The relationship between thymidine kinase (TK) activity and virulence was studied in the mouse using three HSV-1 strains: (1) NIH TK+ (100% activity), (2) NIH TK+/- (25% TK activity), and (3) NIH TK- (0% TK activity). Following corneal inoculation, keratitis, virus titers (eye, trigeminal ganglia brain), survival, and latency were determined for each strain. The most virulent strain, NIH TK+ (3...