Virus titers, virus antigen, and cytokine expression in 1918 H1N1 influenza virus–inoculated ferrets. Ferrets were inoculated intranasally with 10⁶ 50% tissue culture infectious dose (TCID₅₀) of 1918 H1N1 influenza virus. Four ferrets were euthanized at each of the time points 1, 3, 5, and 7 days postinoculation (dpi) and tissue samples were collected for analysis. A, Virus titers in heart, liver, spleen, pancreas, duodenum, jejunum, colon, kidney, and adrenal gland at 1, 3, 5, and 7 dpi. B, Detection of influenza virus nucleoprotein in hepatocytes of the liver of 2 ferrets euthanized at 5 dpi (left panel: ×400 magnification; right panel: ×1000 magnification). C, Fold change expression of tumor necrosis factor alpha (TNF-α), interleukin 6 (IL-6), and interleukin 8 (IL-8) in the heart, liver, and pancreas of ferrets inoculated with 1918 H1N1 virus relative to the mean of 4 Mock ferrets was calculated. Colors indicate up- or down-regulation of cytokines relative to the mean the Mock ferrets. Each square indicates expression levels in an individual ferret. Asterisks in the first square in a group indicate a statistically significant difference (t test) of the group of 4 ferrets as compared to the 4 Mock ferrets. *P ≤ .05; **P ≤ .01; ***P ≤ .001; ****P ≤ .0001.