Development of a high-dose vaccine formulation for prevention of megalocytivirus infection in rock bream (Oplegnathus fasciatus)

Abstract

A formalin-inactivated red sea bream iridovirus (RSIV) vaccine was prepared using the culture supernatant of a persistently infected Pagrus major fin cell line (PI-PMF) with IVS-1 strain (RSIV subtype II Meglaocytivirus). Rock bream (Oplegnathus fasciatus) were injected with a high-dose, ultracentrifuged megalocytivirus vaccine (Ultra HSCMV, 7.0 × 10¹⁰ copies/mL), a high-dose supernatant of cultured megalocytivirus vaccine (HSCMV, 1.0 × 10¹⁰ copies/mL), a supernatant of cultured megalocytivirus vaccine (SCMV, 1.0 × 10⁹ copies/mL), and a low-dose of cultured megalocytivirus vaccine (LSCMV, 1.0 × 10⁸ copies/mL). The vaccine efficacies for the various vaccine formulations were determined done following injection challenge with IVS-1 (1.0 × 10⁴ copies/0.1 mL/fish), and the four different vaccines exhibited cumulative mortalities of 10.0 ± 0.0%, 48.3 ± 7.6%, 75.0 ± 5.0%, and 100.0 ± 0.0%, respectively. Additionally, the dose-dependent vaccine efficacy was also confirmed using two different cohabitation methods that included challenges G (general) and I (individual). When squalene + aluminum hydroxide (SqAl) was used as an adjuvant for the HSCMV or SCMV vaccine, cumulative mortalities of 30.0 ± 5.0% and 48.3 ± 7.6%, respectively, were obtained; moreover, these two adjuvants exhibited the highest efficacy in this study. The observed difference in survival post-challenge for the different vaccine concentrations was not reflected in the differences in neutralizing antibody titers. It was found that the water temperature during immune induction plays a less important a role than the water temperature during the challenge test, in which lowering the water temperature from 25 °C to 21 °C during a challenge improved the level of protection from cumulative mortalities from 35% to 10%. This study demonstrated that protection against mortality using inactivated vaccines against RSIVD in rock bream, which are known to be the most susceptible species to RSIV infection, is dependent upon antigen dose and temperature during the challenge.

Keywords: High-dose vaccine; Megalocytivrus; Red sea bream iridovirus; Rock bream.