| Summary: | Megalocytiviruses affect a broad range of fish species and have caused widespread mortality in aquaculture throughout Asia. The first confirmed report involved mass mortality in red sea bream cultured off Shikoku Island, Japan in 1990. Ornamental fish exhibiting pathology consistent with Megalocytivirus infection have been reported from a range of countries, frequently in association with fish recently imported from south east Asia. Although the relationship between megalocytiviruses in ornamental fish and food fish remains unclear, a potential linkage between a Megalocytivirus from a disease outbreak in Murray cod, Maccullochella peelii, in Australia and those in ornamental fish imported from south east Asia had been demonstrated experimentally. However, no data existed on the strains of virus involved in the first reported cases of Megalocytivirus-like pathology in ornamental fish. Furthermore, the potential for spread from ornamental fish to food fish other than Murray cod was also unknown. This study aimed to characterise Megalocytivirus from archival ornamental fish tissues, and to determine the potential for spread of megalocytiviruses from freshwater ornamental fish to other species, including marine fish. In situ hybridisation (ISH) assays were developed to demonstrate megalocytiviral DNA in tissue sections. The presence of megalocytiviral DNA was confirmed in two archival ornamental fish from 1986 and 1988 which had Megalocytivirus-like pathology. Unlike all other reported cases of Megalocytivirus in ornamental fish, these cases did not involve the Infectious spleen and kidney necrosis (ISKNV)-like genotype. The sequence of the major capsid protein (MCP) and the adenosine triphosphatase (ATPase) genes from the viruses in the archival tissues had a high degree of identity to each other (96.7-100%) and to a turbot reddish body iridovirus (TRBIV)-like Megalocytivirus (96.6-99.9%), reported as a cause of mass mortality in barred knifejaw, Oplegnathus fasciatus, fingerlings in Taiwan. By contrast, MCP ...
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