Fig. 5.
Virulence assay using the intranasal model of infection. Groups of five BALB/c mice were mock-infected or infected with (a) 107 or (b) 4×106 p.f.u. of the indicated purified viruses. Mice were weighed daily, and results are the mean percentage weight change of each group ± sem compared to the weight on the day of infection (upper panels). Animals were monitored daily for signs of illness, scored 1 to 4 (Alcami & Smith, 1992) (lower panels). Data are expressed as the mean ± sem. P values were determined using the Student's t-test and indicate the mean percentage weight changes or signs of illness that were significantly different between groups.