Molecular Characterization and Phylogenetic Analysis of Serotype 8b Fowl Adenoviruses from Commercial Broiler and Layer Flocks with Hepatitis

This study investigated the molecular characteristics and genetic diversity of Fowl adenovirus (FAdV) strains circulating in commercial broiler and layer flocks in the Southern Marmara and Aegean regions of Türkiye between January and December 2025. Liver samples (n = 120) collected from twelve flocks with increased mortality and clinical signs compatible with adenoviral infection were analyzed. Detection was performed using circular amplification technology and PCR targeting the hexon L1 region, and positive samples were sequenced for molecular characterization. BLAST analysis showed that all isolates belonged to Aviadenovirus hepatitidis and were identified as serotype 8b. Pairwise comparisons showed high nucleotide identity among isolates (97.4–100%) and 98.1–100% similarity with the Turkish reference strain MK937075. Only three isolates displayed nucleotide substitutions, while most sequences were identical within the analyzed region. Amino acid similarity ranged from 95.2% to 100%. Phylogenetic analysis revealed that all isolates clustered within a single monophyletic group together with previously reported Turkish FAdV-8b strains. Necropsy findings included hepatomegaly, multifocal hepatic pallor, petechial hemorrhages, gizzard erosion, and serous pericardial involvement. The detection of genetically closely related isolates across multiple provinces suggests regional circulation of a common viral lineage. These findings demonstrate that FAdV-8b is currently the predominant serotype associated with inclusion body hepatitis outbreaks in this major poultry production area and highlight the importance of molecular surveillance and targeted control strategies, including breeder monitoring and region-specific vaccine development.