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Applied and Environmental Microbiology, June 2009, p. 3535-3541, Vol. 75, No. 11
0099-2240/09/$08.00+0     doi:10.1128/AEM.02255-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Phenotypic Diversity of Infectious Red Sea Bream Iridovirus Isolates from Cultured Fish in Japan{triangledown}

Hajime Shinmoto,1 Ken Taniguchi,2 Takuya Ikawa,2 Kenji Kawai,2 and Syun-ichirou Oshima1*

Laboratory of Cell Structure and Function, Division of Marine Bioresource Science, Graduate School of Kuroshio Science, Kochi University, Nankoku, Kochi 783-8502, Japan,1 Fish Disease Laboratory, Department of Aquaculture, Kochi University, Nankoku, Kochi 783-8502, Japan2

Received 1 October 2008/ Accepted 27 March 2009

Megalocytivirus is causing economically serious mass mortality by infecting fish in and around the Pacific region of Asia. The recent emergence of many new iridoviruses has drawn attention to the marked taxonomic variation within this virus family. Most studies of these viruses have not included extensive study of these emergent species. We explored the emergence of red sea bream iridovirus (RSIV) on a fish farm in Japan, and we specifically endeavored to quantify genetic and phenotypic differences between RSIV isolates using in vitro and in vivo methods. The three isolates had identical major capsid protein sequences, and they were closely related to Korean RSIV isolates. In vitro studies revealed that the isolates differed in replication rate, which was determined by real-time quantitative PCR of viral genomes in infected cells and cell culture supernatant, and in cell viability, estimated by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay for infected cells. In vivo studies showed that the isolates exhibit different virulence characteristics: infected red sea bream showed either acute death or subacute death according to infection with different isolates. Significant differences were seen in the antigenicity of isolates by a formalin-inactivated vaccine test. These results revealed that variant characteristics exist in the same phylogenetic location in emergent iridoviruses. We suggest that this strain variation would expand the host range in iridoviral epidemics.

* Corresponding author. Mailing address: Laboratory of Cell structure and Function, Division of Marine Bioresource Science, Graduate School of Kuroshio Science, Kochi University, Nankoku, Kochi 783-8502, Japan. Phone and fax: 81 88 864 5214. E-mail: S-Oshima{at}cc.kochi-u.ac.jp

{triangledown} Published ahead of print on 3 April 2009.

Applied and Environmental Microbiology, June 2009, p. 3535-3541, Vol. 75, No. 11
0099-2240/09/$08.00+0     doi:10.1128/AEM.02255-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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