BVD virus causes a complex of diseases in cattle, the most important of which can interfere with reproduction, affect the unborn calf and ultimately lead to fatal mucosal disease. BVD virus can also cause enteritis during transient infection, which is usually mild but is occasionally severe enough to cause death, even in adult cattle. Transient BVD virus infection is also associated with significant suppression of disease resistance, contributing to outbreaks of other disease such as pneumonia or scours.
The virus is mainly spread by Persistently Infected ( PI) cattle. These animals are infected with the virus in the womb during the first 120 days of pregnancy. The unborn calf's developing immune system does not recognise the virus as "foreign", so does not produce antibodies, instead the animal becomes persistently infected. PIs remain infected with BVD all their lives and they continuously shed large quantities of virus, infecting any unprotected cattle around them. Many die as calves but some live much longer and some PI animals can appear normal. If a PI gives birth to a live calf, it will also be a PI. Removing PI cattle from the national herd is critical to any eradication attempt.
Cattle that are otherwise healthy (non- PIs) can become infected with BVD virus at any point in their lives, which is known as transient infection. They will produce an antibody response to the virus and normally fight it off in 2-3 weeks. They will then have antibodies in their blood and tissue, which persist for years. Reliable tests exist for BVD virus antibodies.
An animal infected with BVD virus will have virus in their blood and tissue. This applies to both PI animals and those that are transiently infected. A number of different tests are available to detect the virus, currently the most commonly used is the antigen ELISA. This detects a component of the virus, the antigen. Animals that test positive in this test are therefore frequently referred to as being "antigen positive". There is a range of reliable, commercially available tests for BVD virus and antibody
If an animal gives a positive result to its first virus test, it is a suspect PI. A second sample taken at least three weeks later will either confirm that the animal is a PI, or if virus is no longer present, will show that the animal was transiently infected with BVD at the time of the first test.
As a consequence, testing for BVD virus is simple but interpreting the results can be complicated. If an animal tests positive for viral antigen, it may be a PI animal or it may be transiently infected. If it has antibodies it has been exposed to infection, is still benefiting from its mother's colostrum or has been recently vaccinated, and is very unlikely be a PI animal. Understanding test results is vital to controlling the disease.
For more information on BVD, please see www.scotland.gov.uk/bvd.
Email: Ian Murdoch, Ian.Murdoch@gov.scot
Phone: 0300 244 4000 – Central Enquiry Unit
The Scottish Government
St Andrew's House
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