National BVDV eradication programme: Assessing the utility of the spot test

We examined the BVDV test records of 3769 farm holdings from one laboratory involved in a national BVDV eradication programme with the objectives (1) to investigate the utility of the spot test to identify BVDV infected herds correctly; (2) to explore the possible reasons for false positive results and (3) to examine the frequency with which herds subsequently had a BVDV persistently infected (PI) calf born. During the 12-month study, 283 (7.5%) farm holdings were identified as BVD not negative based on the presence of antibodies to BVDV. Follow-up testing was carried out in 131 (3.5%) and in 79 (2.1%) of those, results were not consistent with BVDV infection. The main cause of false positive spot tests was transient antibodies in animals younger than 270 days of age, consistent with the persistence of maternally derived antibodies. BVDV PI births were found on 34 farm holdings (0.9%), in herds that were identified by the spot test as BVD not negative, as well as in herds identified as BVD negative. Farm holdings with spot tests with more than 40% of the samples positive for antibody and those with more than 20% and less than 40% were, respectively, 27.7 and 25.0 times more likely to have a PI born than those that had a negative spot test. However, farm holdings with at least one positive, but no more than 20% of the samples positive for antibody, were 3.9 times more likely to have a PI born than those in which no antibody positive animals were detected. We concluded that ignoring the presence of single antibody positives in the spot test could be counterproductive to the objective of national BVDV eradication.