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Bovine viral diarrhoea

BVD stands for Bovine Viral Diarrhoea. It is quite a complex disease.

The outcome of infection

The outcome of infection with BVD virus depends on when infection occurs.

If a healthy, non-pregnant cow is exposed to BVD virus she is likely to be slightly unwell for 2-3 days but will soon develop a strong immunity to the virus. Symptoms of this transient infection include a raised temperature, reduced milk yield, mild diarrhoea and a temporary reduction in fertility; many infections go unnoticed. The animal will respond well to vaccination at a later date.

The BVD virus really starts to cause problems when the cow is first infected during pregnancy. If she meets the virus for the first time in early pregnancy she will abort and return to service; and if she is heavily in-calf both she and the calf will fight off the infection. However, something unusual happens when a cow is infected for the first time in early pregnancy if the foetus survives.

During early pregnancy the calf’s immune system is immature and fails to recognise the virus as something ‘foreign’. If this happens the calf is still infected with the virus when it is born and remains so for life. Some of these calves are born with defects such as brain damage or abnormal eyes but most appear normal. Many will fail to grow properly and fall behind, being stunted or “poor doers”. These persistently infected animals are unable to produce antibodies to the virus and are termed “PI calves”. They never fight off the infection because they can’t produce antibodies to it. It has been estimated that up to 1% of replacements for the national herd are PI calves.

Some PIs look the same as others in the group but many go on to develop mucosal disease, which is a wasting condition caused by the persistent BVD infection. It is characterised by fever, diarrhoea and sometime ulcers in the mouth. Mucosal disease is always fatal and there are no effective treatments so once cases are identified they should be euthansed on welfare grounds. The age at which PI calves come down with mucosal disease is variable and can be as late as eight years of age but more commonly cases of mucosal disease are under two years.

BVD is caused by a pestivirus, the same group that causes Border Disease in sheep. There are strong similarities between the two diseases.

As PI calves are unable to produce antibodies vaccination does not work for them. They produce virus throughout their lives which is shed in the environment infecting other animals. The number of virus particles produced is highest when PIs are clinically ill with mucosal disease but PIs can be considered “‘virus factories” throughout their lives.

Eradicating PI animals is something that will benefit all farms, even those that are already vaccinating because the amount of virus PIs produce stretches the ability of vaccines to prevent disease.

Identifying BVD infection

Dairy farmers can identify BVD infection on their farms by leaving an agitated milk sample for their veterinary surgeon. The antibody level matches the level of infection in the herd. It is also possible to test for antigen (virus) to see if there is a PI in the milking herd on this sample.

The first step to determine if BVD is affecting a beef herd is to blood sample 5-6 calves from each group of young stock (6 to 18 months of age). If the farm is PI free it is 99% certain these calves will come back with no antibody to BVD. Tests are currently approximately £4.50 per calf (ten or more) plus the time taken to collect the samples.

Control of BVD

BVD is difficult to confirm without testing and is spread by PI animals so control measure include:

  • Identify and remove PI animals
  • Vaccinate the herd
  • Monitor the herd to ensure it remains BVD free
  • Take biosecurity measures on-farm including blood sampling stock joining the herd and washing and dipping wellington boots

Identifying PI calves

There are two ways to identify PI calves – tissue samples and blood samples.

If every animal is blood tested and the results are clear the test can be the first qualifying test for BVD accreditation. Each group of young stock is tested annually and a farm becomes accredited a year after the first clear test if all the stock are still negative.

Blood samples from negative animals can be pooled and then tested for antigen to find the PI calves amongst a group. The fee is £25.40 for ten samples pooled together. (It is more expensive to test for antigen than antibody and pooling reduces costs in the hunt for PI calves.)

Tissue sampling is becoming more common and to date 2.8 million calves have been tested this way in Switzerland. The sample is collected as the calf is tagged. You can use management tags or an official pair of tags. The tissue samples are sent to the laboratory in batches at least once a week.

The price depends on which tags and laboratory are used.

BioBest charge £7.50, including a pair of official tags.

Northern Labs charge £7.00, including a pair of official tags.

Northern Labs charge £5.35, including a pair of management tags.

More information about tissue sampling is available by contacting the representative from Allflex tags using the number on their website.


Identifying PI calves is done in combination with vaccination. We currently stock and recommend Bovidec.


If you are buying stock the easiest way to confirm that they are not PIs is by blood sampling them before they are moved on to your farm.