Pestiviruses are a family of viruses with significant economic impact on animal production worldwide. There are 4 species of pestiviruses; Bovine Viral Diarrhoea Virus type 1 (BVDV1), Bovine Viral Diarrhoea Virus type 2 (BVDV2), Border Disease Virus (BDV) and Classical Swine Fever Virus (CSFV). CSFV does not occur in the UK.
The Bovine Viral Diarrhoea Viruses (BVDV) infect cattle and sheep. Acute infection with BVDV in non-pregnant cattle generally causes mild or inapparent disease, though outbreaks of diarrhoea, which may be haemorrhagic and fatal can occur, and the virus is also implicated in bovine respiratory disease complex (calf pneumonia) as it can suppress the immune system, allowing other pathogens to invade.
However, the most dramatic signs of BVDV infection occur following infection of pregnant cattle, as infection at any stage of pregnancy can cause fetal death. In early pregnancy this may appear as infertility; later on abortion and/or fetal deformity may occur. Of special importance, however, is that some calves infected in the first third of pregnancy become persistently infected (PI); these calves do not produce antibody to the virus but can survive and go on to become the most important source of virus for other cattle. Many PI calves develop a fatal enteritis (known as mucosal disease) within the first 18 months of life, and some may appear poorly grown, but others are superficially normal while shedding large amounts of virus. Detection of these PI animals, and their removal from the herd, is critical to any control scheme.
There are 2 types of test available for BVDV. When animals are infected, they produce antibody to the virus which can be detected by testing. Antibody testing can be carried out on blood or milk and will determine whether the virus is circulating in the herd. The second type of test is virus detection, which is used to identify PI animals. Control of the disease can be achieved by a combination of the removal of PI animals, vaccination and biosecurity measures. Control schemes have allowed the eradication of BVDV in some European countries and several control schemes operate in the UK. The Moredun VSU does not operate a control scheme but does provide diagnostic testing in support of SAC Veterinary Services. Since 2000, the VSU and SACVS have typed all BVDV viruses isolated in Scotland by real time RT-PCR; In the ten years to 2010, we have detected only one BVDV2; all remaining BVDV detected has been type 1.
Border Disease Virus (BDV), is A pestivirus closely related to BVDV but which mainly infects sheep. Similarly to BVDV, it mainly causes reproductive disease and the birth of weak lambs known as ‘hairy shakers’, which are persistently infected (PI). Some PI sheep can appear normal and, as for BVDV, these are considered the main source of infection. There are no vaccines and no control schemes for Border Disease, but both antibody and virus detection tests are available at Moredun VSU. Typing of isolates from border disease outbreaks show that 80% are caused by BDV while 20% are caused by BVDV1 suggesting cross-infection from cattle.
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