Neutralizing antibody levels for protection against betanodavirus infection in sevenband grouper, Epinephelus septemfasciatus (Thunberg), immunized with an inactivated virus vaccine

摘要

An inactivated betanodavirus, red-spotted grouper nervous necrosis virus (RGNNV), is a vaccine candidate for viral nervous necrosis (VNN). The present study was conducted to examine inoculation doses of the vaccine and neutralizing antibody titre levels to protect fish against VNN. Young sevenband grouper, Epinephelus septemfasciatus, averaging 25.4 g, were immunized at 25 pC water temperature by a single intraperitoneal injection of formalin-inactivated RGNNV. Fish immunized at vaccine doses of 10i.e, 10i., 10".e, 10". and 10e.e TCID per fish produced antibodies at mean titres of 1:907, 1:511, 1:259, 1:197 and 1:96, respectively, at 20 days post-immunization (p.i.). Neutralizing antibodies were not detected in any control fish (titre <1:80). When fish were challenged with RGNNV (10e. and 10t. TCID/fish) at 20 days p.i., cumulative mortalities of the fish groups immunized with 10i.e, 10i., 10".e and 10". TCID per fish were significantly lower than those of the control group, and the relative percent survival values were higher than 60% in fish groups immunized with 10".e TCID per fish or higher doses. However, no significant differences were found in mortality between the group immunized with 10e.e TCID per fish and the control group. From these results, it was deduced that the minimum effective inoculation dose of the vaccine is 10". TCID per fish and the minimum mean neutralizing antibody titre giving significant protection is approximately 1:200. This antibody titre level is a possible measure of vaccine efficacy against VNN in sevenband grouper, instead of a virus challenge test.