FIG 4.

Viral titers in nasal washes, BALFs, and lungs of progressively passaged rH1N1 virus-infected pigs at 4 dpi. Viral titers of nasal washes (A) and BALF (B) were directly determined by TCID₅₀ assays on MDCK cells. (C) Collected lung tissues were weighed, and 10% (vol/vol) homogenates were prepared in cold PBS. Virus titers in cleared homogenates were determined by TCID₅₀ assays on MDCK cells. All values shown are means ± standard deviations (SD) from three independent experiments for each sample. Representative SD731 virus titers derived from corresponding sites of infected pigs served as positive controls. An asterisk indicates that the virus titer was not significantly different from that of the SD731 virus group (P > 0.05 by ANOVA). The virus detection limit was 10 TCID₅₀.