Application of enzyme immunoassays for the confirmation of clinically suspect plague in Namibia, 1982.

An outbreak of plague occurred in Ovamboland, northern Namibia, late in 1982. Blood cultures, sera and blood clots were tested to obtain laboratory confirmations for clinically suspect cases of the disease. Isolation of the bacillus (Yersinia pestis) was attempted from blood cultures; sera were tested for antibody by passive haemagglutination (PHA) and enzyme-linked immunosorbent assay (ELISA). Sera and clots also were tested by ELISA for the specific F1 plague antigen. All the ELISA procedures were based on a monoclonal antibody to F1 antigen to ensure specificity.Thirty-eight cases were confirmed as plague: 50% by isolation, 34% by antibody responses, and 16% by the detection of antigenaemia. All isolates of Y. pestis were capable of producing F1 antigen, and significant antibody responses were observed in bacteriologically confirmed cases with paired sera. Patients who experienced sero-conversion had a higher IgM titre than IgG titre during the first nine days of hospitalization, while patients hospitalized for 17 or more days had IgG titres that were higher than the IgM titres. The relationship between IgM and IgG antibody titres is discussed with reference to identifying very recent infections. PHA titres increased and declined with IgM titres but were lower and more transient.ELISA procedures increased laboratory confirmations of plague by 23% above the numbers achieved using blood cultures and PHA tests alone. The ELISA to detect F1 antigen accounted for 86% of this increase by confirming cases where bacteriological isolation was not done. This ELISA did not replace the requirement for bacteriological isolation, since seven bacteraemic patients did not demonstrate antigenaemia.