A clinical and laboratory study of smallpox in Brazil. Accuracy of the laboratory diagnosis of smallpox in patients with Brazilian variola minor infection.

Abstract

This study was conducted to determine the accuracy of standard diagnostic laboratory tests for smallpox. From 165 Brazilian smallpox patients whose clinical illness was well documented, 352 specimens of material from lesions were studied. Serological and viral characterization studies were also performed. Electronmicroscopy (EM) and chorioallantoic membrane (CAM) isolation of variola were positive on 100% of vesicopustular and crust specimens. The agar gel-diffusion (AG) test was positive in 98.3 and 97.1% of those specimens. Virus in vesicular fluid smeared on glass slides was not stable. Only 50% of these smear specimens gave positive results for smallpox. EM was five times more accurate than AG test or CAM culture on smears stored at room temperature. Whenever the AG test was positive for pox virus, EM and CAM tests were also positive. No false-positive diagnosis of smallpox was made, and a false-negative diagnosis in smears from 19 of 20 patients was corrected by the study of additional specimens. The Brazilian variola minor virus was excreted in saliva and breast milk. It differed slightly from the international reference variola minor strain.