One hundred and fifty-eight strains of Venezuelan equine encephalomyelitis virus were typed antigenically and classified epidemiologically as either epizootic or enzootic. Plaque sizes for 148 of these strains were determined, and the pH requirements for hemagglutination (HA) of goose erythrocytes of 131 were studied. Only antigenic variant group IABC strains could be classified epidemiologically as epizootic. In vitro these strains were characterized by the formation of small plaques in Vero cells and a relatively narrow pH range for optimum HA reactivity. Experimental studies in horses confirmed the fact that only IABC strains have epizootic potential. We concluded that plaque size in Vero cell monolayers would be a useful method of screening VEE viruses for equine virulent strains. Indirect evidence suggested that small plaques resulted from sensitivity to an anionic substance present in the agar overlay medium.
Present address: Department of Medicine, Section of Infectious Diseases, Louisiana State University Medical School, 1542 Tulane Avenue, New Orleans, Louisiana 70112. Reprint requests should be sent to this address.
Present address: Clinical Research Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142.
Present address: U.S. Army Medical Research Institute of Infectious Diseases, Ft. Detrick, Frederick, Maryland 21701.