Seroepidemiology of human plague in the Madagascar highlands

We conducted a seroepidemiological survey of human plague in the general po pulation using random sampling in the area of Ambositra, the main focus of plague in the central highlands of Madagascar (520 confirmed and presumptiv e cases notified during the past 10 years). Sera were tested using an ELISA IgG F1 assay. Considering the internal validity of the assay and the sampl ing method, the overall corrected prevalence of F1 antibodies was 0.6% (95% CI: 0.2%-1.8%). Being nearly 0 up to the age of 40, the corrected prevalen ce increased markedly after 45 years to 6.2%. Six of 20 individuals who dec lared to have been treated for clinical suspicion of bubonic plague in the past had F1 antibodies. The seroprevalence did not differ according to gend er except in individuals > 60, where antibodies were significantly more fre quent in males. This study suggests that the number of clinically suspected cases of plague provided by the surveillance network was plausible, despit e some true cases being missed and a significant number of false positives. We also confirm that Yersinia pestis infections may occur without marked c linical manifestations and patients may recover without treatment, in accor dance with old observations of pestis minor.