Molecular comparison of iridoviruses isolated from marine fish cultured in Korea and imported from China

We have identified three iridovirus isolates, Sachun and Namhae from diseased fish, and CH-1 from infected imported fish. In order to determine whether these iridovirus isolates are distinguishable from other fish iridoviruses reported as Ranavirus, nucleotide sequence analysis of the K2 region was carried out. This 5905 bp K2 region located between DPOL and RNRS gene includes the 4436 bp K1 region analyzed in our laboratory previously and its flanking regions. The sequences of this region in various iridovirus isolates from fish differed from one another (percent identity = 72∼92) by nucleotide substitutions, insertions, and deletions. Most of the various insertions and deletions, with lengths of up to 163 bp, appeared in clusters in the repeating sequences in both the open reading frames (ORFs) and intergenic regions in K2 region. High homologies, > 71%, were demonstrated in the nucleotide sequences of the ORFs in the K2 regions of iridovirus strains including the infectious spleen and kidney necrosis virus (ISKNV), notably > 91% between isolates Sachun and Namhae isolated in Korea. Thus, these iridoviruses isolated from marine fish might be considered to be members of the tropical iridovirus group, which includes the red sea bream (Pagrus major) iridovirus (RSIV) Ehime-1 and is different from the genera Ranavirus, Lymphocystivirus, and Iridovirus in the family Iridoviridae. Identification of these iridovirus isolates was based on the different sizes of the resulting amplicons in discriminative PCR with primers derived from the K2 region, which contained different lengths of insertion (or deletion) depending upon the isolates. We also have described the possibility of asymptomatic iridovirus infection in fish.