The molecular epidemiology of iridovirus in Murray cod (Maccullochella peelii peelii) and dwarf gourami (Colisa lalia) from distant biogeographical regions suggests a link between trade in ornamental fish and emerging iridoviral diseases.

Iridoviruses have emerged over 20 years to cause epizootics in finfish and amphibians in many countries. They may have originated in tropical Asia and spread through trade in farmed food fish or ornamental fish, but this has been difficult to prove. Consequently, MCP, ATPase and other viral genes were sequenced from archival formalin-fixed, paraffin-embedded tissues from farmed Murray cod (Maccullochella peelii peelii) that died during an epizootic in 2003 and from diseased gouramis that had been imported from Asia. There was almost complete homology (99.95%) over 4,527 bp between Murray cod iridovirus (MCIV) and an iridovirus (DGIV) present in dwarf gourami (Colisa lalia) that had died in aquarium shops in Australia in 2004, and very high homology with infectious spleen and kidney necrosis virus (ISKNV) (99.9%). These viruses are most likely to be a single species within the genus Megalocytivirus and probably have a common geographic origin. Primers for genus-specific PCR and for rapid discrimination of MCIV/DGIV/ISKNV and red sea bream iridovirus (RSIV), a notifiable pathogen, were developed. These were used in a survey to determine that the prevalence of DGIV infection in diseased gourami in retail aquarium shops in Sydney was 22% (95% confidence limits 15-31%). The global trade in ornamental fish may facilitate the spread of Megalocytivirus and enable emergence of disease in new host species in distant biogeographic regions.