Avian infectious bronchitis (IB) is common in all parts of the world and is acute and highly infectious (Jackwood and de Wit, 2013). The infectious bronchitis virus (IBV) belongs to the family Coronaviridae, which consists of RNA-enveloped pleomorphic viruses that possess club-shaped surface projections that give the appearance of a “corona” or fringe (Almeida et al., 1968). The complete virus contains four polypeptides: the membrane protein, the envelope protein, the nucleocapsid protein, and large spike glycoprotein (S) (McMartin, 1993). The S protein is cleaved posttranslationally into S1 and S2 subunits (Jackwood et al., 2001). The section of the S gene that codes for the S1 subunit protein is the site of the majority of the variation between strains (Cavanagh et al., 1992; Adzhar et al., 1996). Only small changes in amino acid sequences are needed to produce a different serotype, which results in a high-evolution rate for IBV. The numerous antigenically diverse strains, almost as many serotypes, and the rapid evolution of yet more variant strains complicate the understanding of this disease (Ignjatovic et al., 2002).
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