Bovine viral diarrhea virus is one of the most important viruses affecting the North American cattle industry.
The virus can cause devastating economic losses in terms of infertility, abortions, stillbirths and calf deaths. As a result, it is not surprising that it is an important and vital component of any vaccination strategy in cow-calf herds and feedlots in all of North America.
Most vaccines for BVD currently on the market contain viruses from both Type 1 and Type 2 viruses to ensure the broadest possible protection. We also have the option of using a modified live vaccine or a killed virus vaccine.
If BVD virus infects an unvaccinated animal with a competent immune system, it may result in some relatively minor clinical symptoms initially such as diarrhea and perhaps a fever.
Some strains of the virus may cause more severe disease, but in most cases these symptoms may be relatively mild and may not even be noticed except by the observant stockperson.
However, BVD virus has a very unique but somewhat complicated strategy for ensuring its survival and persistence within the cattle population.
If a pregnant cow that is susceptible to BVD virus (not vaccinated) becomes infected, it may experience the mild clinical signs of diarrhea and fever and usually recover. The virus then targets the fetus, and depending on the timing of the infection, will produce a variety of clinical syndromes such as early embryonic death, abortion, congenital defects and persistent infections.
If the fetus becomes infected with BVD virus between 40 and 120 days of gestation, this infection will coincide with the development of the calf’s immune system.
One of the first tasks of the immune system is to recognize the normal tissues and cells of the calf so that it will not develop antibodies and attack itself. If BVD virus is present, the calf’s immune system will not recognize the virus as “foreign” and although the calf may be born normal, it will be tolerant to BVD virus or be persistently infected.
These persistently infected calves are permanent carriers of the virus. They are often the source of BVD outbreaks and excrete copious quantities of the virus in their saliva and manure. The calf will not develop immunity to BVD virus but will always be a carrier and shedder of the virus. In some cases, these persistently infected calves are poor doers with stunted growth, but they can also appear normal.
Only 0.3 to 0.4 percent of calves in North America are persistently infected with BVD virus, but they can cause devastating results when they enter a herd and spread this virus among cattle that are not appropriately vaccinated.
A recent study in the journal Vaccine focused on the efficacy of killed BVD vaccines and their ability to provide fetal protection by vaccinating the dam.
Dr. Paul Walz and other authors from Auburn University randomly assigned beef heifers and cows to four vaccination regimes. The heifers and cows were all negative to BVD virus before the start of the trial.
Three groups of cattle each received two pre-breeding doses of different brands of killed vaccines (Cattlemaster, Virashield and Triangle) and one group served as the control group and received only saline injections.
The animals were bred, and once they were confirmed pregnant, they were exposed to cattle that were persistently infected with BVD virus for a 28 day exposure.
The 10 BVD infected cattle that served as the source of infection were infected with a variety of strains of BVD virus.
The authors then followed the pregnant cattle and observed antibody titers, evaluated whether the BVD virus could be detected in the blood of the pregnant cattle and evaluated whether the fetus became infected in each of the cows.
The most important finding was the fetal infection rate. The Cattlemaster vaccinated group had a fetal infection rate of 43 percent, the Virashield vaccinated group had a fetal infection rate of 93 percent and the Triangle vaccinated group had a fetal infection rate of 60 percent. In the control group that received no vaccine, all of the fetuses became infected with BVD virus.
Although this study did not directly compare modified live vaccines, these fetal infection rates are much higher for killed vaccines when compared to similar previous studies for modified live vaccines.
It also demonstrated significant differences between the different brands of killed vaccines in their ability to provide fetal protection.
This is just one scientific study and we always need to be cautious in interpreting the results. Your veterinarian will be in the best position to determine which vaccine will work best for the circumstances in your particular herd and that fits your management practices.
Previous studies would suggest that modified live vaccines administered before breeding probably provide the best fetal protection against BVD virus.
This study shows that killed vaccines provide some fetal protection, but that level of protection can be somewhat variable, depending on the vaccine.
In addition to vaccinating your herd, it is also important to avoid introducing BVD virus by buying animals that could potentially be carriers.
Your veterinarian can help to provide effective means for testing for persistently infected animals through blood tests or more commonly from ear notches.
You should make sure you buy replacement stock from sources with good vaccination protocols and avoid buying suckling calves from unknown sources to replace calves in your herd that have died.