Full genome sequencing, evolutionary dynamics, and pathogenicity evaluation of chicken infectious anemia virus with emphasis on Upper Egypt reveals genetic variability linked to vaccinal strains.

Despite intensive vaccination efforts, Chicken Infectious Anemia Virus (CIAV) remains a formidable threat to the Egyptian poultry industry, primarily through the emergence of vaccine-escape variants. While most studies have focused on Northern Egypt, there is a critical knowledge gap regarding the viral landscape in the Southern provinces. This study provides the most extensive surveillance to date, spanning 10 diverse Egyptian governorates with a strategic focus on Upper Egypt (2024-2025). Out of 400 collected samples, TaqMan-based qPCR revealed a high prevalence of 30% (120/400). Five representative isolates were selected for whole-genome sequencing and molecular characterization. The isolates were classified as Genotype II by phylogenetic analysis, showing a notable genetic divergence (85-87% amino acid similarity) from traditional Genotype I vaccine strains. Notably, comprehensive recombination analysis identified isolate PX96998 as a mosaic variant, with breakpoints indicating intra-genotypic recombination between closely related genotype II lineages, highlighting complex evolutionary dynamics in the field. Furthermore, 3D homology modeling of the VP1 protein identified critical structural alterations in functional loop regions, potentially hindering the binding affinity of neutralizing antibodies and facilitating immune evasion. In an experimental setting, the genotype II isolate demonstrated hypervirulence in SPF chickens, causing severe development retardation and extensive atrophy of lymphoid organs (p < 0.0001). Pathognomonic pale, fatty bone marrow with a sharp drop in PCV% to 18 ± 0.58% indicated severe aplastic anemia. By the fifth week after infection, there was a complete humoral collapse with undetectable ELISA titers due to the huge systemic replication revealed by viral load measurement, which peaked in the thymus (9.50 ± 0.25 log₁₀) and bone marrow (9.00 ± 0.22 log₁₀). These results were supported by histopathology, which revealed intranuclear inclusion bodies and systemic lymphocyte depletion. These findings support the highly pathogenic genotype II CIAV's dominance and aggressive evolution in Egypt. The emergence of recombinant variants with altered structural epitopes underscores the urgent need to replace outdated vaccination plans with genotype-matched immunogens in order to lessen the severe consequences on Egypt's poultry industry.