Bovine Viral Diarrhea Virus (BVDV)

Item No.  212

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Bovine Viral Diarrhea is one of the most economically significant diseases in both beef and dairy cattle. Unlike many other virus species, the BVD virus can infect a bovine fetus in utero by crossing the placental barrier. If that happens during early pregnancy, specifically in the first trimester, the virus establishes itself in the calf at a time when the calf's immune system is still immature and unable to mount an immune response. At the time of birth, the virus will be present in very high numbers in the blood and virtually every organ and tissue of the calf. Without the benefit of an immune response to fight the virus, the calf will harbor BVDV for the rest of its life and is, by definition, persistently infected (PI).

It is not uncommon for a PI calf to be born, and to grow up, uneventfully. During its stay on the premises, however, it will shed BVDV virus through every bodily fluid, including urine, feces, nasal excretions, tear fluid, and saliva, thereby not only contaminating the environment but also potentially infecting every cow it gets commingled with.

This creates a significant risk for a large number of animals to become transiently infected (TI) with BVDV. While those animals generally recover, they do present significant economic losses to the producer, particularly since BVDV has been shown to be a part of the 'shipping fever' respiratory disease complex. PI animals are considered to be the single most important and effective transmission mode of BVDV.

Research has shown that even only a single PI animal in a feedlot pen can cause significant per head losses to the producer. This is due to more cows in the sick pen for a longer period of time, higher costs for prolonged veterinary care and drugs, higher death rate, and reduced weight gain. In a cow-calf operation, undetected PI animals will cause the development of more PI animals as they infect susceptible dams early in their pregnancy. Aside from the generation of PI animals, the immediate effect of a BVDV outbreak can be an abortion storm, resulting in the loss of much of that year's calf crop.

NEOGEN® offers fast and accurate testing for the detection of PI animals. Clients can request a standalone BVDV test or chose the BVDV test offered in combination with subsequent genomic testing.

It is the VDX policy that, in the event of a BVDV positive test result, the client will be notified as soon as possible. Since, generally, the diagnosis of BVDV PI means the animal will be culled, clients are strongly encouraged to conduct follow up testing to confirm the diagnosis. NEOGEN will conduct the follow up testing at no charge, requesting only that a fresh ear notch and whole blood collected in a purple top tube is sent to the lab. Confirmatory testing is done both in-house and by a third party lab until an unambiguous confirmation has been achieved.

NEOGEN can accept several different sample types for testing. While the test method itself is the same for PI detection and TI detection, the distinction is made through the sample type. Since PI animals harbor the virus in their tissues, but TI animals do not, the most appropriate sample types for PI detection are tissue samples, including hair follicles.

In very young animals, serum samples are not a good sample type because there is the potential that maternal BVDV antibodies, passively transferred to the calf through the mother’s milk, will force the virus from the bloodstream into the tissues. This effect is transient since eventually, the maternal antibodies will fade away. If serum is collected during that time of transient passive protection, there will be no virus present and a false-negative diagnosis will be made. If a positive test result is obtained from whole blood, it is impossible to distinguish between PI and TI. Confirmatory testing using a fresh ear notch will have to be conducted to come to an accurate diagnosis of the BVDV status.

BVDV testing is conducted using different methods to accommodate the different sample types and provide optimal test results. The lab can run the PCR test for hair samples, blood cards, and tissue samples. The Allflex TSU samples are processed for the BVDV ELISA. Both tests carry excellent sensitivity and specificity, and the turnaround times are similar.

BVDV testing can be requested in conjunction with genomic testing through Igenity® or through a breed association. In addition, if desired, BVDV testing is available as a standalone test.

Specifications
Species Bovine, Beef, Dairy
Submission Information Acceptable Sample Types:
  • PCR:
    • Hair
    • Blood card
    • Serum and whole blood
    • Tissue
  • ELISA:
    • TSU or TST only

Support

Will the test be positive if I vaccinated my animals?
Generally, the test will not detect vaccinated animals. However, if vaccination occurred very recently, some virus particles can sometimes be found in the blood. When testing the tissue, this small amount of blood can trigger a positive test result. In those cases, confirmatory testing becomes very important to determine the true BVDV status of the animal.

Can I use bull semen as a sample?
No. Semen is not a suitable sample type for testing because the BVD virus is shed only intermittently in semen, so it may or may not provide an accurate result.

Can a persistently infected (PI) animal recover?
No. By definition, a persistently infected animal can never clear the virus from its body. Since it got infected at a time when the immune system was immature and unable to react appropriately, a PI animal cannot mount an immune response and the virus cannot be fought off.

Is a PI animal usually sick?
Sometimes they are, but they don’t necessarily have to be. Many PI animals look completely normal and grow up uneventfully or with only very few clinical signs. Most PI animals cannot be identified by simply looking at them.

Last year I did not have a BVDV problem at all. This year I had several PI animals among my calves. How is that possible?
Several months ago, when the mother cows just became pregnant with the calves that are now born, BVDV entered the farm — either from another animal that was bought without having been tested for BVDV or from a person that had been in contact with a positive animal in another place, i.e. an auction house. When the virus came onto the farm, it infected the cows but the infection remained mild and unrecognized. However, the virus crossed the placenta and infected the fetus, in more than one mother cow, and PI calves were generated.

I have a closed herd and have not bought any animals for some time. How is it possible that I have PI animals on the farm now?
The virus was carried in by people or vehicles. BVD virus can last in the environment for some time and remain infectious.

When is the best time to test?
In a BVDV biosecurity program, all calves born on the premise should be tested right after birth so that any PI animals can be readily identified and removed before they have a chance to comingle much. Generally, there is no particular best time, but the earlier in an animal’s life it is tested the sooner potential PI animals can be removed.

What is the best sample to collect?
Any tissue sample (Allflex TSU, hair card) or whole blood spotted on a card. Serum and semen should not be used for BVDV testing.

If I have a BVDV PI calf, do I need to test the dam for BVDV?
It is very possible that PI cows lead outwardly productive lives and do not appear sick. Even though their fertility is usually reduced, these cows can become pregnant, however, they will always give birth to a PI calf. It is definitely recommended to test the dam to make sure she is not a PI even though most PI calves are generated by transient infection of the mother that will get cleared after a while.

My two-year-old animal was recently diagnosed as a BVDV PI animal. How come I didn’t recognize the status of that animal when it was young?
This doesn’t happen very often but PI calves do not always show signs of disease. These animals can grow up to be adults with unrecognizable BVDV infection unless testing is conducted.

Why do I need to do confirmatory testing when an animal has been found positive for BVDV in one of your tests?
Generally, the diagnosis of BVDV PI has severe consequences since biosecurity considerations require the affected animal to be removed from the herd, usually by slaughter. The lab recommends doing confirmatory testing because not all animals testing positive initially turn out to be PI. Once the VDX lab obtains a positive BVDV result, the animal is called "BVDV suspect" and confirmatory samples (fresh ear notch, whole blood) are requested. The ear notch is tested by immunohistochemistry, a test that actually visualizes the presence of the virus in the tissues, including the skin of the ear notch. If the virus is found there, the animal is called BVDV PI. Occasionally, an initial positive animal will test PI negative in the skin IHC test. This animal, then, is not a PI but a TI and simply needs some time to clear the virus. Not all confirmatory cases, however, are that clear. In cases when initial test result and confirmatory test result combined do not make good sense, we will continue testing until a satisfactory final diagnosis can be achieved. It is important to keep in mind that any presence of BVDV (TI or PI) on a farm is a potentially significant problem that should be addressed.