Evaluation of an enzyme-labeled antiglobulin test for anti-Brucella immunoglobulin G among 3 cattle populations.

Antibody to smooth Brucella abortus lipopolysaccharide antigen on the surface of polystyrene tubes was detected with peroxidase-labeled antibody against bovine immunoglobulin G. The enzyme-labeled antiglobulin test (ELAT) activity of samples was expressed in arbitrary units/0.01 ml by reference to a standard curve based on tests of dilutions of a positive serum pool. Reactions greater than 3.0 U/0.01 ml were classified positive because specificity at this level was 99.8% (417/418 samples correctly classified negative) with agglutination test-negative sera from 33 Brucella-free herds. Results of the ELAT were compared with results of agglutination tests and the complement-fixation test (CFT), using 430 sera from cattle in 7 infected herds. Activity of greater than 5.0 ELAT U/0.01 ml was detected in all 54 sera classified as positive (titer greater than 1:10) by the CFT, including 5 sera classified as negative by the tube agglutination test. Sera from 8 nonvaccinated cows in the infected herds reacted only by the ELAT, whereas reactions were obtained with 25 and 5 sera by only agglutination tests and the CFT, respectively. The ELAT and CFT results were in agreement for 25 of 26 sera from agglutination test-reactor cattle in herds of unknown status. Comparisons of milk ring and whey agglutination tests with the whey ELAT on 146 quarter samples from cows in an infected herd revealed no ELAT activity greater than or equal to 1.0 U/0.01 ml in the 73 samples considered negative by the 2 other tests. Samples (n = 47) that contained greater than or equal to 1.0 ELAT U/0.01 ml included all (n = 40) samples with milk ring or whey agglutination titers greater than or equal to 1:16 and greater than or equal to 32, respectively, and 7 samples that gave weaker reactions to the latter tests.