Epidemiological characterization of fowl adenovirus in China from 2021 to June 2025

Fowl adenovirus (FAdV) causes inclusion body hepatitis and hepatitis-hydropericardium syndrome, posing economic challenges to China's poultry industry. However, the epidemiological characteristics of FAdV in China remain inadequately characterized, particularly following the implementation of strengthened inspection protocols for inactivated FAdV vaccines. Here, a nationwide survey from January 2021 to June 2025 analyzed 18,297 samples from 29 provinces. Overall positive rate was 2.42% (95% CI: 2.20-2.65%) at sample level and 3.09% (95% CI: 2.78-3.41%) at flock level. A pronounced V-shaped temporal trend was observed: positive rates declined from 2.78% in 2021 to a nadir of 0.73% in 2023, followed by a marked resurgence to 5.22% in 2024 and 6.46% in the first half of 2025. This resurgence coincided with policy changes prohibiting the illegal addition of whole FAdV group I virus in inactivated poultry vaccines, suggesting that the removal of vaccine-induced immune pressure created an immunological gap that facilitated viral rebound. Chickens were identified as the primary hosts, accounting for 72.78% (262/360) of FAdV-positive flocks, with broilers representing 67.94% (178/262) of chicken-positive flocks. Co-infections were highly prevalent, involving up to seven additional pathogens in chickens and up to five in waterfowl. Phylogenetic analysis of 125 isolates revealed a complex diversity encompassing eight FAdV serotypes (A1, C4, D2, D3, D11, E7, E8a, and E8b) and duck adenovirus-3, with serotypes C4, E8b, D11, and A1 being predominant. Notably, serotype diversity exhibited a "contraction-expansion" pattern, decreasing from seven serotypes in 2021 to three in 2023, then rebounding to eight in 2024-2025. Furthermore, FAdV-C4 strains diverged into two distinct sub-clusters defined by ten characteristic amino acid substitutions in the hexon gene, predominantly emerging post-2023. This study elucidates the dynamic epidemiology and genetic diversity of FAdV in China, offering crucial insights for targeted surveillance and rational vaccine development.