biotypes can cause clinical disease and the vast majority of vaccine manufacturers include both biotypes of BVD in their vaccines.
There are a variety of vaccines available that provide protection against these two important diseases. These different options may also provide protection against other viral or bacterial diseases and that may be one of the factors that a veterinarian will consider when choosing a particular brand or combination of vaccines. However, another important decision often depends on whether to use a modified live vaccine or a killed vaccine.
There can be good reasons for using either. Modified live vaccines have the viral components of the vaccine altered or changed so they will not cause clinical disease. However, the virus used in the vaccine is still alive and will replicate in the animal after it is given. Most viral vaccines require you to mix together a diluent and a small dried cake supplied in two separate bottles to activate the virus.
The vaccine must be protected from heat and sunlight and used in a specific time period after mixing, while it is still alive or the vaccine will lose its effectiveness.
In many cases, modified live vaccines may provide stronger immunity and greater protection than a killed vaccine. Killed vaccines can also be very effective, but in these cases the viral components that are used in the vaccine are no longer alive. Killed vaccines often require a larger dose of the killed viruses or viral components to stimulate immunity and may require a second booster vaccination in some circumstances. Killed vaccines are also often dependent on an adjuvant, which is a substance included in the vaccine to stimulate the immune response.
Modified live vaccines can be effective against diseases such as IBR and BVD, but they do have some inherent risks associated with them.
Dr. Daniel Givens and Dr. Benjamin Newcomer addressed the risks in a recent paper delivered at the American Association of Bovine Practitioner’s conference in September of 2017.
Administering a live vaccine to calves nursing pregnant cows that have not been previously vaccinated is one of the potential risks.
You will often read in the label precautions for modified live vaccines a warning such as “Do not use in calves nursing pregnant cows unless their dams were vaccinated within the past 12 months.” The potential risk is that the modified live vaccine could multiply within the calves after vaccination and potentially be transferred to the non-vaccinated dams and heifers, which could possibly cause abortions. There have been several studies on this unlikely scenario and the results have been mixed. In some studies, there was no evidence of transmission from vaccinated calves, while in others, there was some potential transmission occurring.
This seems like a very low risk scenario and it is easily prevented by ensuring your cows and heifers are vaccinated.
The second risk is the potential of causing infertility by giving a modified live vaccine too close to the breeding season. Many labels will suggest that they should be administered at least 30 days before breeding.
The IBR virus component of the modified live vaccine has the potential to cause inflammation of the ovaries and could potentially affect the estrus cycle if given too close to the start of the breeding season. The studies described by Givens and Newcomer suggest that this is most likely a risk in animals being vaccinated for the first time. Many of the studies showed no impact of a modified live vaccine given close to the breeding period in previously vaccinated animals, however, it is probably a risk to be aware of and avoided if possible.
The final risk and perhaps the most important is that some modified live vaccines can potentially cause abortions if given to pregnant cows. This is probably a much greater risk in animals that have not been previously vaccinated, but is certainly a significant risk to consider.
Some available modified live vaccines have label claims that allow pregnant cows to be vaccinated, provided they were vaccinated, according to label directions, with this same product within the past 12 months.
It is critically important that if modified live vaccines are given to pregnant cows, that these instructions be followed. Note the phrasing of the instructions, which include “with this same product.” It is extremely important that the identical vaccine from the same manufacturer be given appropriately in the past 12 months.