Viral RNA copy number in swabs collected from chickens infected with either CEF-H9N2 or MDCK-H9N2. Chickens were infected with two different H9N2 clones and viral excretion was detected in swabs collected at 2, 3, 5, 7, 10, 15, and 21 dpi. The data were expressed as mean ± standard deviation. The limit of detection is 2log10 copies/mL. An asterisk indicates statistically significant differences between the two clones (P < 0.05).