Serological Survey for Human Monkeypox Infections in a Selected Population in Zaire

About 3460 persons living in Kole zone of East Kasai, in Zaire, were examined and their sera screened initially by a haemagglutination-inhibition test. Of these, 667 (19%) were positive. Radioimmunoassay adsorption tests for the presence of monkeypox- or vaccinia-specific antibodies gave unequivocal results in 300 of these sera; the remaining 47 were nonspecific. Monkeypox-specific antibodies were found in sera of 27 individuals, of all ages and both sexes, giving an overall prevalence rate of monkeypox virus-specific antibodies of 0.8%. The prevalence rate was four times higher in the 5 to 9 year age group (1.3%) than in children aged 0 to 4 years (0.3%), and was highest (2.4%) in the 15 to 19 year age group. There was no significant difference in the prevalence rates between the sexes. As might be expected, there are substantially higher prevalence rates in persons living in forest galleries than in those in savannah, and among those living in areas where human monkeypox cases had occurred in the past compared with those living in other localities. Nineteen children whose sera showed specific monkeypox antibodies were re-examined. Twelve showed facial and body skin changes suggesting the presence of vesiculo-pustular disease in the past; four of these had been known registered monkeypox cases. Seven children had neither signs nor history of past vesiculo-pustular disease, suggesting that they had suffered from subclinical infection with monkeypox virus.