Molecular characterization, sequence analysis, and taxonomic position of newly isolated fish iridoviruses

Within the past decade, iridoviruses have been identified as the causative agents of systemic disease in a variety of commercially and recreationally important fish. Here we examine nine iridoviruses from fish, reptiles, and amphibians and demonstrate that all isolates were more similar to frog virus 3, the type species of the genus Ranavirus, than to lymphocystis disease virus, the type species of the genus Lymphocystivirus. Comparison of viral protein synthesis profiles, restriction endonuclease digestion patterns, and the amino acid sequence of the major capsid protein indicated that iridoviruses isolated from the same geographic region were similar, if not identical, whereas viruses from different areas were distinct. Moreover, using primers complementary to the conserved major capsid protein, we found that both PCR and RT-PCR successfully amplified virus-specific nucleic acid from all nine isolates. These studies demonstrate that the piscine iridoviruses examined here were members of the genus Ranavirus, and suggest that surveys of pathogenic 'fish viruses' may need to include neighboring amphibian and reptilian populations. In addition, the results indicate that PCR readily identified vertebrate iridoviruses and suggest that PCR will be useful in the diagnosis of fish disease.