Post-vaccination surveillance guidelines and their application David Paton described some ways in which NSPEs (Appendix 14) could be used in combination and the consequences on diagnostic sensitivity and specificity at the herd level. He emphasized that serosurveillance can substantiate rather than demonstrate freedom from infection after an outbreak. Different combinations of tests can be used in series to improve the overall diagnostic specificity but sensitivity remains a limiting factor in achieving the required level of confidence within small herds. Conclusions 1. Probang-sampling followed by RT-PCR is insufficiently sensitive unless three serial samples are tested. 2. Possible solutions to the “small herd problem” include: (i) not vaccinating them; (ii) vaccinating them but increasing the number of small herds “sampled” to increase the probability of detecting infection or (iii) applying a vaccinate-to-kill policy. Recommendations 1. The algorithm combining initial cedi test, retest of positives by cedi and final confirmation by the svanova ELISA currently provides the best available serodiagnostic system within Europe, with respect to diagnostic specificity after vaccination. Combinations, including in-house tests proven to provide equivalent performance, may also be appropriate. 2. Competent authorities should be advised that it is not possible to reach the required level of confidence when testing small herds. 3. If vaccination is not used, the combination of the Ceditest (or other equivalent NSP test) as a screening test and SPCE as a confirmatory test can do away with the necessity to handle live virus in performing the VNT. Designing post-vaccination surveillance Matthias Greiner presented a new software tool (Appendix 15) to optimise herd sensitivity whilst maintaining minimum herd specificity by altering the numbers of samples taken per herd and the cut point at which a farm would be considered positive. He explained with regard to serial testing it is advantageous to select tests that are non-covariant with regard to specificity and covariant with regard to sensitivity. He modelled two scenarios for serial testing using herd population data from Denmark. Conclusions 1. A combination of serial testing with a high specificity can still result in a high herd level sensitivity although the sensitivity at individual animal level is lower. 2. The current NSPEs are fit for purpose to substantiate freedom of FMD at a 5% prevalence at herd level and 2% prevalence between herds. 3. Further work is required to refine and validate the proposed tool by modelling the uncertainties of the data. Recommendation 1. The economic advantage of this optimised sampling strategy should be compared with the more conventional approaches. Use of likelihood ratios in analysis of NSP test results Dónal Sammin presented the possible use of likelihood ratios (Appendix 16) in interpretation of diagnostic test results and provided a means of combining results obtained with two of more tests.
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