A comparison of two ELISAs for the detection of antibodies to bovine leucosis virus in bulk-milk.
OBJECTIVE
To estimate the sensitivity, specificity and detection limits for two bulk-milk enzyme-linked immunosorbent assays, the Svanovir BLV-gp51-Ab and the Lactelisa BLV Ab Bi indirect tank 250, for the detection of antibody to bovine leucosis virus in milk.


METHODS
Milk samples from 27 cows known to have enzootic bovine leucosis (EBL) were serially diluted with milk from a herd known to be free from the disease. The dilution at which antibodies could no longer be detected by each test was determined. A total of 1959 bulk-milk samples submitted to a laboratory for the Victorian (EBL) eradication program were tested with both the Svanovir and the Lactelisa assays. A Bayesian approach was used to calculate maximum-likelihood estimates of test sensitivity and specificity. An additional 660 bulk-milk samples were tested with both the Svanovir and the Lactelisa assays. Herds that had positive results on either or both of the assays were subjected to blood or milk testing of individual cattle.


RESULTS
The dilution of milk at which the Svanovir assay failed to detect enzootic bovine leucosis antibody in half of the samples was 1 in 40, whereas the comparable value for the Lactelisa was 1 in 200. Computer modeling of the operating characteristics of the Svanovir assay indicated that the sensitivity of that assay would be considerably lower than that for the Lactelisa, and the specificity was estimated to be higher. Evaluation of the assays using 660 bulk-milk samples showed that the Lactelisa assay detected four infected herds that were not detected by the Svanovir test. No false positive results were recorded for either assay.


CONCLUSIONS
Use of the Lactelisa assay in the Victorian EBL eradication program will enhance disease detection and eradication, but may also result in an increased frequency of false positive bulk-milk test results.
Ridge
S E
SE
CVO Unit, Department of Primary Industries, 475 Mickleham Road, Attwood, Victoria.
Galvin
J W
JW
D000818:Animals; D000914:Antibodies, Viral / Q000032:analysis*; D002417:Cattle; D003612:Dairying; D016583:Enzootic Bovine Leukosis / Q000175:diagnosis* / Q000821:virology*; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods / Q000662:veterinary; D005260:Female; D001909:Leukemia Virus, Bovine / Q000276:immunology* / Q000302:isolation & purification; D008892:Milk / Q000737:chemistry*; D011237:Predictive Value of Tests; D012680:Sensitivity and Specificity
D000914:Antibodies, Viral