Chlorine-based disinfection for controlling horizontal transmission of VNN in a seawater recirculating aquaculture system growing European seabass

An innovative physicochemical approach for treating the recycled water within a seawater recirculating aquaculture systems (RAS) has been recently developed, which includes inherent disinfection within the ammonia electro-oxidation cycle. The efficiency of the inherent disinfection was tested in comparison to a RAS consisting of a bio-treatment unit (which did not include a disinfection component), both systems operating under similar fish densities and makeup water flow rates. Neural necrosis virus (NNV) was chosen as a model pathogen, due to its pathogenicity to sea bass (Dicentrarchus labrax) and many other aquaculture species. The course of infection (exposure → immune response → infection) was monitored in a cohabitation experiment, testing the horizontal transmission of NNV using immunological and molecular methods, in addition to recording clinical signs. Results indicate that under the conditions prevailing in the RAS in which the physicochemical method was applied, the NNV horizontal transmission pathway was blocked. No immune response to viral neural necrosis (VNN) was detected in the fish sampled during the experiment, in contrast to a significant and classic immune response observed in the fish held in the RAS based on bio-treatment. Challenged fish in both systems showed typical outbreak profile, however in the bio-treated RAS, a slow disease course was observed, hypothesized by us to be the result of a high concentration of humic substances present in the water. The results indicated that the off-host phase of the NNV can be controlled by effective chlorine-based disinfection. The disinfection effect in the physico-chemically-treated RAS and C∙t (chlorine concentration times the retention time) value quantified in the present work are most likely sufficient to reduce infection of a wide range of other aquaculture pathogens.