Genetic Characterization of Co-circulated Classic and Very Virulent Infectious Bursal Disease Viruses in Commercial Broiler Flocks of Egypt

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In recent years, the reintroduction of the infectious bursal disease virus (IBDV), particularly its severe strains, has imposed considerable cost on the Egyptian poultry industry. The goal of the current study was to investigate the molecular features of IBDV in Egypt from June 2019 to April 2021. A total of 30 field samples (bursa of Fabricius) were collected from broiler farms in which the chickens were vaccinated (Transmune 2512 s/c) at hatching. A highly variable region encompassing VP2 gene was targeted for IBDV screening utilizing reverse transcription-polymerase chain reaction (RT-PCR). Of 30 tested samples, 16 were positive by PCR. To isolate the virus, the bursal suspension was injected into 10-11 day embryonated chicken eggs via the chorioallantoic membrane. Five current positive isolates from 2021 were chosen for nucleotide and amino acid (aa) sequence analysis. Phylogenetically, three of the strains under study belonged to the very virulent (vvIBDV) strains, with 97-98% resemblance to Giza 2008 belonging to the (Genogroup 3) IBDV strain. The remaining two strains were identified as a vaccination strain (genotype 1) that matched the winter field 2512 vaccine strain by a similarity percentage of 98. Mutations in the antigenic locations of (P) domain loops were discovered when the sequencing samples were compared to the existing IBD vaccines. The circulating strains were found to be very similar to vvIBDV serotype 1 genotype 3 strains with mutations in the P domain loop providing a potential reason for the circulation of vvIBDV viruses in Egyptian broiler farms despite the vaccination program.