Several diagnostic tests are available for aiding in the control and prevention of the BVD virus. Cattle producers are encouraged to consult with their animal health professionals on how best to position BVD virus-testing strategies in their herd health program. Daniel L. Grooms, Michigan State University College of Veterinary Medicine, lists some of the methods used to diagnose BVD virus.
Virus isolation has been the most common method for identifying cattle infected with BVD virus. The virus is relatively easy to isolate from a variety of specimens including serum, buffy coats (white blood cells), nasal swabs and tissue samples.
The most common application of antigen detection is the enzyme-linked immunosorbent assay (ELISA). The ELISA can be used for detecting virus in blood, nasal swabs and skin samples such as ear notches. Recently, immunohistochemistry (IHC) and fluorescent antibody testing has been used to detect BVD virus in skin samples of cattle persistently infected (PI) with the BVD virus.
Nucleic acid detection
The polymerase chain reaction (PCR) is the most commonly used nucleic acid detection assay. It's estimated PCR is 10 to 1,000 times more sensitive than virus isolation.
PCR has been used in screening protocols for PIs where samples from multiple animals are pooled together. This strategy takes advantage of the high sensitivity of the assay while reducing the cost per animal tested.
Serological assays commonly have been used to diagnose viral infections. However, the nearly ever-present exposure of cattle in the U.S. to the BVD virus (naturally or by vaccination) has made serology at times difficult to interpret.
Before attempting to identify individual PI cattle, it is essential strong evidence exist that PI cattle are likely on the operation.
“This evidence may include detection of the BVD virus in necropsy samples, identifying the virus in pooled PCR samples or serology unrelated to vaccination,” Grooms explains. “Once a rancher decides to screen individual animals, several strategies can be considered — including testing all calves at one time, such as at birth, branding or calfhood vaccination.”
Bob Larson, director of University of Missouri-Columbia's Veterinary Medical Extension & Continuing Education, says all calves, replacement heifers, bulls and non-pregnant dams without calves should be tested for PI status.
“Any female still pregnant at the time the herd is tested should be isolated from the breeding herd and kept isolated until her calf is tested and found to be negative,” Larson says.
If the dam is identified as PI, she should be sold for slaughter immediately.
Pooled PCR testing
PI monitoring can be accomplished with pooled blood or skin biopsy (usually ear notch) samples for PCR testing.
“By pooling samples, the expense of screening herds with few PI animals is minimized,” Larson explains. “A single PI animal is detectible in pools of 50 to 250 negative samples.”
Animals contributing to negative samples are all assumed to be non-PI, whereas positive pools may contain samples from PI animals or transiently infected animals. If the initial pool is PCR-positive, it must be split and retested to find the one or more individuals in the pool infected with BVD virus.