Abstract: Canine adenovirus type 2(CAV-2) is an important respiratory pathogen in Canine. The aim of this study was to investigate the prevalence and pathogenicity of CAV-2 in parts of Sichuan Province and Chongqing Municipality. The nasal swabs of 121 canines with respiratory tract diseases were collected from 2021 to 2022 for detecting CAV-2 by specific PCR primers, and the epidemic strain was isolated for further study, including its genome characteristics and pathogenicity. The results showed that the positive rate of CAV-2 in canine was 29.8%. Notably, 69.4% of the positive samples had identical 9 consecutive nucleotide deletions (1 035-1 043 nt) in the E3 genes, which resulting in 4 amino acid deletions in the E3 genes (345-348 aa). One strain with natural E3 deletion was successfully isolated, and the half infection value of MDCK cells was 10^-9.46TCID₅₀·0.1 mL^-1 after plaque purification. The puppies inoculated with the isolation showed typical respiratory symptoms, such as sneezing, runny nosing and coughing. The genome of the isolate was 31 786 bp, and shared the homology of 98.8%-98.9% in nt with the CAV-2 genomes in GenBank. Compared with the known CAV-2 genomes, besides the unique mutations of the E3 gene, the isolate in this study also had unique amino acid mutations in Fiber, Hexon, and E1A, which made this strain distinguish from the known CAV-2 strain in the genomic evolutionary tree. This study obtained the genome sequence of the Chinese CAV-2 strain for the first time, and found the strain with natural deletion in E3 genes, which is helpful to understand the prevalence and genetic evolution of CAV-2.

Key words: canine adenovirus type 2, E3 gene natural deletion strain, isolation and identification, genome, pathogenicity