Abstract
Brucella abortus strain 19 vaccine (S19) is used throughout the world to protect cattle from brucellosis. If used widely in a population it is extremely effective in rapidly reducing the prevalence of the disease. It suffers from a major disadvantage as vaccination stimulates an immune response indistinguishable, using classical tests, from that following infection with a virulent field strain of B. abortus. Consequently, veterinary authorities are inhibited from using it for fear of confusing the diagnosis of field infection. Groups of animals were vaccinated with S19 and infected with a virulent strain to mimic the field situation. Both groups of animals responded as expected, but they could not be distinguished using classical tests or by ELISA using anti-isotype reagents. A difference was observed in the normalised ELISA result when the animals were tested using lipopolysaccharide (LPS) antigens coated at different concentrations. This phenomenon was investigated further and a significant difference between the ELISA reactions of serum from vaccinated and infected animals was confirmed. This difference in binding could not be attributed to a difference in the avidity of antibodies arising following vaccination and infection. A significant difference in the binding of monoclonal antibodies to high and low concentration antigens (conventional and modified ELISA) provided strong evidence that some O-polysaccharide epitopes were becoming obscured or their conformation was changing when the LPS was coated at a low concentration. Validation of the modified ELISA in field trials showed that the test had an excellent diagnostic sensitivity and specificity when used in non-vaccinated animals. In animals sampled immediately following vaccination with S19, using a different cut-off, the diagnostic sensitivity and specificity were still good. The modified ELISA is suitable for diagnosis world-wide where S19 vaccination is used in a controlled or an indiscriminate manner.