Pathogenicity of mutant viruses in mice. Groups of 5 mice were infected intranasally with 106 EID50 (A, B) and 104 EID50 (C, D) of mutant viruses. Mice were monitored daily for weight loss and signs of disease over a 14-day period. Data represents the mean body weight change (%) ± SD. Statistical analysis between the wild-type virus and the mutant viruses was performed using Mann–Whitney test (**p < 0.01).