The in vitro cytopathology and the neurovirulence to inbred mice demonstrated by five variants originally derived from one laboratory strain (Miyama) of type I herpes simplex virus (HSV-1) were studied comparatively. Three of the variants are syncytial [+GC (LPV), +GC (SPV), +GC (81)] and two are non-syncytial [-GCr and -GCf]. The size of plaques produced by the five variants was found to be in the order of +GC (LPV)>+GC (81)>+GC (SPV)>-GCf>-GCr. The pathogenicity of these variants was compared in three kinds of inbred mice (AKR, C 3 H/He and C 57 BL) after intraperitoneal (IP) or intracerebral (IC) inoculation. The +GC (LPV) variant was the most virulent as shown by the highest mortality of mice by either route of inoculation. The other four variants caused death of mice only after IC inoculation, and among these variants, +GC (81) was shown to be the most virulent. These data indicate that so far as these five variants of the Miyama strain of HSV-1 are concerned, neurovirulence is positively correlated with their cell fusion activity or the size of plaques which they produce. Pre-IP-inoculation with any of the less virulent variants [-GCr, +GC (SPV) and +GC (81)] protected mice from subsequent lethal infection with +GC (LPV) by the same route of inoculation.
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