Bovine viral diarrhoea virus (BVDv) is a pestivirus, from the family Flaviviridae, related to classical swine fever and border disease (sheep).
The virus exists as multiple strains, which can be broadly classified into two groups, type 1 and type 2. Both can trigger acute disease of varying severity.
- Pestiviruses have the ability to establish persistent infection in calves during gestation, which often goes unnoticed.
- These persistently infected (PI) animals shed a huge amount of virus throughout their life and are the main reservoir of infection. Some PIs succumb to mucosal disease while others can reach breeding age and produce PI calves and infect other pregnant and non pregnant animals whilst they remain in the herd.
- When non PI animals are infected via transmission of infected bodily fluids they shed virus for a short period of time but can be a risk to naïve animals.
Acute disease occasionally gives rise to common symptoms (fever, anorexia, depression and diarrhoea) but its main role in non pregnant animals is to affect the immune system and make it less able to resist other infections including respiratory disease, mastitis and scour. Mucosal disease affects some PI animals and is characterized by ulcerations of the mouth and digestive tract, profuse scour leading ultimately to death. BVDv infection reduces reproductive performance of a herd by causing early embryonic death and abortion.
- In pregnant animals BVDv has the ability to cross the placenta and infect the foetus and in the first trimester of pregnancy results in the birth of persistently infected (PI) calves, which may later develop mucosal disease.
Diagnosis may be made on clinical signs and epidemiology, but clinical suspicion must be confirmed by laboratory examination of blood, milk or tissue samples. These can be tested for antibody or antigen.
Symptomatic treatment of mucosal disease is futile.
The most important components of a BVD control program to reduce or eliminate BVD are
- screening tests
The primary objective for vaccination of breeding animals with a BVDv vaccine is to prevent the birth of persistently infected calves because of the epidemiological threat they pose to the other animals in the herd.
BVDv is also included in respiratory vaccines for young animals as BVDv also contributes to the bovine respiratory disease complex.
Exposure to BVDv is widespread with 95% of dairy herds showing serological evidence of exposure1 although the percentage of herds with active infection at any one time will be less than this.
In a recent Scottish study carried out in 2007 by the SAC2
From a bulk milk sample survey of 400 dairy herds:
- 22% of study farms showed no evidence of recent BVD exposure;
- 42% of study farms showed high antibody titre, indicating recent BVD exposure or vaccination; and,
- 36% of study farms showed intermediate antibody levels.
From a blood sample survey of 300 suckler cow herds collected:
- 62% of study farms showed no evidence of recent BVD exposure; and,
- 17% of study farms had Persistently Infected animals.
It causes economic losses which are often underestimated as some may not be obviously attributable to the BVDv infection.
- PI animals are a source of loss, as typically, PI's fail to reach their genetic potential, exhibiting decreased weight gain, increased disease susceptibility, and reduced fertility. They shed the virus, causing reproductive loss and immunosuppression in naïve animals.
- BVDv infections also cause fertility losses due to an increased risk of embryonic and foetal death resulting in lower conception and pregnancy rates and reduced reproductive performance.
In a naive beef suckler herd BVDv infection can cost £37.00 per cow per year over a ten year period peaking at £65/cow/year3.
http://www.afbini.gov.uk/adds-cattlebvd.pdf (RISK MANAGEMENT OF BVD AND IBR David Graham)
http://www.scotland.gov.uk/Resource/Doc/915/0099594.doc. (BVDV BEST PRACTICE SCOTTISH INDUSTRY STANDARDS)
http://www.checs.co.uk/ (Checs Technical document)
Will an inactivated BVDv vaccine have any effect on a PI?
No, it will continue to shed and be a threat to the herd, plus if it is retained for breeding it will produce a PI calf.
If my herd has active BVDv will vaccination alone be enough?
No, herd owners in association with their veterinary surgeon need to find and cull any PI animals within their herd.
What does a high positive bulk milk antibody titre mean?
It means that there is either current or recent BVDv circulating within the herd.
How do I know if I have a PI animal in my herd?
Your veterinary surgeon can blood sample 5 animals from each management group between 9 and 18 months of age and depending on these results it may be necessary to test further animals
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