SUMMARYKorean field strain of fowl adenovirus (FAdV) 8b was isolated from chickens showing high mortality. Isolated FAdV-8b strains with the hexon and fibre genes were genetically analysed. The Korean FAdV-8b (K194/19) strain isolated in 2019 showed higher sequence identity with FAdV-8b strain isolated in China but lower sequence identity with Korean FAdV-8b (K187/08) strain isolated in 2008. K194/19 strain formed a distinct sub-cluster within the FAdV-8b cluster in a phylogenetic tree based on hexon and fibre genes. As FAdV can infect day-old chicks through vertical transmission, blood samples were obtained from 54, 60, and 63-week-old parent chickens. FAdV specific antibody levels were investigated using ELISA and virus neutralization (VN) tests with the K194/19 and K187/08 strains as antigens. In VN tests, all sera neutralized the K187/08 strain. However, K194/19 strain was neutralized by sera collected from 60 and 63-week-old chickens but not sera obtained from 54-week-old chickens, indicating natural infection. Finally, to determine the pathogenicity of K194/19 strain, 1-day-old and 4-week-old SPF birds were infected with K194/19 and K187/08 strains. No significant difference in pathogenicity was observed between the two strains. Although K194/19 strain showed similar pathogenicity with K187/08 strain, differences in nucleotide and amino acid sequences of the hexon and fibre genes may determine the evasion ability of the K187/08 neutralizing antibody, indicating the need of development of novel FAdV vaccine.

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