The chicken embryo fibroblast cell line (DF1, ATCC, Manassas, VA, USA) was grown in Dulbecco's minimal essential medium (DMEM) with 10% fetal bovine serum (FBS) and maintained in DMEM with 5% FBS. The African green monkey kidney Vero cell line (ATCC, Manassas, VA, USA) was grown in Eagle's minimum essential medium (EMEM) containing 10% FBS and maintained in EMEM with 5% FBS. Primary chicken neuronal cells were grown in Neurobasal medium with B-27 supplement (Invitrogen).