Multicycle virus growth kinetics were performed in MDCK cells infected with the indicated viruses at multiplicity of infection (MOI) of 0.001. Virus titers in the tissue culture supernatants were determined by immunofocus assay (fluorescent focus-forming units, FFU/ml) using mouse monoclonal antibodies (MAbs) against IAV (HT103) (O’Neill et al„ 1998) or IBV (B017; Abeam) NP, and a fluorescein isothiocyanate (FITC)-conjugated anti-mouse secondary antibody (Dako). Mean value and standard deviation (SD) were calculated using Microsoft Excel software.