Abstract

Infectious Bursal Disease Virus (IBDV) is highly infectious and causes severe economic losses in the Egyptian poultry industry. In the present study, 40 samples of bursa Fabricius tissue were collected from various poultry flocks residing in six governorates during 2020 in Egypt (8 from El-Daqhlia, 10 from El-Sharquia, 10 from El-Qaliobiyea, 4 from EL-Behera, 6 from Alexandria, and 2 from El-Gharbia). Among these flocks, the chicken suffered from depression, dehydration, and ruffled feather with high mortality rates (20-50%) leading to the haemorrhagic and enlarged bursa of Fabricius. Reverse transcription-polymerase chain reaction (RT-PCR) was performed, targeting the hypervariable region of the VP2 gene of IBDV. The 30 samples were detected positive by RT-PCR (8 from El-Daqhlia, 7 from El-Sharquia, 6 from El-Qaliobiyea, 3 from EL-Behera, 5 from Alexandria, and 1 broiler chicken from El-Gharbia). A total of 10 strains were selected for genetic analysis, representing different governorates. All identified strains belonged to a very virulent IBDV with 95.7-96.7% nucleotide identity and 98.2-99.4% amino acid identity with very virulent IBDV strains from Europe and Asia. Phylogenetically, the Egyptian strain was divided into two subgroups. All strains identified in the present study belonged to the phylogenetic subgroup I with new eight nucleotide mutation mutations when compared with HK64 and other Egyptian strains. All sequenced viruses had G254S mutation. Moreover, Y220F mutation was detected in major hydrophilic region A, in two strains (EGY/SN5 and EGY/SN10), compared with HK64. These mutations may increase viral pathogenicity and antigenicity. The Egyptian strains in the study were distinct from the vaccinal strain. Furthermore, they may explain the recent IBDV outbreaks reported in vaccinated flocks. The current study highlighted the importance of continuous monitoring of mutations in IBDV, and the assessment of their effects on virus virulence and vaccine efficacy against newly evolved strains.