Molecular and pathological characterization of fowl adenovirus serotype 2 linked to inclusion body hepatitis in broiler chickens.

Fowl adenoviruses (FAdVs) are significant viral agents in poultry, frequently linked to diseases such as inclusion body hepatitis (IBH) and hydropericardium syndrome. This research focused on the characterization of FAdV strains isolated from broiler chickens in Egypt through integrated pathological, molecular, phylogenetic, and structural analyses. Sixty-six clinical samples from broiler flocks showing IBH signs were screened for FAdV. Suspected cases were propagated in specific-pathogen-free embryonated chicken eggs (SPF-ECE) and assessed via gross pathology and histopathology. PCR targeting a 727 bp region of the hexon gene confirmed FAdV infection. Positive sequences were compared with all Egyptian FAdV sequences retrieved from NCBI for phylogenetic and amino acid analyses. Hexon protein 3D modeling was performed to assess the effect of hypervariable region (HVR) substitutions on antigenic structure. Two samples tested PCR-positive. Naturally infected birds showed pale, enlarged livers with focal necrosis and hydropericardium. Infected embryos had greenish liver discoloration and pock lesions on CAM. Histopathology revealed classic IBH features including hepatocellular necrosis, basophilic intranuclear inclusion bodies, and nuclear icosahedral virions. Phylogenetic analysis showed both isolates belong to FAdV-D serotype 2. Analysis of NCBI sequences confirmed the dominance of FAdV-11 and -2 in Egypt. 3D modeling indicated conserved structure in FAdV-2, while FAdV-11 displayed amino acid substitutions in HVRs affecting loop polarity and flexibility. This study confirms the circulation of genetically conserved FAdV-D serotype 2 in Egypt and highlights its association with IBH in broiler chickens, supported by molecular, pathological, and structural findings.