BVD Tests: Important Tools
K-State veterinarian discusses how BVD can affect the herd and the tests to identify a problem.
Bovine viral diarrhea (BVD) virus can cause a host of problems — from diarrhea to abortion and susceptibility to other diseases. Acute infection with the virus triggers an immune response, the body fights back and the animal recovers. This situation is not as concerning as persistent infection (PI) in which the animal can never get rid of the virus and is constantly shedding it for the rest of its life.
Gregg Hanzlicek of the Kansas State University (K-State) Veterinary Diagnostic Laboratory says the thing to worry about most with BVD is that it causes immunosuppression.
“Thus, it contributes to higher sickness rates with other important diseases, such as bovine respiratory disease,” he says.
“On the cow-calf side we worry about reproductive issues, and on the feeding side we are concerned with immunosuppression and respiratory disease. The reason the PI cattle are dangerous is because they usually don’t have any symptoms. They are born with the virus and are constantly shedding it in huge numbers, but they appear normal. All the animals around them are exposed to the virus. In addition, if the exposed animals are pregnant — in the first 120 days of gestation — this presents the possibility that the fetus will become a PI calf,” he explains.
Testing for BVD
There are several ways to diagnose BVD.
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“We are looking for animals that are carriers of the virus, therefore the tests we use are looking for the organism itself,” Hanzlicek says. “There is one technique called virus isolation in which we actually grow the virus (from a blood sample or tissue sample). The downside of that test is that it takes a little time to grow and isolate the virus.”
Another test, called immunohistochemistry, is completed on tissue samples, using a chemical reaction and looking at the sample microscopically. There are two ELISA tests that can be used.
“One is called an antigen-capture ELISA,” he explains. “This test sets up a reaction between the virus in the sample and the antibody to the virus contained in the test kit. If the virus is present, it causes a reaction that can be visually seen.”
Recently, another ELISA test has come into use. This is called a BVD SNAP® test. It can be completed chuteside, with results available within a few minutes. There are SNAP tests being utilized for other diseases, as well.
“The test with the most recent technology behind it, and the one being used more and more frequently, is the PCR (polymerase chain reaction) test. One outstanding feature of the PCR is that it can quantify the amount of virus in the sample,” he says.
By using the PCR and assessing the number reported on the test result, which is called the Ct number, you have a good idea whether that animal is a PI or a TI (transiently infected) animal.
“The rule of thumb is that if you have an animal with a positive test result, you should probably retest that animal in about three to four weeks, because both the ELISA and the PCR will find not only the cattle that are persistently infected, but also the animals that are transiently, temporarily infected. Those transiently infected animals will fight off the infection and no longer be shedding any virus, usually within a few weeks,” he explains.
Sometimes producers complain that the PCR test is too sensitive in that it finds both PI and TI animals, but Hanzlicek assures that both the ELISA and PCR tests will find PI and TI cattle.
“We need to talk to producers more about this problem. People think it’s a hassle to go back and retest in 3 to 4 weeks, but it’s best to remove both PI and TI animals from your herd. A TI on the retest will be negative, and it would then be safe to put those cattle back in the herd. By contrast, any PI cattle should be sold to slaughter,” he says.
It makes just as much sense to remove the TI cattle in addition to the PI cattle from the group, because the TI animal is shedding the virus for a while.
“Whether the virus exposure is coming from a PI or TI makes no difference to the calf or cow that is exposed,” he says.
Editor’s Note: Heather Smith Thomas is a freelancer and cattlewoman from Salmon, Idaho.