Bovine viral diarrhea virus is one of the most important viruses affecting the North American cattle industry.
I have witnessed a variety of ways in which this virus can cause devastating economic losses in terms of infertility, abortions, stillbirths, and calf deaths. 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.
BVD virus has two main forms of infection. The first is the transient or primary infection where the virus infects an unvaccinated animal with a competent immune system. This transient infection is usually characterized by relatively minor clinical symptoms such as diarrhea for a few days. However, these transient infections can also suppress the immune system of the animal and make them more susceptible to other infectious diseases such as respiratory disease.
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If this transient BVD infection occurs in a pregnant cow, she may come down with diarrhea but she will probably recover uneventfully.
However, the virus targets the fetus in the pregnant cow and depending on the timing of the infection, the virus can cause a variety of clinical syndromes such as early embryonic death, abortion, congenital defects or the second form of infection, which is called a persistent infection .
If the fetus becomes infected with BVD virus between 30 and 120 days gestation, this viral infection will coincide with the development of the fetal 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 completely normal, it will be immune-tolerant to BVD virus or persistently infected.
These persistently infected animals are a major source of the virus because they secrete almost 1,000 times more virus than the transiently infected animals. They are also infected for life and will continually shed the virus.
Many of these animals eventually die from the clinical syndrome that we call mucosal disease, which is characterized by diarrhea, oral ulcers, and a muzzle with a burned appearance. There is no treatment for the fatal syndrome of mucosal disease. In the rare event that a persistently infected animal survives until breeding age and becomes pregnant, her offspring will always be persistently infected as well.
The control of BVD virus largely depends on preventing these persistent infections from occurring and from avoiding transient infections in our pregnant cows.
It is important to understand that the only way persistently infected calves can be created is by being infected with the virus as a fetus during that early window of gestation (30-120 days of pregnancy).
Therefore, our vaccine strategies must protect the fetus by vaccinating the cow appropriately. The majority of the commercial vaccines have been proven to provide reasonably good levels of fetal protection if used appropriately. If you aren’t vaccinating for BVD, make sure you consult with your veterinarian to design a vaccination strategy that fits your herd’s management system.
If we suspect BVD has entered a herd and there may be persistently infected animals present, your veterinarian may recommend a diagnostic testing strategy for identifying those persistently infected animals. The test that is currently used at most of our diagnostic labs is a polymerase chain reaction (PCR) test that is performed on a small skin biopsy, which is usually taken from the ear of the calf.
You can take the sample by using a pig ear notcher, or I like to use a brisket punch to take a small round sample of skin out of the ear. The samples are submitted to the lab either fresh or frozen.
The PCR test can identify viral particles in the skin and if the skin biopsy is strongly positive, that tells us the calf is persistently infected with BVD virus. Transient infections don’t tend to have lots of virus in the skin and so they won’t have a strong positive reaction.
We will focus our testing on the young stock in the herd because few persistently infected animals survive until adulthood (although it does happen from time to time). I usually suggest testing all calves as well as replacement heifers and any cows that may have aborted or lost their calf. If a calf or heifer tests positive, then we usually follow up and test the dam of that animal to make sure that it isn’t persistently infected as well.
You should disinfect your instrument between ear notches so you don’t spread other viruses, such as bovine leucosis, between animals. After you disinfect the instrument, you should rinse it with clean water because the disinfectant can cause false negative test results.
One of the major advantages of the PCR test is that the laboratory can pool samples. It can often test up to 10 calves as a pool, which helps to reduce the cost of testing. If a pooled sample is positive, then the laboratory has to follow up and test the individual samples in that pool to determine which of the calves are positive.
Testing for BVD virus can be confusing because there are different tests that can be used for different circumstances. The blood test for BVD antibodies is only useful in identifying recent transient infections and doesn’t help in identifying persistently infected animals. The PCR test can also be used on blood samples but may require a second test after 14 days to confirm that positive animals are truly persistently infected.
If you suspect you have BVD in your herd, your veterinarian can help you set up a diagnostic testing plan to identify persistent infection, as well as a vaccination program to prevent them from occurring.
John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.
