Comparison of virulence between w81 and ma81 virus infection in C57BL/6 mice. Groups of 18 C57BL/6 mice were inoculated intranasally with 5 MLD50 of wild type (w81) or mouse-adapted (ma81) H5N2 avian influenza virus. Viral growth kinetics in mouse lungs were determined by TCID50 at 1, 3, 5, and 7 dpi (A). Data are expressed as mean × SD titers from 3 lungs per time point. Mean body weight changes (B) and survival rates (C) were monitored in six remaining mice for 14 days. *, P value ≤0.05; MLD50, mouse lethal dose 50%.