11 April 2008

3 minute read

It causes significant gastrointestinal, respiratory and reproductive disease in cattle Dr Dan Givens from the Auburn University College of Veterinary Medicine told the recent US National Cattlemen's Beef Association Convention.

He said that if an infection reaches 34 per cent of a herd, losses have been estimated at $20 per calving if it is a low virulent strain and this can go up to $57 per calving if the strain is highly virulent.

The losses could be even higher because of hidden loss, such as suppression of the immune system, or low fertility caused by BVDV.

He said the significance of the disease could be seen by the large number of different vaccines that are on the market - estimated at more than 180.

However, Dr Givens said that vaccination is only half the answer, as vaccinating cows can only cannot completely protect developing foetuses from infection and the calves can act as reservoirs for BVDV and remain carriers for the rest of their life.

If foetuses are infected with BVDV in the first 150 days of gestation, then they risk being aborted or their immune system can be altered so that they have the viral infection for life.

Dr Givens said that these infected animals shed a lot of infectious viruses through faeces, urine, saliva, nasal discharge, hair and secretions from the reproductive tract. These animals which hold a persistent infection are more common than many people believe, Dr Givens said, and they are a critical link in the continued transmission of viruses and they are a continual economic burden on the business.

He showed that numerous tests on herds in the US and in the UK among beef and dairy herds had shown a prevalence of persistently infected animals ranging from under one per cent ( 3 out of 1,769 tested in the UK) to more than 10 per cent (59 out of 559 tested in Wyoming in the US).

Identification and Removal

Apart from vaccination, these persistently infected animals need to be removed, to control BVDV, and to do this the herds need to be tested regularly to identify the infected animals.

The producer can work out a strategy for controlling BVDV by working out a testing programme together with the vets.

DR Givens said that there are also ELISA (enzyme linked immunoabsorbent assay) tests that can be carried out on ear notches after the sample has either been frozen or refrigerated. There are also immunohistochemistry tests that can be carried out on samples in formalin. Dr Givens said that thee tests are not affected by ingestion of maternal antibodies in colostrums, so they can be carried out on animals under six months old.

An antigen capture serum test, which isolates the virus from serum, can be affected by the ingestion of maternal antibodies, so it should be carried out on calves over six months old.

However, Dr Givens added that while the tests are reliable, no test is perfect and if a positive reaction is found then the animals should be retested in 21 days to verify whether the calf is BVDV positive or not.

Transient Infection

If a cow, which is not pregnant, is transiently infected then the actual clinical impact might not be severe and there is no real need to remove the animal from the herd.

However, Dr Givens said that if a pregnant animal is transiently infected the foetus could become persistently infected during the first 150 days of gestation. These persistently infected animals pose a severe threat to all animals, particularly pregnant cows or heifers. Persistently infected animals need to be removed either by being humanely killed on farm or by being taken to a slaughterhouse.

Further Reading

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Find out more information on BVDV by clicking here.