Genomic Characterization and Pathogenicity of a Novel Birnavirus Strain Isolated from Mandarin Fish (Siniperca chuatsi)

Background: Birnaviruses infect a wide range of aquatic and terrestrial hosts, including several economically important fish species. This study aimed to isolate and characterize a novel birnavirus strain from mandarin fish (Siniperca chuatsi), a high-value freshwater species in Chinese aquaculture. Methods: A novel strain, designated mandarin fish birnavirus (MFBV), was isolated from diseased fish and propagated in SCK cells. The complete genome was determined using high-throughput sequencing and RACE. Viral replication kinetics, tissue distribution, and pathogenicity were assessed through in vitro infection, RT-qPCR, histopathology, and experimental challenges. In addition, disinfectant sensitivity and environmental stability were evaluated. Results: The MFBV genome comprises two segments (A: 3539 bp; B: 2719 bp), and phylogenetic analysis revealed close relatedness to largemouth bass birnavirus (LBBV) and Lates calcarifer birnavirus (LCBV). MFBV displayed rapid replication in SCK cells, completing a replication cycle in 8–10 h. In juvenile and fry fish, an experimental infection caused acute disease with cumulative mortality ranging from 41.8% to 83.6%, with fry showing higher susceptibility. Viral RNA was detected in multiple tissues (7.9 × 10⁶–7.9 × 10⁷ copies/μg RNA), and histopathological lesions were observed in the intestine, spleen, and kidney. MFBV was highly sensitive to glutaraldehyde (20 ppm), while other disinfectants showed reduced efficacy. Viral half-life ranged from 36.5 to 144.5 h at room temperature. Conclusions: These findings demonstrate that MFBV can induce acute systemic infection in mandarin fish. The results offer new insights into the genomic and biological features of birnaviruses, contributing to improved disease management and viral taxonomy.