Assay of variola virus by the fluorescent cell-counting technique.

A quantitative assay for infective variola virus particles was developed which is based on the enumeration of cells containing fluorescent viral antigen after infection of McCoy cell monolayers. The direct fluorescent-antibody technique was employed to stain cells. The efficiency of virus adsorption was markedly enhanced by centrifugation of virus inoculum onto McCoy cell monolayers at 500 x g for 15 min. By this procedure, a proportionality was obtained between the number of fluorescent cells and volume of inoculum. Observations on the sequential development of viral antigen within cells and counts of fluorescent cells showed that the optimal time for enumerating fluorescent cells was after an incubation period of 16 to 20 hr. A linear function existed between virus concentration and cell-infecting units. Fluorescent cells were distributed randomly in infected cover slip cell monolayers. The assay was demonstrated to be highly sensitive, precise, and reproducible.