Preparation of HSV-1 and HSV-2 Strains with Enhanced Virulence

Serial passages of herpes simplex virus type 1 (HSV-1), strain Victoria, and type 2 (HSV-2), strain Bja, in MDBK cell monolayer and suspension cultures resulted in high viral titer in vitro and enhanced viral virulence in vivo in newborn white mice. The infectious viral titer in MDBK cells was found to correlate with viral virulence (LD50) among infected newborn mice. For example, the 4th passage of HSV-1 in MDBK suspension cell culture with a titer of 107.5 TCID50/ml caused an infection in newborn mice with a titer of 107.65 LD50/0.02 ml, while the 6th passage of HSV-2 in monolayer cultures with a titer of 107.5 TCID50/ml inoculated into newborn mice had a titer of 107.22 LD50/0.02 ml. The viral titer determined by the neutral red (NR) colorimetric method was distinctly lower (Dlog10=3) than the titer determined by microscopic read-out of the cytopathic effect (CPE) observed in the virus-infected monolayer.