Isolation, Molecular, and Histopathological Patterns of a Novel Variant of Infectious Bursal Disease Virus in Chicken Flocks in Egypt

Simple Summary Infectious bursal disease virus (IBDV) is a potent immunosuppressive, persistent pathogen that can thrive in a range of environmental conditions and withstand even potent disinfectants. The common circulating pathotypes of IBDV in Egypt are classical virulent, attenuated, very virulent IBDV, and novel variant (nVarIBDV). The current study describes the incidence of nVarIBDV in 18 Egyptian chicken flocks although they were vaccinated against IBD. Partial sequence analysis of viral protein 2 (VP2) in two obtained isolates identified them as nVarIBDV (genotype A2d) exhibiting 100% identity to SD-2020 and 99.5-98.1% similarity to ZD-2018-1, QZ, GX and SG19 strains. Similarity to USA variant strains was 95.5-92.6%. Moreover, the similarities in the aa of the VP2 hypervariable region of both isolates to vaccine strains were 86-90.4%. Histopathological examination of both the bursa of Fabricius and spleen collected from diseased chickens in flock no. 18 revealed severe atrophy. In conclusion, this study identified the nVarIBDV genotype which is identical to the nVarIBDV circulating in China. Further studies are required to investigate the epidemiological situation of this novel genotype across the country, and to assess various vaccine protections against nVarIBDV. Additionally, it is crucial to incorporate nVarIBDV into the inactivated vaccines administered to breeder chickens prior to egg production to ensure complete protective and specific maternal immunity in their offspring during their first three weeks of life.Abstract After an extended period of detecting classical virulent, attenuated, and very virulent IBDV, a novel variant (nVarIBDV) was confirmed in Egypt in this study in 18, IBD vaccinated, chicken flocks aged 19-49 days. Partial sequence of viral protein 2 (VP2) [219 aa, 147-366, resembling 657 bp] of two obtained isolates (nos. 3 and 4) revealed nVarIBDV (genotype A2d) and OR682618 and OR682619 GenBank accession numbers were obtained. Phylogenetic analysis revealed that both nVarIBDV isolates were closely related to nVarIBDV strains (A2d) circulating in China, exhibiting 100% identity to SD-2020 and 99.5-98.1% similarity to ZD-2018-1, QZ, GX and SG19 strains, respectively. Similarity to USA variant strains, belonging to genotypes A2b (9109), A2c (GLS) and A2a (variant E), respectively, was 95.5-92.6%. Also, the VP2 hypervariable region in those two, A2d, isolates revealed greater similarities to Faragher 52/70 (Vaxxitek (R)) at 90.4% and to an Indian strain (Ventri-Plus (R)) and V217 (Xtreme (R)) at 89.7% and 86-88.9% in other vaccines. Histopathological examination of both the bursa of Fabricius and spleen collected from diseased chickens in flock no. 18 revealed severe atrophy. In conclusion, further studies are required to investigate the epidemiological situation of this novel genotype across the country, and to assess various vaccine protections against nVarIBDV. Additionally, vaccination of breeders with inactivated IBD vaccines including this nVarIBDV is essential to obtain specific maternal antibodies in their broilers.