Improved competitive enzyme immunoassay for the diagnosis of bovine brucellosis

A modification of the competitive enzyme-linked immunosorbent assay (C-ELISA) for differentiating the antibody response of cattle vaccinated with Brucella abortus strain 19 and B. abortus infected cattle is described. This assay utilizes lipopolysaccharide as the antigen, immobilized on a polystyrene matrix, and a monoclonal antibody (M84) with specificity for an epitope of the O-polysaccharide. A goat anti-mouse IgG antibody-enzyme conjugate is used for detection. The specificity of the modified assay was 99.7% when 1446 sera from brucellosis free herds were tested and it correctly identified 636 sera from B. abortus infected cattle as positive, using a cut-off of 30% inhibition, for a sensitivity estimate of 100%. No reactions were noted among 261 sera from vaccinated cattle. However, in testing 1147 sera that gave positive reactions in the buffered plate antigen test, the indirect ELISA, the complement fixation test or a combination of these tests from the serum bank, 31 gave positive reactions. Twenty-seven of the 31 sera originated from recently vaccinated cattle. The overall specificity for sera from vaccinated cattle was 97.3%. Because of the sensitivity and specificity of this procedure and its ease of performance, it would be a reasonable alternative as a single assay for serological diagnosis of brucellosis.