Emerging and characterization of a novel fowl adenovirus 4 strain with open reading frame 19 (ORF19) in diseased chickens from China

The highly virulent genotype fowl adenovirus 4 (FAdV-4), associated with severe hydropericardium hepatitis syndrome (HHS) in poultry from China, is characterized by a large deletion of 1966 bp (1966-del), including losses of ORF19 and ORF27. However, how this virus originated remains unclear. In this study, a novel FAdV-4 strain, HNU-XXY-2019, from diseased chickens, which has a genome size of 45669 bp and is approximately 1966 bp longer than the known FAdV-4 genome from China, was isolated and characterized. Forty-eight unique amino acid substitutions were revealed in protein-coding regions, including 10 substitutions in the Fiber 2 protein, which is reported to reside in the virulence gene. Phylogenetic analyses revealed that HNU-XXY-2019 was located in an evolutionary branch between clusters of highly pathogenic FAdV-4 1966-del strains and traditional FAdV-4 strains, with a closer relationship to Indian and Parkistan strains, suggesting that it may be an intermediate strain or an ancestral strain of the FAdV-4 1966-del strains. Further animal experiments demonstrated that HNU-XXY-2019 has moderate virulence and is pathogenic to SPF chickens at high challenge doses. This study is the first discovery and characterization of a novel FAdV-4 strain with a genome similar to that of traditional FAdV-4 strains but with a close genetic relationship to the FAdV-4 1966-del strains in China, indicating that it may play an important role as an intermediate evolutionary link between the highly virulent FAdV-4 1966-del strains prevalent in China and the traditional strains circulating outside. These data also help to understand the confounding circulation of FAdV-4 and to develop new prevention methods.