RGNNV/SJNNV reassortant betanodavirus outbreaks in a sea bream and sea bass farm.

Introduction: The Mediterranean aquaculture has suffered significant economic losses due to viral nervous necrosis mainly caused by RGNNV betanodavirus genotype primarily involving sea bass (Dicentrarchus labrax). Recently, a RGNNV/SJNNV reassortant betanodavirus, har- bouring the RNA1 segment of RGNNV genotype and the RNA2 segment of SJNNV genotype, emerged. So far, the reassortant strain has caused a negative economic impact mainly on sea bream (Sparus aurata) hatcheries sparing the sea bass farming sector. Methodology: Multiple mortality outbreaks occurred in an Italian marine farm involving both sea bass and sea bream at different life stages. Batches of sea bass and sea bream involved in the outbreaks (December 2017, May 2018 and August 2018) were investigated through a complete microbiological and molecular investigation. Results: The cumulative mortality rates recorded during the outbreak occurred in December 2017 were 10% and 100% in larvae of sea bass and sea bream, respectively. In May 2018, sea bass survived the first outbreak (weight 4 g) showed a further outbreak with 10% of mor- tality. Moreover, in August 2018 a newly introduced batch of sea bream suffered a further outbreak, which led to 100% of mortality. All the batches were negative for parasites and bacte- ria. Betanodavirus was isolated on SSN-1 cells from all batches. Betanodavirus-typical lesions have been also found at histology. The molecular characterization of the strains isolated during all the outbreaks reported 100% nucleotide and amino acid identities, showing the involvement of the same viral strain during the different outbreaks. The phylogenetic analysis has demon- strated that the strain detected in both sea bream and sea bass involved in the multiple mortality outbreaks was a RGNNV/SJNNV reassortant betanodavirus. Conclusion: The microbiological and molecular analyses allowed identifying a RGNNV/ SJNNV reassortant betanodavirus strain as the causal agent of the outbreaks. This is the first investigation of a field mortality outbreak caused by a RGNNV/SJNNV reassortant betano - davirus involving sea bream and sea bass simultaneously. Sea bream has recorded the highest mortality rates, but sea bass seems to act as asymptomatic carriers and viral source for other susceptible species such as sea bream.