Isolation, Characterization, and Pathogenicity of a Novel Recombinant Fowl Adenovirus 8a Strain in China

Inclusion body hepatitis (IBH), a disease predominantly associated with fowl adenovirus serotypes 8a (FAdV-8a) and 8b (FAdV-8b), poses a significant challenge to poultry production on a global scale. This study reports a severe outbreak of IBH that occurred in 2021 at a chicken layer farm in Hunan Province, China. Liver tissues were collected from visibly affected layer chickens for viral isolation in LMH cell cultures. Subsequent phylogenetic analysis was conducted based on DNA sequencing and bioinformatics data to determine the genetic relationships of the isolated virus. Pathogenicity trials were performed on specific-pathogen-free (SPF) chickens. A novel FAdV-8, designated HuN21, with recombination between serotypes FAdV-8a/FAdV-8b, was isolated. Recombination and phylogenetic analyses revealed that this recombinant strain harbored a genomic backbone and fiber gene from FAdV-8a and FAdV-8b, respectively. HuN21's pathogenicity differed significantly by age groups, showing high pathogenicity (100% mortality, classic IBH signs) in 1-day-old SPF chickens, compared with no mortality despite viral replication in 28-day-old chickens. These data provide robust evidence for recombination between different serotypes of FAdV-8. Notably, the unique genetic and pathogenic characteristics of the HuN21 strain indicate potential as a candidate strain for the development of a bivalent inactivated vaccine or a natural vaccine against FAdV-8a/FAdV-8b.