European sea bass (Dicentrarchus labrax) is an important farmed marine species for Mediterranean aquaculture. Outbreaks of betanodavirus represent one of the main infectious threats for this species. The red-spotted grouper nervous necrosis virus genotype (RGNNV) is the most widely spread in Southern Europe, while the striped jack nervous necrosis virus genotype (SJNNV) has been rarely detected. The existence of natural reassortants between these genotypes has been demonstrated, the RGNNV/SJNNV strain being the most common. This study aimed to evaluate the pathogenicity of different RGNNV/SJNNV strains in European sea bass. A selection of nine European reassortants together with parental RGNNV and SJNNV strains were used to perform in vivo experimental challenges via intramuscular injection. Additional in vivo experimental challenges were performed by bath immersion in order to mimic the natural infection route of the virus. Overall, results on survival rates confirmed the susceptibility of European sea bass to reassortants and showed different levels of induced mortalities. Results obtained by RT-qPCR also highlighted high viral loads in asymptomatic survivors, suggesting a possible reservoir role of this species. Our findings on the comparison of complete genomic segments of all reassortants have shed light on different amino acid residues likely involved in the variable pathogenicity of RGNNV/SJNNV strains in European sea bass.

Despite the negative impact of viral hemorrhagic septicemia (VHS) and infectious hematopoietic necrosis (IHN) on European rainbow trout farming, no vaccines are commercially available in Europe. DNA vaccines are protective under experimental conditions, but testing under intensive farming conditions remains uninvestigated. Two DNA vaccines encoding the glycoproteins (G) of recent Italian VHSV and IHNV isolates were developed and tested for potency and safety under experimental conditions. Subsequently, a field vaccination trial was initiated at a disease-free hatchery. The fish were injected intramuscularly with either the VHS DNA vaccine or with a mix of VHS and IHN DNA vaccines at a dose of 1 µg/vaccine/fish, or with PBS. At 60 days post-vaccination, fish were moved to a VHSV and IHNV infected facility. Mortality started 7 days later, initially due to VHS. After 3 months, IHN became the dominant cause of disease. Accordingly, both DNA vaccinated groups displayed lower losses compared to the PBS group during the first three months, while the VHS/IHN vaccinated group subsequently had the lowest mortality. A later outbreak of ERM caused equal disease in all groups. The trial confirmed the DNA vaccines to be safe and efficient in reducing the impact of VHS and IHN in farmed rainbow trout.

Viral nervous necrosis (VNN) is a disease caused by Betanodavirus that leads to outbreaks mortality in wild or farmed marine fish. This study aimed to investigate the presence of Betanodavirus in grouper from different sites along the Algerian coast and to characterize the isolates using molecular tools. A total of twenty-three individuals were examined: one dusky grouper (Epinephelus marginatus Lowe, 1834) and twenty-two goldblotch groupers (Epinephelus costae Steindachner, 1878). The RT-qPCR detected the presence of VNN in seven individuals of Epinephelus costae, three from the center of the Algerian coast (two from Algiers and one from Gouraya) and four from the west (Mostaganem). Phylogenetic analyses of the partial sequences of RNA1 and RNA2 viral genome fragments revealed the presence of the red-spotted grouper nervous necrosis virus (RGNNV) genotype, closely related to strains previously detected in Algerian groupers and wild grouper from Spain. The results of this study confirm further that VNN disease is endemic among grouper populations in Algeria, which could be a determining factor in the observed decline of this species.