Genetic Characterization of Infectious Bursal Disease Viruses Associated with Gumboro Outbreaks in Commercial Broilers from Asyut Province, Egypt

Ten infectious bursal disease virus (IBDV) field strains were isolated from 15 broiler flocks located in various parts of Asyut, Egypt. Seven strains were subjected to comparative sequencing and phylogenetic analyses to help provide optimal control program for protection against IBDV infection. Sequence analysis of a 530？bp hypervariable region in the VP2 gene revealed that the rate of identity and homology was around 95.6~99.1%. Sequence characterization revealed the 7 strains identified as vvIBDV with the four amino acids residues typical of vvIBDV (242I, 256I, 294I, 299S). The BURSA-VAC vaccine was the nearest vaccine in sequence similarity to the local examined IBDV strains followed by CEVACIBDL then Bursine plus and Nobilis Gumboro indicating its probable success in the face of incoming outbreaks when using these vaccines. Phylogenetic analysis revealed that the presence of three clusters for the examined strains and are grouped with reference very virulent IBDVs of European and Asian origin (Japanese and Hong Kong) strains suggesting the different ancestors of our isolates. The antigenic index showed a number of changes on the major and minor hydrophilic antigenic peaks of the virus surface structures indicating a new genetic evolution of the surface structure epitopes that may lead to vaccination failure and reemergence of the disease. 1. Introduction Infectious bursal disease (IBD) is an acute, highly contagious viral disease of young birds characterized mainly by severe lesions in the bursa of Fabricius causing fatal condition and immunosuppression in chickens [1]. Infectious bursal disease virus (IBDV) belongs to the family Birnaviridae and has nonenveloped capsid. Since the first report in 1989, IBDV has two subtypes; the first one is variant and the other is the classical subtype that has been subdivided into 3 pathotypes: attenuated, virulent, and very virulent (vvIBDV) [2]. These vvIBDV strains were reported to break through high levels of maternal antibodies in commercial flocks, causing up to 60–100% mortality rates in chickens and producing lesion typical of IBDV [3]. The IBDV genome is divided into segments A and B: segments A (3.4？kb) and B (2.8？kb). The large segment A encodes 4 viral proteins, the two capsid proteins VP2 (48？kDa) and VP3 (32–35？kDa), the viral protease VP4 (24？kDa), and a nonstructural protein VP5 (17–21？kDa), while the smaller segment B encodes VP1 (90？kDa), an RNA-dependent RNA polymerase. Expression/deletion studies have shown VP2 aa positions 206 to 350 to represent a major conformational, neutralizing antigenic