VETERINARY RISK ASSESSMENT - IS THE ADDED LEVEL OF RISK REDUCTION SUFFICIENT TO WARRANT THE INTRODUCTION OF COMPULSORY BTV POST-MOVEMENT TESTING OF RUMINANTS FROM THE BTV RESTRICTED ZONE IN ENGLAND , OVER AND ABOVE THE CURRENT REQUIREMENT FOR PRE-MOVEMENT TESTING WITH NEGATIVE RESULTS
Introduction
1. The risk of causing bluetongue ( BTV8) outbreaks in Scotland by allowing movements of animals (to live) from bluetongue Restricted zones during the "vector free" period, and thereafter, was addressed in an earlier Risk Assessment of 18 December 2007. The conclusion was that the risk of transporting infected livestock to Scotland from Restricted zones, in the "vector free" period, under current rules, with negative pre-movement testing, is considered to be very low (very rare but cannot be excluded).
2. This assessment attempts to quantify the extent of any added benefit gained by introducing compulsory post-movement BTV testing in Scotland, for ruminants from the BTV restricted zone in England.
Test Sensitivity and Specificity
3. Under Annex 3 of Regulation 1266/2007, conditions are provided for the movement of ruminants from Restricted zones to Free areas during the "vector free" period. The current "vector free" period began on 20 December 2007, as declared by the Chief Veterinary Officer (UK), based on an assessment of vector trapping and climatic data. During this period movements of animals out of the Restricted zone in England are allowed, provided the animals are tested before movement take place. There are two testing options:-
(a) Waiting 14 days after 20 December and applying an "agent Identification test" ( PCR test) with negative results, or
(b) Waiting 28 days after 20 December and conducting a negative serological test ( ELISA test).
4. Advice from the Institute of Animal Health Laboratory at Pirbright ( OIE International Reference Laboratory for bluetongue) is that the sensitivity and specificity of both tests is in the region of 99.9% and it would be extremely unlikely for either test to fail to identify a positive animal. Where a test is applied before movement to animals in restricted zones, with negative results, there is very little added benefit, in terms of risk reduction, from repeating the test as post-movement test during a "vector free" period when the risk of infection after negative pre-movement testing is remote.
Disease Prevalence in the Restricted Zone in England
5. Epidemiological reports from Defra indicate that the within herd prevalence of BTV affected animals on confirmed premises in the Restricted area is very low - less than 10% on average for cattle herds and under 3% for sheep flocks as determined by serological surveillance. This is thought to reflect the relatively short time of exposure to infected midges and that cattle are relatively more attractive to midges than sheep are. The prevalence of infected herds in the Restricted zone is unknown but it is likely to be low, which further reduces the risk of introducing disease to Scotland when an effective pre-movement test is applied with negative results.
Incentives and Disincentives to test.
6. A compulsory post-movement testing BTV regime would impose a cost on recipient producers in Scotland of around £15 per animal for PCR testing, or £2.50 an animal for ELISA testing. These costs would be selectively applied to farmers who have already complied with notification requirements and paid for pre-movement testing. They could potentially encourage illegal movements in an attempt to avoid all costs, and thereby act as a disincentive to compliance with the all important pre-movement testing.
Laboratory Capacity
7. Colleagues at the IAH Pirbright laboratory have indicated that there would be insufficient laboratory capacity to provide routine post-import PCR testing, although there should be capacity for extra ELISA testing at VLA, and possibly at other private laboratories.
Duration of "Vector free" period
8. The "vector free" period is constantly being monitored. Midge trapping and climatic data are being assessed to help inform a decision about when to end the "vector free" period, however there is an element of prediction with this work and a cautious approach is advisable in making that judgement. It might be that the "vector free" period will need to be ended in late February or the beginning of March. After that time, all movements of susceptible ruminants from restricted zones will stop, as required by Regulation 1266/2007. If compulsory post-movement testing was introduced, the very small element of added risk reduction would apply to a very short period that could end in a matter of weeks.
Livestock Movements
9. The number of movements of susceptible ruminants from the Restricted zone to Scotland in what remains of the "vector free" period is difficult to predict. Scottish Government has introduced a new requirement for keepers receiving animals from a bluetongue Restricted zone to complete a post-movement notification document and send it to Pentland House. The details will be used by Scottish Government to monitor the numbers of these animals and to inform future risk assessments. Numbers of sheep and cattle moving from the Restricted area last year will be available from the official movement databases shortly, however, if Scottish farmers heed official advice to be cautious about where they source their livestock, and if the current testing requirements prove too onerous or costly, we could anticipate far fewer movements than in a "normal" year.
Conclusions
10. The degree of further risk reduction achieved by introducing compulsory post- movement testing for ruminants from the Restricted zone in England is considered to be very small and difficult to justify on veterinary and animal health grounds. The pre-movement test is the important test that prevents positive animals leaving the Restricted zone.
11. The risk of introducing an undetected untested animal, or a positive animal, to Scotland could be mitigated by introducing a post-movement checking regime. There are two main approaches to such a regime. Firstly, notifications of cattle and sheep moved from the Restricted zone to Scotland could be checked against national cattle and sheep movement databases to provide reassurance that animal movements from the Restricted zone are being notified. Secondly, the two laboratories conducting pre-movement testing ( IAH Pirbright and VLA) could provide test results to confirm that animals notified as moving from the Restricted zone have indeed been tested negative. The proportion of movements that would need to be checked in either case should be the subject of a statistical assessment based on the actual numbers of animals moving. If numbers of movements are small resources might allow for 100% checks, otherwise a statistical sample could be checked.
12. Overall, there would appear to be an insufficient level of further risk reduction to justify introducing compulsory BTV post-movement testing in Scotland as a disease control measure, and there is a risk that putting added cost on those who are already complying with the important pre-movement tests could encourage illegal movements. The greatest risk to Scotland comes from illegal movements of untested livestock.
DR MARTYN BLISSITT
Veterinary Adviser
21 January 2008