Figure 3. Virulence of mutant viruses in mice.

Four mice per group were intranasally inoculated with 10³, 10⁴, 10⁵, or 10⁶ PFU (each in 50 μl) of the indicated viruses. Body weight was monitored daily for 14 days. Mice that lost more than 25% of their baseline weight were euthanized. The values represent the average body weight compared with the baseline weight ± SD from four mice.