Molecular characterization and evaluation of field fowl adenovirus isolates in specific pathogen free chicken embryonated eggs

Fowl adenoviruses (FAdVs) pose a substantial economic threat to global poultry production due to outbreaks of inclusion body hepatitis (IBH) and gizzard erosion. The present study aimed to characterize four Malaysian FAdV serotype 8b isolates (UPM2101, UPM2102, UPM2103, and UPM2401) through sequence analysis of the hexon and fiber genes to identify unique variations, while concurrently assessing viral infectivity via propagation in specific pathogen free (SPF) chicken embryonated eggs. The four FAdV field isolates were obtained from commercial broiler chickens in Malaysia presenting classical lesions. Isolates UPM2101, UPM2102, and UPM2103 were collected from farms in Johore in 2021, whereas UPM2401 was collected from Kedah in 2024. UPM2101, UPM2103, and UPM2401 originated from swollen, pale, and friable livers with petechial hemorrhages, whereas UPM2102 was isolated from a gizzard erosion case. Polymerase chain reaction (PCR) amplification confirmed the presence of FAdV in all samples. Sequence analysis revealed high nucleotide identity among the isolates. In the hexon gene, a nucleotide insertion at position 33 in UPM2101 and a substitution (C→T) at position 428 in UPM2401 resulted in non-synonymous amino acid changes. UPM2101 exhibited multiple amino acid substitutions at positions 225–234 with an additional glutamine at position 235, whereas UPM2401 showed a single substitution (A93T). In the fiber gene, nucleotide substitutions and a deletion were observed in the knob region, resulting in amino acid changes including S149T and R150A in UPM2103 and R150S in UPM2401. Phylogenetic analysis classified all isolates within serotype 8b, forming a closely related cluster. Propagation in embryonated eggs demonstrated efficient viral replication, with UPM2101 exhibiting earlier embryonic mortality than UPM2401, indicating potential differences in virulence. The findings of the current study indicated that the Malaysian FAdV serotype 8b field isolates are closely related yet exhibit distinct molecular variations that may influence viral replication efficiency and infectivity in embryonated eggs.

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