Experimental transmission and virulence of a megalocytivirus (Family Iridoviridae) of dwarf gourami (Colisa lalia) from Asia in Murray cod (Maccullochella peelii peelii) in Australia

In February 2003 there were 90% losses of Murray cod (Maccullochella peelii peelii) fingerlings in a Victorian aquaculture facility. The disease was caused by a megalocytivirus (Family Iridoviridae) closely related to the recognized species Infectious Spleen and Kidney Necrosis Virus (ISKNV) and strain dwarf gourami iridovirus (DGIV), neither of which had previously been reported from farmed or wild fish in Australia. Experimental transmission trials were undertaken to test the hypothesis that the outbreak could have arisen through introduction of a virus with ornamental gouramis imported from South East Asia. Intraperitoneal injection of Murray cod fingerlings using filtered tissue homogenates from dwarf gourami (Colisa lalia) positive for megalocytivirus DNA by polymerase chain reaction (PCR) resulted in > 90% mortality. Mortality was also induced by cohabitating Murray cod fingerlings and dwarf gourami; as the fish were physically separated, the virus spread between the two species via water. Histopathology revealed lesions identical to those reported in the Victorian outbreak, 125–130 nm icosahedral virions were observed in lesions and most exposed fish were PCR positive. DNA sequencing confirmed 99.9 to 100% homology of major capsid protein and ATPase nucleotide sequences between DGIV, ISKNV, the viral inoculum obtained from dwarf gourami and virus present in experimentally infected Murray cod. These findings confirm that Murray cod are highly susceptible to a megalocytivirus present in ornamental fish imported from South East Asia. The implications for aquaculture, conservation of native fish and quarantine policy are discussed.