FIG. 4.
Effect of 5,000 PFU of rA59/S→3′UTR MHV-1 viruses on A/J mice. Groups of 14 mice were infected intranasally with 5,000 PFU with two independent rA59/S→3′UTRMHV-1 recombinants, rA59, or MHV-1, and followed clinically as described in Materials and Methods. Mice judged to have severe disease were euthanized. (A) Survival curve. (B) Three mice were selected randomly from each group at 36 h, 4 days, and 7 days postinfection and euthanized. Lung homogenates in DMEM (10% [wt/vol]) were prepared, and virus titers were determined by plaque assay. Results are expressed as PFU/g of tissue. The limit of detection was 170 PFU/g.