Advancing BVD Control in the UK
21st April 2008
Category: Health Fact Sheets
Advancing BVD Control in the UK:
Introduction A national or regional control strategy for the control and eradication of bovine virus diarrhoea virus (BVDV) rests on four principals that are to a degree interdependent.
1. Management to prevent the introduction of BVDV to a herd through the purchase of replacement breeding stock
2. A structured programme to demonstrate and maintain freedom from endemic BVDV infection for herds selling BVDV-free replacement breeding stock
3. A structured programme to eliminate BVDV from endemically infected herds
4. The availability of reliable and affordable diagnostic tests.
The perceived high prevalence of BVDV herd infection in the UK and an overly optimistic view on the value of herd vaccination against this disease has led to a general impression that the implementation of national control measures is neither practical nor required in this country. In contrast BVDV has been eradicated from Norway and Sweden for several years and recently Denmark, a country with similar cattle density to the UK, has also achieved national freedom from BVDV. Indeed many other EU countries have well developed national strategies either in place or at an advanced stage of planning. From this it can be inferred that no technical issue exists as a barrier to significantly advancing BVDV control within the UK from its present position.
Over the past eight years in the UK there have been BVDV control programmes in operation. These have been informed by the design and practical experience of the programmes elsewhere in Europe and influenced most strongly by the Scandinavian experience. More recently the Cattle Health Certification Standards (CHeCS) has ensured standardised BVDV programmes exist throughout the country. There are now a moderate number of herds able to demonstrate and maintain herd freedom from BVDV infection. There have also been regional initiatives, most notably in Shetland that have successfully eradicated BVDV from a significant geographical area. These experiences have shown that the BVDV control methodology is robust and applicable to UK farming conditions. Health declarations supported by veterinary signature were introduced to advertise the health status of herds at the major sales. These have now been sanctioned by a partnership that includes NFU-Scotland, BVA, CHeCS, major breed societies and auctioneers. Using this system no breeding animal can be marketed with a health declaration in Scotland unless it either has been screened by a suitable test prior to sale and shown to be blood test negative for BVDV or it is being sold from a herd accredited free of BVDV.
The following discussion and methodology reflects the SAC experience with BVD control.
SAC’s BVDV programme formed the basis of the CHeCS control programme; it was the programme used in Shetland and in Orkney where the Hi Health programme was originally designed and managed by SAC. SAC’s cattle health scheme is branded as the Premium Cattle Health Scheme and has members throughout the United Kingdom.
1. Management to prevent the introduction of BVDV to a herd through the purchase of replacement breeding stock The cattle industry in the UK has traditionally had a high reliance on the purchase of herd breeding replacements through auction markets and there has been little interest in statutory measures to prevent the sale of animals that are carrying infectious disease agents. This then places the responsibility for tackling the first point (above) on the individual herd owner. There is sufficient technical knowledge of the qualitative risks purchased stock pose for the introduction of BVDV; the use and limitations of screening tests for BVDV infection; and the quarantine requirements for BVDV to provide the individual with appropriate advice. However there is no clear indication that a significant number of herd owners seek advice from their veterinary surgeons on this area or that veterinary surgeons are sufficiently informed to provide that advice. In a national or regional control programme this area may be improved by enacting legislation to introduce more formal control at the point of sale, either through a clear compensatory mechanism or straight prohibition of the sale of breeding stock that are of unknown BVDV status. Either to support this or as an alternative strategy clear information in the form of technical literature may be produced for the industry, the herd owners and the veterinary profession on measures to control the spread of BVDV through the sale of stock. This should include a risk assessment of the varying categories of stock that are traded, information on the disease screening and quarantine strategy. An example of such information can be seen in the technical note on the purchase of breeding cattle produced by SAC. In appendix 1 there is an outline for an appropriate risk assessment for the different classes of stock that may be purchased by breeding herds. Control of BVD in finishing units is not considered since BVDV disease in these units is essentially an end stage problem i.e. by controlling disease in breeding herds the disease will be controlled in finishing units. This area of disease control is further facilitated by a critical mass of herds supplying breeding stock that have achieved BVDV free accredited status (see below). As yet that critical mass does not exist in the UK.
2. A structured programme to demonstrate and maintain freedom from endemic BVDV infection for herds selling breeding stock As explained above such programmes have existed in the UK for several years and the supporting methodology can be considered to be proven. The details of the CHeCS programme are provided in the CHeCS technical document. In summary, herds must adhere to rules on biosecurity that demand 3 metre boundary fencing between neighbouring cattle as well as quarantine and screening of added animals. Demonstrating freedom requires each management group of calves to be screened for antibody to BVDV once they have reached 9 months of age. The absence of antibody in the group indicates that there has been no exposure to BVDV and there is no BVDV viraemic animal present in the group. A clear annual herd test is achieved if in the course of one year all groups are judged to be free in this way. To achieve accredited status there must have been two consecutive clear testing years. Accredited status lasts for one year and is renewed if the screening tests are again clear and the herd owner and herd veterinary surgeon sign off that all the biosecurity rules of the scheme have been adhered to. Bulk tank antibody testing and screening of any animals showing disease signs that could be attributed to BVD infection are also included. In the beef herd the weakness with this accreditation system is that the test of exposure to BVD reflects breaches in biosecurity that have occurred at least 15 months previously. However as very few animals are sold from such herds until they are older than 9 months of age this does not materially affect the level of assurance that the scheme provides to the market. In the dairy herd, bulk milk tank monitoring provides a more immediate indicator of the incursion of BVDV to the herd, particularly where this encompasses age cohort screening such as first lactation bulk milk antibody test. There is no bar to the use of BVDV vaccine within the accredited programme. Herd owners and their veterinary surgeons concerned about the impact of BVDV incursion frequently opt to vaccinate the breeding herd to minimise this potential impact particularly where herds consist of either a naïve population of animals or an increasing proportion of naïve animals in what was once a herd with high seroprevalence. Furthermore the existing BVDV vaccine components stimulate relatively little antibody that is detected by the antibody tests that are routinely used for diagnosis and therefore the antibody response due to vaccination in groups of cattle has proved to be easily differentiated from that arising in response to infection with BVDV. For herds that do not wish to adhere to the standard on boundary biosecurity there is a lower status of accreditation where herd vaccination alone is used. This is termed BVDV vaccinated monitored free. The programmes are constantly under review by the technical committee of CHeCS and show a degree of evolution that reflects the changing knowledge of the biology of BVDV infection. This also means that there is little reason to design or develop a new accreditation procedure to support a national BVD strategy. In appendix 2 there is a flow diagram of the steps in progressing with accreditation.
3. A structured programme to eliminate BVDV from endemically infected herds The CHeCS programmes also provide methodology for eradicating BVDV from a herd. The main difference between accreditation and eradication within CHeCS is that in the former the CHeCS rules must be adhered to in order to satisfy the certification procedure while in the latter the methodology provides a framework that can be adapted to the individual herd and is not subject to certification. Where active BVDV infection has been demonstrated a herd can only enter the accreditation procedure once it has passed through eradication. Essentially eradication involves screening all stock on the unit for BVDV on one occasion in its life. This process is continued for all young stock born for the 12-month period after the last persistently viraemic calf has left the herd. The herd then moves on to the accreditation programme. Modifications to this methodology have arisen because of the extremely low prevalence of BVDV persistently viraemic animals in the adult herd. To prevent resources being wasted on unnecessary testing, cows that have given birth to calves that have tested negative for virus need not be screened. This is a practical modification of value in the beef cow herd, but of less relevance in the dairy herd where fewer calves are available for testing as most male calves are either culled or sold early in their life. However in the dairy herd bulk tank screening for virus by PCR can provide an effective screen on the adults in the herd. As the detection limit of this test is in the region of one in a hundred cows individual milk samples may need to be collected in large herds and the screening may have to be repeated to screen cows that were not in milk when the test was carried out. If a positive result is achieved then individual animal blood screening is required on the animals that made up the positive pool. In appendix 3 there is a flow diagram illustrating a common approach to herd eradication of BVDV infection.
4. The availability of reliable and affordable diagnostic tests The Scandinavian approach to both accreditation and eradication initially relied on the use of the antibody ELISA to indicate exposure to BVDV infection. Where herd infection had been identified or was suspected individual animals that had tested with low or negative antibody titres could then be tested for the virus by culture methods, usually confirmed by immunoperoxidase (IPX) staining. The development of the BVDV antigen ELISA provided a test that was cheaper and had a comparable sensitivity to the culture and IPX methods. ELISA testing allows savings to be made through large scale testing and automation. Despite the development of a cheap and reliable ELISA to detect viraemia it has remained common practice to operate a two stage screening relying on antibody testing first to screen for those that require to be tested for BVDV viraemia. The advantage to this is that it provides information on the seroprevalence within each group that is tested which indicates the probability of finding one or more viraemic animals. The disadvantage is that this increases the costs of testing and doubles up the risk of identification errors at testing. The largest concern over screening for viraemic animals has been test sensitivity. Failure to pick up a viraemic animal can result in a failure in the eradication programme that may take up to a year to recognise. To overcome that the Animal Health Service in the Netherlands has advocated pooling and testing by PCR. Where pools are positive the animals that made up the pool are individually tested by the antigen ELISA. However as the sensitivity of the available PCR tests and antigen ELISAs are likely to vary and be relatively poorly defined by the manufacturer of the product it is difficult to conclude which testing system provides the best sensitivity. Of the commercially available antigen ELISAs the test kit produced by IDEXX appears to allow testing young colostrum fed animals at the earliest opportunity. Traditionally antigen screening was only advocated once calves reached three or four months. By this time maternally derived antibody had decayed to the extent that uncomplexed BVDV could be detected in blood samples. The IDEXX kit uses a protein that is produced by cells infected with BVDV rather than components of the virus itself. This has allowed testing down to one month of age. This advantage is of great value in the beef cow herd as it allows viraemic calves to be identified before they begin to be major components in the spread of infection and allows them to be excluded from the herd before the breeding period begins so reducing the chances of a new generation of viraemic calves. In North America immunoperoxidase staining on skin biopsies has been a technique widely applied to eradication of BVDV in beef herds as, because it is not affected by the presence of maternally derived antibody, it also allows the detection and removal of viraemic animals before the breeding season begins. Tagging systems have been produced for use in UK (Dalton) that produce a tissue core labelled with the calves ear number in the course of the statutory ear tagging. These tissue cores are preserved by a dessicant and can be rehydrated. The resulting tissue fluid can be used in the antigen detection system. The attraction of this approach is that it prevents misidentification at sampling and lay people can reliably collect samples in the course of routine husbandry practices. While this technology is available it has not been used in the UK. The final important point for consideration is the sensitivity of tests carried out on single animals compared to the sensitivity of the entire accreditation animal originating from a herd that is accredited through the CHeCS accreditation programme will provide a higher level of assurance than will an animal from a herd with no BVDV status that has tested negative to an individual antigen ELISA. The tests used for the diagnosis of BVD are listed in appendix 4 with an indication of the expected sensitivity, specificity, how the test is used and the expected commercial cost.
Conclusion A voluntary BVDV control programme has existed in the UK for the best part of a decade. This has supported successful regional eradication programmes and disease free accreditation in an increasing number of pedigree herds throughout the UK. From this it can be inferred that the methodology is robust and practical. The diagnostic tests are largely reliable, amenable to large scale testing and automation and therefore affordable. Regional or national control programmes may result in rapid progress in BVDV control in the UK. This would move the BVDV disease status on to a similar footing as many of our European competitors. However, rapid progress may also be made if control is applied directly at the point of sale of breeding animals. This would in turn stimulate herds producing breeding livestock for sale to participate in the BVDV eradication and accreditation programmes that are already in place. These two approaches are not mutually exclusive and greatest progress could be expected if both approaches are adopted.