Mandarin fish ranavirus (MRV) infection poses a substantial challenge to the mandarin fish culture industry as no effective preventive or therapeutic measures currently exist. The creation of a highly permissive cell line from a natural host is crucial for developing a vaccine for MRV and understanding its pathogenic mechanisms. In this research, the mandarin fish (Siniperca chuatsi) kidney cell line (SCK) was isolated from mandarin fish kidneys. Subsequently, SCK-a to SCK-g monoclonal cell lines were derived from the SCK cell population, distinguished by morphological variations. Notably, MRV infection induced an advanced cytopathic effect (CPE) in almost all cells of the SCK-f clone. Further tests showed that MRV achieved a peak viral titer of 10^10.7 50% tissue culture infectious dose (TCID₅₀)/mL and consistently exceeded 10¹⁰ TCID₅₀/mL across nine passages in SCK-f cells. Electron microscopy verified the MRV virion integrity within SCK-f. In vivo experiments revealed that MRV infections led to cumulative mortality rates of 86.9% in mandarin fish and 88.9% in largemouth bass (Micropterus salmoides). Such results suggest that SCK-f is highly permissive to MRV. This study underscores the importance of cellular diversity in developing viral permissive cell lines. The SCK monoclonal cell line pool may offer potential for generating highly permissive cell lines for other mandarin fish viruses.

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.

Birnaviruses infect a wide range of hosts, including economically important fish species. In this study, we report the isolation and characterization of a novel birnavirus, named mandarin fish birnavirus (MFBV). We obtained the full-length sequence of the MFBV genome. The segment A is 3539 bp, and the segment B is 2719 bp in length. Phylogenetic analyses revealed its close relation-ship with Largemouth bass birnavirus (LBBV) and Lates calcarifer birnavirus (LCBV) of the Birnaviridae family. Further, characterization of MFBV revealed its rapid replication kinetics that replication cycle is about 8-10 hours in mandarin fish kidney cells. Both intraperitoneal in-jection and immersion infection routes resulted in severe symptoms in juvenile or fry fish. Mortal-ity was occured within one day post-infection (dpi), with cumulative mortality rates ranging from 41.8% to 83.6% depending on the challenge method. Notably, compared to juvenile fish, fry fish exhibited higher susceptibility to infection. Viral RNA copies in multiple tissues ranged from the highest of 107.9 to the lowest of 106.9 per μg total RNA revealed that MFBV has a broad tissue tropism. Histopathological examination showed characteristic lesions, including villus damage in the intestine, necrosis in the spleen and kidney cells. MFBV was most sensitive to glutaralde-hyde, with a final concentration of 20 ppm resulting in an 8 log-unit reduction in titer. While povidone-iodine, formaldehyde, sodium hypochlorite, hydrogen peroxide, and potassium per-manganate required concentrations in the 10³ ppm range to achieve similar efficacy. The half-life of the virus during different phase was 36.5 hours to 144.5 hours.