Bovine brucellosis: evaluation of field sera by a competitive and superimposable ELISA utilising a monoclonal antibody against Brucella abortus lipopolysaccharide

With the aid of a horseradish peroxidase (HRP) tagged monoclonal antibody against smooth lipopolysaccharide from Brucella abortus (Bruce 1), a competitive and superimposable ELISA test procedure for bovine brucellosis has been evaluated for its ability to discriminate between Strain 19-vaccinated (S19-Vacc) and Biotype 1-infected (B1-Inf) cattle. In the competitive assay, all sera from S19-Vacc animals competed effectively against HRP-Bruce 1 (low HRP activity), while 10 out of 40 B1-Inf animals competed less effectively with Bruce 1 (high HRP activity). Successful competition by cattle antibodies would result in an increased proportion of cattle Igs binding to the assay antigen. This was confirmed by superimposing an alkaline phosphatase conjugated rabbit anti-cattle Ig after the competitive ELISA had been completed. With the superimposable assay, alkaline phosphatase activity was correspondingly high for S19-Vacc animals, and low for 36 out of 40 B1-Inf animals. The superimposable ELISA had therefore improved the discriminatory capabilities of the assay procedure from 75% to 90%.