Protection of Japanese flounder Paralichthys olivaceus from viral hemorrhagic septicemia (VHS) by Poly(I:C) immunization

In immunization of fish with polyinosinic-polycytidylic acid (poly[I:C], a synthetic double-stranded RNA, injection of Poly(I:C) followed by challenge with a live virus induces a transient, non-specific antiviral state by interferon activity. When exposed to a virus while in this antiviral state, the fish acquire a specific and protective immunity against the corresponding viral disease and survive. In the present study, the effiacy of Poly(I:C) immunization was investigated in japanese flounder Paralichthys olivaceus using viral hemorrhagic septicemia virus (VHSV) as a model; the minimum dose of Poly(I:C) required for inducing protection and the duration of the antiviral state were determined, and a potentially curative effect of Poly(I:C) administration was assessed. The antiviral state was induced by administration of Poly(I:C) doses ranging from 12.5 to 200 mu g fish(-1). Minimum dose to induce the antiviral state (relative percentage survival, RPS: 90%) was 12.5 mu g fish(-1). No curative effect of Poly(I:C) was observed in fish pre-infected with VHSV. Fish injected with 200 mu g Poly(I:C) fish(-1) were highly protected (RPS: 100%) from an artificial challenge with VHSV, and specific antibodies against VHSV were detected. The corresponding high level of antiviral state against VHSV was attained 1 d post Poly(I:C) injection, lasted for 6 d and susequently decreased. Moreover, the surviving fish were highly protected from re-challenge with VHSV (RPS: 100%). Thus, it was considered that an immunity against viral hemorrhagic septicemia was induced in the Japanese flounder by injecting live VHSV following Poly(I:C) administration.