Tag and Test for BVD
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Whenever producers look up the classic symptoms of BVD, they read about the most severe form of the disease and decide that they do not have the problem in their herd. Symptoms can include low birth weight calves, poor fertility, early embryonic death, calf death, respiratory disease, poor conception rates and thus high numbers of return to service. However, poor health in calves or poor fertility in adults is often a sign that BVD is underlying the problem; getting rid of BVD always improves health and productivity.
A PI calf remains infected with BVD throughout its entire life – which is usually comparatively short. The persistent infection comes about as a result of foetal infection in the first 120 days of gestation. This occurs if the dam herself is PI or, if a naive dam becomes infected with the virus in early pregnancy. PI calves cannot produce an immune response to combat the virus. Consequently they shed virus for their entire life.
Vaccination is rarely 100% protective. If you vaccinate a PI calf, it will have no effect on the amount of virus that is shed by that calf resulting in a constant viral challenge on the farm. In some cases for various reasons some animals do not respond to the vaccine effectively if this does occur then she may be at risk of producing a PI calf in subsequent gestations. Vaccination plays an important role in eliminating BVD from the farm but should be carried out in conjunction with appropriate on-farm surveillance and, when they are present, identification and removal of PI animals.
Our partners, NML, currently offer an antibody BVD ELISA test for milk or blood, an antigen ELISA test for tissue or blood samples and a qPCR antigen test on bulk milk samples. The former measures the host response (antibodies) to the infection (and thus whether an animal has been exposed) and the latter measures a specific protein component of the virus itself. Nordic's Tag and test utilizes NMLs antigen ELISA virus test.
PCR is another type of virus test. It detects BVD viral RNA. NML are able to perform the BVD PCR service on bulk milk samples and will shortly be launching a BVD PCR service for blood and tissue sampling. Watch this space.
There are many ways to search for a PI animal. It is often best to start your PI search with the calves and the youngstock but older animals within the herd can also be PI's and should never be overlooked.
Testing calves and youngstock also gives you information about the BVD status of the dams from the respective calves. Any calf that tests negative for virus in tissue should not be a PI. Furthermore, the dam of the calf can be regarded as not a PI (as PI dams always have PI calves). If any calves test positive for virus, they should be isolated and retested in 3 weeks. Their dam should also be tested. Remember, if you find a PI and cull it, calves born after that date should be tag and tested at birth for an additional 12 months to establish if there has been a lasting effect from virus exposure.
When PIs are identified, they should be culled. It is important to retest positive PI animals after 3 weeks to confirm the animal is PI. Please ensure potential PIs have no contact with in-calf animals.
You can tag and test an animal of any age, even an aborted foetus. There are more cost effective ways to find PI animals in a breeding herd than to tag and test every animal in the herd.
Tag and test can be used for calves from the day of birth onwards. There is a slight potential of a reduced amount of virus due to maternal antibodies but a PI should still be evident from the test results.
Vaccination does not affect tag and test results. Vaccination has no effect on the presence of virus in PI animals and thus they will still test positive after vaccination.
Please note: There have been occurrences within literature where animals have not exhibited standard virus and antibody patterns. These occurrences are usually rare but we recommend that herd exposure is monitored in conjunction with your vet.