Passive surveillance of infectious bursal disease virus circulating in Egyptian chicken flocks in 2025

Abstract

Abstract Infectious bursal disease virus (IBDV) causes severe immunosuppression diseases in poultry, resulting in substantial economic losses for the global poultry industry. This study aimed to characterize circulating IBDV strains in Egyptian chicken flocks and assess their potential impact on poultry health and production. We conducted a passive surveillance study by collecting bursal samples from a total of 30 flocks, including commercial broilers, layers, and Baladi chickens, across nine Egyptian governorates in 2025. These flocks exhibited clinical signs of depression, along with kidney and bursal lesions indicative of IBDV infection. Pooled bursal homogenates were tested using RT-PCR with VP2-specific primers, revealing that 20 flocks (66.6%) tested positive for IBDV. Field and vaccine strains were distinguished by phylogenetic clustering of the VP2 hypervariable region (HVR) sequences against reference vaccine strains (D78, Winterfield 228, VAXXITEK), strains that cluster closely with reference vaccine strains are classified as vaccine-origin, whereas field strains are distinct branches indicate circulating wild-type or vvIBDV. Ten representative positive samples were inoculated in specific pathogen-free embryonated chicken eggs (SPF-ECE). The inoculated embryos exhibited haemorrhage, skull swelling, and liver necrosis with a pale-yellow appearance, along with congestion and thickening of the chorioallantoic membrane (CAM). Phylogenetic analysis of the partial VP2 gene classified the isolates according to the unified genogroup nomenclature: the majority of isolates belonged to the A3 (vvIBDV) genogroup, while one isolate clustered within the A2d (nVarIBDV) genogroup. These findings highlight the co-circulation of both virulent and variant IBDV strains in Egyptian chicken flocks, complicating disease control and demanding a continued surveillance and periodic re-evaluation of vaccination programs.