FIG. 2.
Virulence of influenza viruses for C57BL/6 mice. Groups of 5 mice were infected with 105 PFU of BJx109 or PR8 (A) or reverse-genetics virus PR8-BJx109 HA or PR8-BJx109 NA (B) via the intranasal route. Control mice received an equivalent volume of PBS. (Top) Mice were weighed daily, and the results are expressed as the mean percent weight change of each group ± standard error of the mean (SEM) compared to the weight immediately prior to infection. Animals displaying evidence of pneumonia and/or having lost >25% of their original body weights were euthanized. (Bottom) Survival of mice following intranasal infection with influenza viruses. The data shown are from one experiment and are representative of two or more independent experiments. The P values for survival proportions were obtained using the Mantel-Cox log rank test. BJx109 versus PR8, P < 0.01, and PR8-BJx109 HA versus PR8-BJx109 NA, P < 0.01, in 2 independent experiments.