BVD virus (BVDV) is closely related to the viruses that cause classical swine fever in pigs and border disease in sheep.
BVDV causes a complex of diseases in cattle, the most important of which interfere with reproduction, affect the foetus and lead to mucosal disease. BVDV infection causes significant suppression of disease resistance and so contributes to disease complexes in calves such as pneumonia and neonatal diarrhoea.
The disease is maintained by a small population of animals that become persistently infected (PI) with the virus. These PI animals are the major reservoir of BVDV and arise after becoming infected whilst in the uterus during early pregnancy. Such infections remain throughout the pregnancy and, after birth, for the lifetime of the animal.
Cattle exposed to BVD virus may show few clinical signs, producing protective antibodies within three to four weeks. In some situations, BVD virus infection may temporarily lower immunity to other infectious diseases exacerbating these clinical infections particularly in young calves.
BVD virus during early pregnancy causes embryonic death and return to oestrus, foetal death/abortion, mummification of the foetus, birth defects of the nervous system and eyes, weak/premature calves, and live persistently-infected calves.
Three inactivated BVD vaccines are available in UK. Initial vaccination comprises two doses 3-4 weeks apart before first service followed by booster vaccination at 12 months’ intervals. If all breeding females are vaccinated then this will control disease by preventing BVD infection of the developing foetus during pregnancy and production of PI calves.
It is estimated that more than 90% of UK herds have had exposure to bovine virus diarrhoea virus (BVDv). Losses result from reduced fertility, poor production and increased susceptibility to other infections especially in young calves.
The economic losses from an uncontrolled outbreak of BVD can be very high. It was calculated in 2004 that in a 100-cow beef herd these can exceed £45,000 over a ten-year period, while losses in the dairy herd have been estimated at twice this level. In most outbreaks, reproductive losses are the most significant, although mucosal disease cases are the most obvious.
Control and prevention can only be achieved through adhering to strict biosecurity procedures, vaccination and long term control strategies. BVD eradication is possible following whole herd blood testing and elimination of all PI carrier animals. If farmers go for eradication then strict herd biosecurity measures must be maintained to prevent re-introduction of virus infection as the herd will soon become naïve and therefore fully susceptible to infection.