Viral Nervous Necrosis (VNN) causes high mortality and reduced growth in farmed European sea bass (Dicentrarchus labrax) in the Mediterranean. In the current studies, we tested a novel Pichia-produced virus-like particle (VLP) vaccine against VNN in European sea bass, caused by the betanodavirus “Red-Spotted Grouper Nervous Necrosis Virus” (RGNNV). European sea bass were immunized with a VLP-based vaccine formulated with different concentrations of antigen and with or without adjuvant. Antibody response was evaluated by ELISA and serum neutralization. The efficacy of these VLP-vaccine formulations was evaluated by an intramuscular challenge with RGNNV at different time points (1, 2 and 10 months post-vaccination) and both dead and surviving fish were sampled to evaluate the level of viable virus in the brain. The VLP-based vaccines induced an effective protective immunity against experimental infection at 2 months post-vaccination, and even to some degree at 10 months post-vaccination. Furthermore, the vaccine formulations triggered a dose-dependent response in neutralizing antibodies. Serologic response and clinical efficacy, measured as relative percent survival (RPS), seem to be correlated with the administered dose, although for the individual fish, a high titer of neutralizing antibodies prior to challenge was not always enough to protect against disease. The efficacy of the VLP vaccine could not be improved by formulation with a water-in-oil (W/O) adjuvant. The developed RGNNV-VLPs show a promising effect as a vaccine candidate, even without adjuvant, to protect sea bass against disease caused by RGNNV. However, detection of virus in vaccinated survivors means that it cannot be ruled out that survivors can transmit the virus.

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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.

At the end of October 2018, a mass fish mortality occurred in Iraq, involving thousands of tons of cultured and wild common carp (Cyprinus carpio) along Euphrates and Tigris rivers. Fish were found dead or moribund along rivers coasts, showing lethargy, dyspnoea and flared gills. At necropsy, discoloration patches were noticed on the gills. Wet preparations showed rare metacercariae and Dactylogyrus spp. Samples were subjected to bacteriological tests and virological investigation through real‐time PCR and nested PCR. Both were positive for koi herpesvirus (KHV) and carp oedema virus. Results obtained were confirmed by the OIE reference laboratory of KHV disease (KHVD) at Cefas (UK) and by sequence analysis. This is the first report on the detection of both viruses in Iraq.

Mortality in wild fish populations represents a challenging issue for public fish health inspectors. When a single fish species is involved, an infective aetiology is frequently suspected, with focus on viral notifiable diseases. However, other viral agents not subjected to regulation and causing mortality in common carp have been reported such as carp edema virus (CEV). In mid‐June 2020, a severe common carp mortality was observed in an artificial lake in north‐east of Italy. Sleepy fish were noted some days before the beginning of the mortality itself, which lasted several days and involved over 340 adult specimens. During the outbreak, water temperature was around 15°C, water quality was normal, and no adverse meteorological events were reported in the area. Four specimens, which showed severe cutaneous hyperaemia and increased mucus production on skin and gills, were tested by bacteriological methods and virological analysis targeting the main carp pathogens. Molecular analysis performed on gills, kidney and brains from all the fish analysed resulted positive for CEV, which, based on anamnestic information and laboratory findings, was considered the responsible for the mortality event herein described.