DAO 83:115-122 (2009)  -  DOI: https://doi.org/10.3354/dao02001

ABSTRACT: Viral nervous necrosis (VNN), caused by a fish nodavirus, is one of the most serious fish diseases worldwide. Here we report a unique vaccination method in sevenband grouper Epinephelus septemfasciatus using a synthetic double-stranded RNA polyinosinic polycytidylic acid (Poly(I:C)), an interferon inducer, followed by challenge with a live fish nodavirus. Fish injected with Poly(I:C) at 200 µg fish^–1 were highly protected from artificial challenge with red-spotted grouper nervous necrosis virus (RGNNV) (relative percentage survival, RPS: 100%), and specific antibodies against RGNNV were detected in sera from survivors. Moreover, the surviving fish were protected from re-challenge with RGNNV (relative percent survival RPS: 100%). Thus, it was confirmed that specific immunity against RGNNV was established in sevenband grouper by injection with live RGNNV following Poly(I:C) administration. Antiviral state was induced in fish by injection with Poly(I:C) at ≥50 µg fish^–1, but no toxic response was observed in the fish even if Poly(I:C) was injected at a dose of 200 µg fish^–1. In fish injected with Poly(I:C) at 200 µg fish^–1, a high level of antiviral state of > 90% RPS against RGNNV challenge lasted for at least 4 d after Poly(I:C) injection. However, no curative effect by Poly(I:C) injection was observed in fish already infected with RGNNV. It is considered that the present immunization method using Poly(I:C) followed by a live virus injection could offer protection against various viral infections in a broader range of fish species.

KEY WORDS: Poly(I:C) · Immunization · Fish nodavirus · Viral nervous necrosis · VNN · Sevenband grouper · Vaccine · Antiviral state