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. Transient BVD virus infection causes significant suppression of the immune system, leading to outbreaks of other disease such as pneumonia and scours.
The virus is mainly spread by Persistently Infected (PI) cattle. These animals are infected as calves in the uterus during the first 120 days of pregnancy. At this stage, the unborn calf’s developing immune system does not recognise the BVD 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..
An animal infected with BVD virus has virus in its blood and tissue. This applies to both PI animals and those that are transiently infected. 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.
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