Diagnostic usefulness of antibodies against ribosome recycling factor from Brucella melitensis in human or canine brucellosis

The diagnostic usefulness of an enzyme-linked immunosorbent assay (ELISA) using a purified recombinant ribosome recycling factor from Brucella melitensis (CP24 antigen) was tested in human and canine infections caused by smooth and rough Brucella species, respectively. Anti-CP24 antibodies were detected in 9 (43%) of 21 consecutive cases of canine brucellosis and in 8 (53%) of 15 dogs followed for 60 days after the diagnosis of acute brucellosis. Among eight patients with acute brucellosis, anti-CP24 antibodies were detected in four in the 10 weeks following diagnosis, but the remaining four were negative during the whole follow-up (22 weeks). The frequency of anti-CP24 antibodies was also low among 24 patients with subacute brucellosis and 23 patients with chronic illness (29 and 26%, respectively). While all patients positive for anti-CP24 antibodies were also positive for antibodies to total cytoplasmic proteins of Brucella (CP), five were negative for antibodies to another cytoplasmic protein, the Brucella lumazine synthase (BLS). When a larger sample of 35 human sera negative for anti-BLS antibodies was assayed, 85.7% were positive for anti-CP24 antibodies, suggesting that the combined measurement of both reactivities could yield a higher sensitivity than any test alone. To test this hypothesis, an ELISA combining both antigens was designed. The percentage of positive results among chronic cases was higher for this assay than for the individual measurement of anti-CP24 or anti-BLS antibodies (83 versus 26 and 65%, respectively) and was closer to the value obtained for anti-CP antibodies (91%). The frequency of anti-CP24 antibodies is low in both canine and human brucellosis. In the latter case, however, an ELISA combining CP24 and BLS is more sensitive than assays measuring anti-CP24 or anti-BLS antibodies separately and almost as sensitive as the ELISA using CP.