Viral nervous necrosis is caused by the nervous necrosis virus (Betanodavirus genus), a naked virus with two positive-sense RNA segments. Betanodaviruses are classified into four species, being RGNNV predominant in the Mediterranean area. RGNNV causes high mortality in several fish species, including European seabass. There are two vaccines to protect seabass against RGNNV infection. In this regard, the development of strategies to protect different fish species against different viruses, such as the use of probiotics, is a key issue for the aquaculture industry. Shewanella putrefaciens Pdp11, SpPdp11, is a fish probiotic with proven positive effects on seabream and Senegalese sole, protecting those species against bacterial pathogens; however, its antiviral activity is unknown. This study evaluates the anti-RGNNV activity of sonicated-SpPdp11 extracts in vitro and in vivo.
The in vitro evaluation was performed on E11 cells following 3 assays: (i) neutralization, (ii) 6-h pre-adsorption, (iii) post-adsorption, determining the inhibition of RGNNV-induced CPEs and quantifying viral replication. The immunostimulatory activity of SpPdp11 extracts was also examined, analysing the transcription of mx, hsp70, tnfα, e3 and tlr3 in E11 cells.
For the in vivo evaluation, two European seabass groups were established: (i) control group, (ii) experimental group, fed with commercial pellet supplemented with SpPdp11 extracts. Animals were fed for 30 days and subsequently challenged by intramuscular injection.
SpPdp11 extracts compromised RGNNV replication in E11 cells (67.3% and 55% CPE inhibition in pre-adsorption and post-adsorption assays, respectively), and modulated the transcription of all the immune-related genes examined. The highest induction was obtained for mx gene.
Regarding the in vivo results, 82% of fish fed with the SpPdp11-supplemented diet survived to RGNNV infection, whereas the survival rate of fish fed with the control diet was 64%.
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