Interspecies transmission between Solea senegalensis and Sparus aurata of reassortant Nervous Necrosis Virus (NNV) strains and effect of stress on the outcome of the infection

Viral Encephalopathy and Retinopathy (VER) episodes in Southern Europe have led to the isolation of several reassortant NNV strains from different fish species, including Senegalese sole and gilthead seabream. Polyculture of both species is being developed to optimize available resources. However, this farming technique can be threatened by NNV horizontal transmission between diseased fish or asymptomatic carriers and non-infected individuals from both fish species, which could lead to a VER outbreak in the facility and seriously affect fish production. Therefore, in this study we have assessed the susceptibility of gilthead seabream and Senegalese sole to two reassortant NNV strains isolated from each of the two fish species, and the possibility of interspecies transmission by cohabiting infected and naive individuals. Our results showed that both NNV isolates caused moderate mortality rates and replicated in both fish species. In the cohabitation challenges, infective NNV particles were recovered from naive cohabitants, demonstrating interspecies transmission from infected individuals that shed NNV into the water column. In addition, cumulative mortality in sole cohabitants was significantly higher, presumably due to the stress provoked by the aggressiveness of gilthead seabream. This is supported by the analysis of the hsp70 gene, a stress biomarker overexpressed in the sole cohabitants, especially in those that died on the first day of cohabitation. Therefore, despite the numerous advantages of polyculture, the risk of VER outbreaks represents a serious constraint for the implementation of this technique in Mediterranean aquaculture.