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Herpesviral hematopoietic necrosis caused by cyprinid herpesvirus-2 (CyHV-2) is highly prevalent in gibel carp (Carassius gibelio Bloch) and is associated with high mortality and great economic loss. Currently, no efficient methods of prevention and control of CyHV-2 infection are available. Increasing evidence suggests that antigen-specific egg yolk immunoglobulins (IgY) are highly effective against various pathogens in farmed fish. Therefore, the aim of this study was to develop a specific IgY against CyHV-2 infection in gibel carp. We first produced a recombinant D4ORFs protein that encodes four open reading frames of CyHV-2 and then generated specific IgY by immunizing laying hens with this protein. The eggs laid by immunized hens were processed into spray-dried egg powder containing D4ORFs IgY. The D4ORFs IgY showed high neutralizing activity in vitro against CyHV-2, as evidenced by a significant reduction in cytopathic effect in gibel carp brain cells cultured in the presence of the IgY antibodies. Whole-egg powder containing IgY specific to D4ORFs protein was incorporated into the basal diet at levels of 0, 0.1, 0.3, and 0.6% to prepare experimental diets for use in intervention studies in gibel carp. Another group received non-specific IgY. For preventive intervention, all fish were fed these experimental diets for 25 days before being immersion challenged with CyHV-2, after which the fish were switched to the basal diet for 21 days. For therapeutic intervention, fish were first challenged with CyHV-2 and then fed the diet with 0.6% D4ORFs IgY. Fish that received D4ORFs IgY prior to CyHV-2 challenge had significantly higher survival rates and significantly lower viral load in the trunk kidney compared to those that received non-specific IgY. Improved survival and reduced viral load were also observed in CyHV-2-challenged fish that received dietary supplementation of D4ORFs IgY. Hematological and serum biochemical analysis also showed improved health and enhanced antiviral defense in fish that received D4ORFs IgY. Overall, these findings demonstrated both preventive and therapeutic effects of anti-D4ORFs IgY against CyHV-2 infection. • D4ORFs encoded by four tandem ORFs of CyHV-2 showed high immunogenicity. • Anti-D4ORFs IgY can effectively neutralize CyHV-2 in vitro. • Dietary inclusion of anti-D4ORFs IgY improves survival in CyHV-2-infected gibel carp. • Anti-D4ORFs IgY is prophylactically and therapeutically effective against CyHV-2 infection. [ABSTRACT FROM AUTHOR]