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Betanodaviruses are causative agents of viral nervous necrosis (VNN), a devastating disease of cultured marine fish worldwide. Virus particles contain a single type of coat protein that spontaneously assembles into virus-like particles (VLPs) when expressed in a baculovirus expression system. In the present study, the immunogenicity of betanodavirus VLPs and the protection they confer against VNN in the European sea bass Dicentrarchus labrax were investigated. Enzyme-linked immunosorbent assay and seroneutralization tests performed on plasma from fish vaccinated intramuscularly with doses as low as 0.1 g of VLPs indicated that the VLPs elicited the synthesis of specific antibetanodavirus antibodies with neutralizing activity. Moreover, fish vaccinated with VLPs were protected from challenge with live virus. Both the immune response and the protective effect against viral challenge were dose dependent. Reverse transcription-PCR data indicated that higher doses of vaccine also reduced the number of fish containing detectable quantities of betanodavirus RNA on day 30 after challenge. Taken together these data strongly support the hypothesis that VLPs obtained in the baculovirus expression system may represent an effective vaccine against VNN.

A commercial P-glucan known for its ~mmunostimulatory effects in several fish species was tested in turbot ScophthalniusmaximusL.. both as an oral immunostimulant and as an adjuvant for oral vaccination. Some non-specific immune parameters were tested after a 5 wk feeding period with a commercial diet mixed with yeast P-glucan. Furthermore, during the last 5 d of the feeding period, half of the fish were orally vaccinated by mixing the commercial pellets with an anti-vibriosis vaccine (Vibriffa bain ND). The oral administration of P-glucan induced no reduction in mortality after a challenge with a virulent Vibrio anguillarum (strain 408). In contrast, a single oral vaccination resulted in protection against V anguillarum. The use of P-glucan as an adjuvant did not reduce the mortality rate more than did the single vaccination. An increase in white blood cell count was observed after the administration of only P-glucan. The plasma complement activity was not influenced by any of the treatments. In contrast, lysozyme activity was enhanced after administration of the adjuvanted vaccine. An increase in the chemiluminescent response of opsonised zymosan-stimulated head-kidney leucocytes was less obvious and no significant results were recorded.