The causative agent of IB is infectious bronchitis virus (IBV), a non-segmented, positive-sense, single-stranded RNA virus, which belongs to the genus Gammacoronavirus, family Coronaviridae, in the order Nidovirales [4]. The full length RNA genome of IBV is 27.6 kb in length and encodes non-structural proteins, four accessory proteins, and four structural proteins (spike [S], matrix [M], nucleocapsid [N], and envelope [E]). The S protein is cleaved into S1 and S2 as two subunits at the S1/S2 cleavage site. The S1 protein contains the receptor-binding domain and mediates viral attachment to host cells, whereas S2 is responsible for membrane fusion [5], [6].