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Bovine Viral Diarrhoea Virus – the beginning of the end?

01 September 2016, at 12:00am

BVDFree England, a national scheme which aims to eliminate Bovine Viral Diarrhoea virus (BVDv) from all cattle herds in England, was launched on July 1 2016. Compulsory BVDv elimination programmes have been in operation in Scotland and the Republic of Ireland since 2013 and in Northern Ireland since March 1 2016.

BVDv is endemic in cattle in the UK and Ireland and infection results in major economic losses in infected herds. BVDv elimination has been identified as an industry priority by the Cattle Health and Welfare Group (CHAWG, www.chawg.org.uk) given its significant financial cost to the dairy and beef sectors. These losses are a result of high prevalence in combination with negative effects on reproduction and general health in affected herds. BVDv has remained endemic despite attempts to limit the losses associated with BVD through vaccination in individual herds. 

BVDv Transmission

BVDv has evolved an ingenious strategy to help it spread between animals and to persist in a herd. When a naïve pregnant cow is infected with BVDv in early pregnancy the infected cow’s mature and intact immune system responds and clears the virus from her system. However, the virus crosses the placenta and if the foetus is infected between 30 and 140 days of pregnancy the immature immune system of the foetus is damaged and creates immunotolerance, which causes the offspring to be born persistently infected (PI) with the virus. This PI calf will be immunotolerant to BVDv, in general seronegative, and shed large quantities of virus in all bodily fluids throughout its life. In most bovine populations the prevalence of PIs are estimated to be around one per cent, with some variation.

Acutely infected animals may be a source of horizontal infection but as they shed much lower amounts of virus and only for a few days during acute infection, their importance for viral transmission and persistence of the infection within the herd is limited. Seroconversion will occur resulting in natural immunity, which generally is considered lifelong.

BVDv spreads between herds through buying in or contact with infected animals, particularly PIs, or through movement into herds of cows carrying PI foetuses (PI carriers). Infection can also be introduced to a susceptible herd by indirect means through contaminated fomites, or infected embryos or semen.

Self-clearance

PI animals do not always succeed in establishing additional persistent infections before they are removed from the herd (due to death, trade or culling). This is thought to be more frequent in smaller herds, as a result of herd immunity developing quickly due to the efficient spread of virus from PI animals to the surrounding group. It can also occur in larger herds, possibly explained by an increased risk for early death in PI animals if there are greater secondary disease challenges. As a result, in any BVDv infected population (regardless of the herd-level BVDV seroprevalence), and at any given point of time, a large proportion of the herds will be free from infection due to self-clearance.

BVDv elimination programmes

The BVDv elimination programmes in the ScandinaviBVDv elimination programmes in Scandinavian countries, which started in the 1990s, have been very successful, and within approximately 10 years those countries were either free, or almost free from BVDv. The programmes focused on prevention of foetal infection in early gestation and elimination of PI animals. The core elements of the strategy in Scandinavia were biosecurity (to prevent introduction of infection into free herds), elimination of PI animals in infected herds to reduce virus circulation, and continuous monitoring of free herds for early detection of reinfection. 

The Swiss BVDv national eradication campaign launched in 2008 used testing of by ear notch samples for BVD virus using antigen ELISA or RT-PCR to identify PI animals instead of serology to identify herds in which BVD virus was circulating. The rationale behind the approach chosen was the very high initial seroprevalence, high cattle density and level of cattle movements, and the use of shared summer grazing in mountain pastures. National BVD Elimination programmes based on compulsory BVDv antigen testing of all calves started in Germany in 2011, Republic of Ireland in 2013, Belgium in 2015 and Northern Ireland in 2016. Significant year-on-year progress can be seen in the results for the Republic of Ireland (Fig. 1).

BVDFree England

BVDFree England builds on the experience from the national elimination programmes in Europe and a range of existing initiatives to control BVD. BVD control programmes have been offered for almost 20 years by Cattle Health Schemes operating to Cattle Health Certification Standards (CHeCS). The key elements of the programmes are biosecurity, monitoring herd status using serological or antigen tests and elimination of PI animals. Vaccination is permitted in CHeCS BVD programmes. A number of other initiatives ranging from individual vet practice schemes based through to regional and national programmes have also helped raise awareness and support action to control BVD on farms.

BVDFree England is an industry-owned scheme which has been developed through partnerships between the livestock industry, veterinary profession, science sector and government. A key aim is to ensure PI cattle are identified and removed to slaughter, increasing the number of herds from which the BVD virus has been eliminated, working towards regional and ultimately national elimination. Good biosecurity is an essential prerequisite to protect herds free of BVD from a breakdown.

Farmers who sign the BVDFree Charter make the following commitment:

  1. To actively engage in BVD control in order to eliminate the disease from their herd 
  2. To report all BVD testing results from their herd to the national database
  3. To allow herd status and/or individual animal statuses to be openly accessible through the BVDFree database (without any specific details of farm name or keeper being shown)
  4. Not to move Persistently Infected (PI) animals other than directly to slaughter (or through a dedicated red slaughter market)

The BVDFree Scheme is based on the same principles as other successful elimination programmes. The core of the Scandinavian strategy is captured in the four step ‘ADAM’ approach to implementing the BVDFree programme on a farm:

  • Assess the level of biosecurity and disease risk on your farm
  • Define the BVD status of your herd
  • Action plan for control of BVD on farm put in place 
  • Monitor progress – annual status check

Every vet’s expertise (EVE) will be key to successful delivery of the BVDFree ADAM principles. BVDFree England allows farms to choose whether to use serological testing to define herd status on the basis of evidence of exposure to BVD virus or to use antigen testing to detect PI carriers. Results of testing are recorded in the BVDFree publicly searchable database (available on www.bvdfree.org.uk). BVDFree assigns a status to individual animals and to a herd, where appropriate.

A minimum of two years of testing will be required to assign a BVDFree herd status unless an individual status is available for all animals in the herd, or if the herd already has a CHeCS accredited status for BVD. The CHeCS accredited status is recorded and recognition provided in the BVDFree database.

The overall aim of BVDFree England is to deliver the next generation of cattle on farms in England free of BVD virus to the next generation of farmers. The first target is to engage the majority of the cattle industry in taking action to control BVD by March 28, 2019, 100 days after the launch of BVDFree England on July 1, 2016. BVDFree will then press policymakers to protect the investment of the majority by recommending to ministers that legislation to ensure everyone takes action should be put in place. Other countries have shown that BVD elimination is possible but enthusiasm, commitment and a sustained effort from vets, farmers and others will be needed to achieve this objective. Much of the knowledge on the pathogenesis and epidemiology of BVDv has been gifted to us by previous generations of UK researchers, notably Joe Brownlie. 

It is time for this generation to stand up and make its commitment to deliver BVDv elimination for the next generation. To paraphrase, ‘we gotta stop wearing our wishbone where our backbone oughtta be.’

Dairy Lead Veterinary Science Expert at AHDB

Derek joined AHDB Dairy in 2014 having worked with the Meat and Livestock Commission and the Agriculture and Horticulture Development Board since 1998. In his current role, Derek is responsible for the strategic agenda on the control of endemic diseases.

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