Two plaque-purified strains of Langat virus were studied to find markers that would distinguish strains of reduced pathogenicity suitable for vaccines. The following properties were examined: 1) pathogenicity in mice, 2) plaque morphology in chick-embryo cell cultures and the course of multiplication and antigen production, 3) thermal-inactivation rates, 4) chromatography on calcium phosphate columns, and 6) antigenic differences shown with the modified hemagglutination-inhibition (HI) test of Casals. The plaque assay, with the serum-free agar-overlay medium, was found to be as sensitive as the titration of the infectiousness of virus in mice, but plaque morphology was much the same for the two viruses. The two strains could, however, be differentiated on the basis of degree of peripheral pathogenicity in mice, thermal-inactivation rates, and difference in elution patterns and in antigenicity demonstrable by the modified HI test. The markers presently known still appear to be inadequate for the definition of Langat strains as suitable candidates for live vaccines against the tick-borne complex of group B arboviruses.
Present address: c/o Dr. David F. Clyde, Institute of International Medicine, 660 West Redwood Street, Baltimore, Maryland 21201.