EVALUATION OF A BULK-MILK ELISA TEST FOR THE CLASSIFICATION OF HERD-LEVEL BOVINE LEUKEMIA-VIRUS STATUS

The results of a commercial bulk-milk enzyme-linked immunosorbent assa y (ELISA) test for herd-level bovine leukemia virus (BLV) status were compared to results obtained from individual agar-gel immunodiffussion (AGID) testing on sampled cattle. A positive herd was defined as a he rd having one or more AGID-positive animals. The estimated true herd s tatus was based on the sensitivity and specificity of the AGID test an d the number of cattle sampled per herd. Ninety-seven herds were used, with a mean of 13 cows sampled per herd. The AGID test indicated an a pparent herd prevalence of 70.1%. After accounting for the number of c ows sampled and the sensitivity and specificity of the AGID test, the estimated true herd prevalence of BLV was 52.3%. The ELISA test identi fied 79.4% of herds as positive for BLV, and had an apparent sensitivi ty and specificity of 0.97 and 0.62, respectively. However, after acco unting for the sensitivity and specificity of the AGID test in individ ual animals, the specificity of the ELISA test was 0.44. The ELISA tes t was useful for identifying BLV-negative herds (i.e., ruling out the presence of BLV infection in test negative herds). With the moderately low specificity, herds identified as positive by the ELISA test would require further testing at the individual or herd level to definitive ly establish their BLV status. (C) 1997 Elsevier Science B.V.