Bovine viral diarrhoea (BVD) is probably the most economically-significant viral disease effecting cattle in the UK. Frank Stephen, from DS McGregor and Partners, looks at the risks and controls.
The cost of infection in a 100-cow beef herd has been shown to be £45,000 over a 10-year period, with losses in a similar-sized dairy herd up to double that figure. But with judicious use of testing and vaccination, this disease can easily be eradicated.
The disease
BVD can have a wide range of effects on a herd. When first infected, a transient diarrhoea, milk drop and, in calves, a mild pneumonia may be the only signs seen. Bulls infected with BVD can become infertile for several months and spread the disease in their semen.
It is when BVD infects the herd during the breeding season and pregnancy that the main problems arise. BVD virus passes across the placenta to infect the unborn calf causing early embryonic loss, abortions, infertility and the birth of deformed calves.
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| A naïve herd infected for the first time with BVD can lose more than 25% of the calf crop |
Infection of a cow, which has not before met the disease, during the first half of pregnancy, can lead to the birth of a calf which is persistently infected with BVD. These calves known as PI calves (persistently infected), are born normal but excrete virus all their lives. Due to the immunosuppressive action of the virus groups of calves with a PI among them have increased problems and mortality due to scour and pneumonia.
A PI calf will often develop mucosal disease between six and 24 months of age (they have mouth and foot lesions, scour profusely, lose condition and die) for which there is no treatment. A naive herd infected for the first time with BVD can lose more than 25% of their calf crop to mucosal disease. Some PIs however live normally for years and are the main source of infection in the national herd. A cow which is a PI will, if she breeds successfully, always produce a PI calf.
Testing
Although testing for BVD can be carried out on a range of samples, blood and milk are the most commonly used. The samples are used to either detect antibody which shows that the animal has come in contact with BVD or antigen which shows that the animal is at that time infected with the virus.
When the antigen test is positive it is normal to repeat the test three weeks later to show whether it is a transient infection or a PI. If it was a transient infection the test will now be antibody positive. Animals which test negative for antibody and antigen have not met the disease and are classed as naive to BVD.
There is now also a test which can detect virus in a plug of tissue taken from the ear of a calf at tagging.
Control and eradication
When developing a control strategy for BVD the first thing to do is to ascertain the BVD status of the herd. In the dairy herd the bulk milk tank should be tested for antibody and antigen. If it is antibody positive at least some of the herd has been exposed to BVD.
If it is also antigen positive then there is a likelihood of a PI being present among the milking cows.
Efforts should be made either by use of milk or blood samples to find the PI and cull her from the herd. All replacement heifers should be blood tested to discover their status before entering the herd. Again any PIs should be culled. A vaccination policy if not already in place should then be instigated.
In the beef herd five to 10 cattle aged nine to 18 months should be tested for antibody from each distinctly managed group in the herd. If they are antibody positive it is proof there is active infection present in the herd. One can then test all calves over six months and cull any PIs.
The mothers of any PIs and any barren or aborted cows should also be tested to find PIs. All PIs should be culled. Another approach in the known infected beef herd is to test all breeding animals and replacements, remove any PIs and initiate a vaccination policy.
It is essential that no PI or unvaccinated naive animal is presented to a bull for this method to be successful. In our practice we have eradicated BVD from more than 25 herds this way.
Health schemes
There are several health schemes which can either confer BVD free accredited or BVD vaccinated, monitored free status on herds. These schemes are members of Cattle Health Certification Standards (CHeCS), an industry body which ensures quality control of the schemes. Youngstock are tested annually to prove continued freedom from BVD.
Biosecurity
Herds which are free of BVD must try to avoid its introduction by implementing strict biosecurity measures.
The costs of the disease far outweigh the time and effort required to keep a herd free of the disease. A closed herd with 3m double fencing to avoid nose-to-nose contact with cattle on neighbouring farms would be the ideal scenario.
Only buy animals from herds either accredited free or vaccinated and monitored free from BVD. Any bought in breeding animals from herds of unknown status should be isolated for one month and tested for BVD.
You must be aware that in calf, antibody positive cows or heifers may be carrying a PI calf in their wombs or if they are sucking a calf it may be a PI. Bought in replacement calves should be avoided as they are a common way of introducing infection into a herd.
An infected bought in bull could also have devastating effects on a herd so his status must always be checked. Don't forget that a bought-in animal which has been vaccinated should also be checked to prove that it is not a PI.
Visitors to the farm are also a potential source of infection so ensure cleaning and disinfection is carried out before they mix with your stock. Cattle on rented grazing or reared away from the farm are another risk factor in contracting the disease as are animals going to a show or market and returning to the farm.
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| A beef cross calf showing BVD signs – dull coat, diarrhoea and runny nose. |
Vaccination
As the disease is so widespread vaccination is a vital part of the biosecurity of the herd. The vaccines available have proven to be highly efficacious. They must be used according to the manufacturer's recommendations and ideally given close to the breeding season. The primary course is two doses followed by an annual booster.
BVD is an economically important disease which many countries in Europe are now controlling and eradicating. With good reliable tests and efficacious vaccines, eradication of BVD is certainly possible.
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