[Factors affecting plaque formation by Lassa virus in Vero cells].

The method of Porterfield and Allison was adapted for titration of the infectious activity of Lassa virus by the plaque formation in Vero cells. The virus was cloned, and the effect of the time of adsorption, pH, temperature, as well as polycations (DEAD-dextran, protamine sulphate) dimethylsuphoxide (DMSO), and trypsin added during adsorption or into the agar overlay on the effectiveness of plaque production by Lassa virus (virus titres, plaque size) were studied. The optimal adsorption time was found to be 1 1/2-2 hours, pH 8.0. The number of plaques produced by the virus was approximately similar at 35 degrees C. The substances under study did not enhance the efficacy of plaque formation, on the contrary, DMSO and high concentrations of polycations decreased plaque size.