The ornamental fish trade is suspected of facilitating the spread of an emerging viral disease.
The virus, which affects the Dwarf gourami, Colisa lalia, is so similar to one that infects farmed Murray cod, Maccullochella peelii peelii, that the two are believed to be a single species with a common geographic origin.
A team of Australian scientists, headed by Professor Richard Whittington of the University of Sydney, sequenced the genes of a virus that killed Murray cod in a disease outbreak in 2003 and compared the sequence to that of viral genes from imported Asian Dwarf gouramies, that had died in Australian aquarium shops.
When the sequences of the two viruses were compared, they had almost completely homology over 4527 base pairs with 99.95% of the sequences identical.
The two viruses, which are known as Murray cod iridovirus (MCIV) and Dwarf gourami iridovirus (DGIV) shared remarkable similarities - 99.9% - with a third virus known as infectious spleen and kidney necrosis virus (ISKNV).
All three viruses are believed to represent a single species within the Megalocytivirus genus and are thought to have originated from the same original location.
The study, which was published in the journal Molecular and Cellular Probes, also describes how a new PCR primer was developed for rapidly identifying the MCIV DGIV ISKNV virus.
A test of this primer revealed that around 22% of Dwarf gouramies in Australian aquarium retail stores were infected with the virus, raising fears that the ornamental fish trade may help spread the disease.
The authors wrote: "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".
All of the gouramies examined had been imported from suppliers in Singapore.
The vast majority of Dwarf gouramies in the UK are imported from the same region.
Spread by waterA follow-up study, which has just been published by Richard Whittington and Jeffrey Go in the journal Aquaculture, has shown that the Dwarf gourami iridovirus can be spread from infected fish through the water.
Whittington and Go injected Murray cod with filtered tissue homogenates from Dwarf gourami and cohabited the fish in the same water.
They found that the Murray cod subsequently tested positive for Megalocytivirus DNA through PCR analysis. The infection with the Dwarf gourami virus resulted in 90% mortality.
Whittington said: "Other species may be susceptible to megalocytivirus infection and act as carriers.
"For example, Mosquito fish (Gambusia affinis) is widely distributed across Australia and is closely related to poeciliid aquarium species such as Swordtails (X. hellerii) and Mollies (P. latipinna), which are known to be susceptible to infection by megalocytiviruses."
Dwarf gourami iridovirusA Dwarf gourami iridovirus has been known about for several years, but it was not previously known that a single species was capable of apparently infecting other species.
A separate study by scientists from the University of Florida in 2003 found that a Dwarf gourami iridovirus caused clinical signs including lethargy and the darkening of body colouration.
The affected fish stopped eating, sometimes had a distended abdomen and, internally, an enlarged spleen, reddened intestine and a clear amber fluid in the body cavity.
Practical Fishkeeping has been aware of health issues in imported Dwarf gouramies for many years, however, recent reports from readers have suggested a rise in mortalities and a decrease in lifespan.
A number of major importers of fish from south east Asia confirmed to this magazine that Dwarf gouramies have been of inferior quality for at least the past ten years, but they had not noticed an unusal rise in mortalities.
Many of these companies had already switched to gouramies from non-Singapore suppliers due to health problems with imported livestock.
Health problems in Dwarf gouramies have historically been blamed on resistant bacterial infections, fish TB and Nocardia-like infections.
For more information see the paper: Go J, Lancaster M, Deece K, Dhungyel O, Whittington R (2006) - 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. Molecular and Cellular Probes. 2006 Jun-Aug;20(3-4):212-22.
Go J and R Whittington (2006) - Experimental transmission and virulence of a megalocytivirus (Family Iridoviridae) of dwarf gourami (Colisa lalia) from Asia in Murray cod (Maccullochella peelii peelii) in Australia. Aquaculture 258 (2006) 140-149.