Asymptomatic iridovirus infection in various marine fishes detected by a 2-step PCR method

Iridovirus is a major fish pathogen in marine aquaculture of Asian countries including Korea. In this study we present the first report of the presence of asymptomatic iridovirus infection in various cultured marine fishes in Asian countries. The presence of iridovirus in 250 samples taken from fish of market size of 4 different farmed species for each month of 2003 was determined using a 2-step PCR technique. The proportion of iridovirus-positive fish varied from month to month, but the overall prevalence was 37.2% (93 / 250), and differed depending upon the species of fish: 79.7% (51 / 78) for rock bream (Oplegnathus fasciatus), 31.3% (20 / 64) for rockfish (Sebastes schlegeli), 27.3% (15 / 55) for sea perch (Lateolabrax sp.), and 13.2% (7 / 53) for red sea bream (Pagrus major). In a genomic analysis, we found that i) the DNA concentration of the iridovirus in asymptomatically infected tissues was approximately 10− 5 times of that of moribund fish infected clinically, ii) a comparison of three different genomic regions of iridoviruses causing clinical and asymptomatic infections, respectively, revealed no differences, iii) these nucleotide sequences were the same as those of the iridovirus Sachun (IVS-1), which is known to be the major iridovirus strain in Korea, except for a 48-bp deletion of the repeating sequences in the ORF-1 gene in the K1 region. In a challenge experiment with the iridovirus (IVS-2) obtained from moribund rock bream in 2003, it was confirmed that a protective immune response against a second clinical viral infection was not evoked in hosts infected by asymptomatically infectious iridovirus (ASIV), whether these were adult fish of market size or young-of-year fish. This is the first report of a high level of asymptomatic iridovirus infection in various marine cultured fish. Its implication for the potential risk of clinical infection with iridovirus is discussed.