Segregation of mouse virulent and avirulent ECHO virus type 9 strains and the correlation of mouse-virulence with the "temperature" (rct/40) marker

A quantitative study of mouse virulence of ECHO virus type 9 strains, isolated from cases of aseptic meningitis was carried out using the plaque technique. Plaques obtained directly from ECHO virus type 9 isolates exhibited marked variation in mouse virulence; the majority of plaques was of high or low virulence, and only 3 out of 80 plaques tested were completely mouse avirulent. Subplating of these mouse avirulent plaques yielded strains which were predominantly mouse avirulent (45 out of 47 plaques). However, the passage in undiluted form of the avirulent plaque progeny in MKCC and especially in infant mice was selective for mouse virulence. The rct/40 marker was employed for the study of mouse virulence of the ECHO virus type 9 strains. An excellent correlation was found between the rct/40 marker and mouse virulence; rct/40⁺ strains were mouse virulent whereas rct/40^- strains were avirulent. The rct/40 marker is suggested for use in the determination of mouse virulence of ECHO virus type 9 isolates.