Betanodavirus, the etiological agent of viral encephalopathy and retinopathy (VER), is a significant pathogen in Mediterranean aquaculture, particularly affecting European sea bass (Dicentrarchus labrax). This positive-sense RNA virus belongs to the Nodaviridae family and includes four genotypes: Redspotted grouper NNV (RGNNV), Striped jack NNV (SJNNV), Tiger puffer NNV (TPNNV), and Barfin flounder NNV (BFNNV). Reassortment events between RGNNV and SJNNV have resulted in the emergence of novel viral strains, complicating disease management, particularly in the larval stages of gilthead seabream (Sparus aurata). Co-infections with these genotypes in wild populations suggest that viral nervous necrosis (VNN) may evolve in other species in the future.
This study reports the isolation and molecular characterization of a reassortant RGNNV/SJNNV strain from European sea bass in Tunisia, following a mortality event in July 2023.
The virus was isolated from brain tissue of affected sea bass using the striped snakehead (SSN-1) cell line, and RT-PCR confirmed the presence of Betanodavirus in the cell culture supernatant. Full-length RNA1 and RNA2 were amplified by reverse transcription polymerase chain reaction (RT-PCR), sequenced bidirectionally, and the resulting sequences were assembled to reconstruct the complete viral genomes.
Phylogenetic analysis revealed that RNA1, encoding the RNA-dependent RNA polymerase, clustered with RGNNV, while RNA2, encoding the capsid protein, aligned with SJNNV, confirming the reassortant nature of the virus.
This is the first complete RGNNV/SJNNV reassortant genome identified in D. labrax in Tunisia.
These findings emphasize the need for ongoing surveillance of Betanodavirus evolution and reassortant strains. They also underscore the urgency of developing targeted vaccination strategies to reduce disease prevalence, enhance host resilience, and ensure the long-term viability of aquaculture systems.