Genetic point mutation inducing antigenic drift in hypervariable region of a very virulent IBDV isolate in chickens in Egypt during 2014-2016

Infectious bursal disease (IBD) is a highly contagious viral disease affecting young chickens causing immune suppression, high morbidity and mortality. Its economic significance is recognized worldwide. In this study, suspected IBD samples (bursa of Fabricious) were collected from 45 chicken flocks in 3 Egyptian governorates from 2014 to 2016. The virus was inoculated in embryonated chicken eggs via chorio-allantoic- membrane (CAM) route inducing specific IBDV lesions in the embryos. Viral identification was carried out through Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) targeting VP2 gene. Fourteen positive IBDV isolates (31%) were confirmed by RT-PCR. Three pure IBDV isolates were subjected to partial VP2 gene sequence analysis from which 2 IBDV isolates No. 1 and 3 (Accession No. KX827589.1 and MK906027) were defined as a very virulent IBDV (vvIBDV) genogroup 3, while the third isolate (Accession No. KX827588.1) was closely related to a vaccine strain in cvIBDV genogroup 1. Nucleotide and amino acid sequence analysis and blast of the IBDV isolates indicated a close relationship with the previously recorded Egyptian IBDVs with 96 to 99% identity. Point mutation or amino acid substitution in positions P202M (conserved region); and A211T, D212Y (hypervariable region) of the VP2 gene in the isolate No. 3 vvIBDV (Accession No. MK906027) that differ from all the previously recorded Egyptian isolates in GenBank were present.