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.