Low pathogenicity of flounder iridovirus (FLIV) and the absence of cross-protection between FLIV and rock bream iridovirus.

Abstract

The genus Megalocytivirus is known to infect a wide range of cultured marine fish. In this study, we examined the pathogenicity of FLIV (Megalocytivirus from olive flounder, genotype III) and RBIV (Megalocytivirus from rock bream, genotype I) to their homologous and heterologous host species. Olive flounder (7.5±1.3cm) injected with FLIV [major capsid protein (MCP) gene copies, 6.8×103 -6.5×106 /fish] at 24°C did not die until 90days post-infection (dpi). The average virus replication in the spleen peaked (1.27× 106 /fish) at 20dpi. Rock bream (6.5±1.5cm) injected with FLIV (8.8×105 and 6.5×106 /fish of MCP copies) showed no mortality until 50dpi. The rock bream that survived after FLIV infection were rechallenged with RBIV at 50dpi had 100% mortality, showing that there is no cross-protection between FLIV and RBIV. Temperature shifting (26°C and 20°C at 12h intervals) did not cause FLIV-specific mortality into olive flounder, but higher virus copies were observed in the fish exposed to higher stocking density. This study demonstrates that FLIV and RBIV have different antigenic and pathogenic characteristics and that FLIV has low pathogenicity to olive flounder.