The relative expression ratios of target genes in tested tissues vs. those in control tissues were calculated by the 2-ΔΔCT method using the chicken housekeeping gene glyceraldehyde-3-phosphate-dehydrogenase (NM_204305) as the endogenous reference gene in order to normalize the level of target gene expression (Livak and Schmittgen, 2001). Standard deviations were determined by using the relative expression ratios of three replicates for each gene measured. Differences of virus titers and mRNA expression levels were statistically analyzed with an unpaired non-parametric test and paired Student’s t-test, respectively, using GraphPad Prism version 6.0 (GraphPad Software Inc., La Jolla, CA, United States) software. Compared to the mock-infected control, p < 0.05 and p < 0.01 were considered to indicate a statistically significant difference unless stated otherwise.