Molecular Characterization of Megalocytiviruses from Diseased Fishes in Korean Aquatic Farms from 2013 to 2017

Abstract

We identified structural and non-structural gene regions encoding major capsid protein (MCP) and DNA polymerase (DPOL) of megalocytiviruses collected from infected cultured fishes in RBIVD outbreak farms in 2013-2017 in Korea. With the two PCRs using 1-F/1-R and 4-F/4-R primer sets of the Manual of Diagnosis Tests for Aquatic Animals of the World Organization for Animal Health (OIE), amplicons were generated from the spleen and kidney tissue from approxmately ~30 fishes, including rock bream (Oplegnathus fasciatus), red sea bream (Pagrus major), and rock fish (Sebastes schlegeli), from 15 outbreak regions in the aquatic farms of the South Sea and Jeju Island. In phylogenetic analysis, complete MCP and partial DPOL genes belonged to RSIV type-subgroup2. Interestingly, these genes formed a cluster indicating closer relatedness to GSIV-K1, RIE12-1, and RBIV-C1, which were previously isolated from Japan and China, than with RBIV-KOR-TY1 isolated from Korea. However, the nucleotide sequence identities of the MCP and DPOL genes of these viruses were high, at 99.8% and 99.7%, respectively, compared with RBIV-KOR-TY1. Comparisons of nucleotide and amino acid sequences showed minimal differences between the obtained strains in the MCP gene, however, one or two nucleotide sequences substitutions of the DPOL gene were detected in nine strains, including a silent mutation detected in five strains. These findings suggest a slow rate of evolution of megalocytiviruses in this region, but the potential for mutations and new pathogenic strains warrants continuous surveillance.