Characterization of fowl adenovirus isolates in Taiwan: genome, antigenicity, and pathogenicity

Fowl adenoviruses (FAdVs) are important avian pathogens associated with inclusion body hepatitis and hydropericardium-hepatitis syndrome, contributing to significant economic losses in the poultry industry. Despite an increasing number of cases in Taiwan, the comprehensive characterization of circulating strains remains limited. This study investigated the molecular features, antigenic properties, and pathogenicity of FAdVs isolated in Taiwan between 2020 and 2025. Among suspected cases from broiler, layer, breeder, and colored broiler flocks, ten FAdV isolates were obtained. Whole-genome sequencing revealed six serotypes (FAdV-2, -3, -4, -8a, -8b, and -11) belonging to three species (C, D, and E), with one isolate exhibiting intertypic recombination. Cross-neutralization assays confirmed distinct serotypes with minimal antigenic overlap, highlighting the diversity of FAdVs circulating in Taiwan. A representative strain (NTU/C1120/2023) was selected for experimental infection in specific-pathogen-free chickens to evaluate pathogenicity. The isolate induced mild clinical signs and characteristic hepatic lesions in infected birds. Histopathology and immunohistochemistry further confirmed marked hepatic involvement. In addition, co-infection experiments with infectious bronchitis virus (IBV) were conducted to investigate potential interactions between the two viruses. Under co-infection conditions, FAdV viral loads in tissues were lower, whereas viral shedding tended to persist longer and antibody responses were reduced. In vitro assays using chicken embryo kidney cells further demonstrated reciprocal effects on viral replication. Collectively, this study provides new insights into the genetic and antigenic diversity of FAdVs in Taiwan and suggests that interactions with IBV may influence viral replication dynamics.