A mandarinfish Siniperca chuatsi infection and vaccination model for SDDV and efficacy evaluation of the formalin-killed cell vaccine in yellowfin seabream Acanthopagrus latus

Abstract

Scale drop disease virus (SDDV) is a distinct member in genus Megalocytivirus. Knowledge underlying pathogenesis of SDDV is still very limited. Here, a mandarinfish Siniperca chuatsi infection and vaccination model for SDDV was established. Artificial infection either by intraperitoneal or by intramuscular injection caused over 80% lethality to various sizes of mandarinfish juveniles. Cohabitation infection also resulted in over 70% lethality in the same infectious recycling water system. All data demonstrated that mandarinfish is a very highly susceptible fish species for SDDV. Real-time quantitative PCR showed that SDDV loads in infected-mandarinfish was the highest in spleen, followed by heart, gill, kidney, liver, muscle, intestines and brain, which was further confirmed by histopathology, immunofluorescence and immunochemistry. Histological analysis showed that although SDDV caused very high mortality to mandarinfish, its histopathology was not so typical as by ISKNV-infected mandarinfish. Formalin-killed cell (FKC) vaccines with 107.2 TCID50/0.1 mL, 106.9 TCID50/0.1 mL and 106.5 TCID50/0.1 mL SDDV antigens were prepared to immunize mandarinfish. As a result, the survivals of immunized fish were 100% (40/40), 97.5% (39/40) and 62.5% (25/40), respectively. By contrast, all un-immunized fish died upon SDDV challenge. Effectiveness of SDDV FKC vaccine was evaluated by immunizing 11,520 and 1038 yellowfin seabream and >90% protective effects were observed in both individual tests at 30 days post vaccination. Collectively, mandarinfish was confirmed as an ideal fish model for studying the infection and vaccination of SDDV, and MFF-1-based FKC vaccine renders highly effective protection against SDDV infection both in mandarinfish and yellowfin seabream.