Megalocytivirus
In 2003 there was an outbreak of iridovirus in farmed Murray cod in Victoria that caused 90% losses. In 2004 these viruses were found in imported ornamental fish - gourami. These have established breeding populations in the wild in Australia and therefore may pose a threat to biodiversity in the Murray-Darling Basin and other systems in which Maccullochella sp. still exist. These viruses are not host specific and all Murray-Darling Basin sp. may be susceptible. Due to the risk to aquaculture, recreational fisheries and biodiversity, there is need to develop and validate diagnostic tests for GIV in order to enable rapid accurate detection of GIV, facilitate surveys of Murray cod and imported ornamental fish to improve risk assessment and conduct surveys of free-living and farmed fish in Australia. The specific tests required include a robust and accurate PCR, and cell culture. These tests need to be able to distinguish GIV from related iridoviruses such as RSIV.

CyHV-2 of goldfish
In the last 10 years imported goldfish numbers have continued to grow, in part, due to their competitive price. The imported goldfish have been subjected to the AQIS requirements for import certification and a 3 week holding period before release. Anecdotal evidence suggests that mortality rates of domestically produced goldfish increase when these fish are mixed in retail shops with imported goldfish. Investigations have revealed pathology consistent with CyHV-2. Imported goldfish may carry latent CyHV-2 infections. This project aims to provide the diagnostic capability and a sampling regime to further investigate these concerns.

The project relates directly to Strategies 1, 2 and 3 in Aquaplan 2005-2010 and meets key research area 7.2.1 in the FRDC Aquatic Animal Health Sub Program Research and Development Plan 2002-2008 (updated June 2005).

Management of many Australian freshwater recreational fisheries involves supplementation of existing populations or attempts to establish new populations through the release of fingerlings. Conservation management of threatened freshwater species also relies heavily on the release of fingerlings to establish new populations. Australian and overseas studies have repeatedly demonstrated that fingerling stockings succeed in a limited number of cases and often the success of these programs is difficult to evaluate with conventional approaches. The factors and stocking strategies that can enhance the chances of fingerling stockings being successful are only just starting to be investigated in Australia (see FRDC Project 1998/221 “Impoundment stocking strategies for eastern and northern Australia”).

This project provides an opportunity to explore the possibility of re-establishing adult cod populations through seeding with fewer but much larger individuals (not fingerlings). The research has particular relevance to Australian freshwater cods as apex predators that may serve as indicators of river system health. Additionally, the study species, Trout Cod is Australia’s most imperiled cod species. It was once an important recreational species and a component of the inland commercial fishery of the Murray-Darling Basin. Today extensive efforts to recover the species and establish new populations remain limited by our lack of understanding of what happens to stocked individuals during sub-adulthood.

Susceptibility of freshwater fishes to barramundi nodavirus.

- There is a need to address concerns about the risk of possible lethal transmission of barramundi nodavirus to freshwater fishes already under threat in their natural habitat.
- Important freshwater fishes will be exposed to the nodavirus from cell cultures to determine their susceptibility.
- By using the OVL isolation facility in Townsville, well outside the Murray-Darling region, there is minimal risk to the natural fish populations.
- Confirmation that barramundi nodavirus can cause lethal infections in freshwater fishes will strengthen the application of strict licence conditions on barramundi farming in southern Australia.
- Confirmation that barramundi nodavirus does not affect freshwater fishes will possibly allow expansion of the barramundi farming into regions needing new sustainable economic development.

A barramundi infection model.

- To establish a realistic virus dose and route of infection for the challenge trials, an infection model using barramundi will be developed.
- OVL has ready access to barramundi larvae and fry of all ages.
- The model is also necessary to help quantify the effect of virus exposure to disinfectants and different environmental conditions.

What is the viability of barramundi nodavirus?

- Fish health management requires good information on how to effectively decontaminate facilities following outbreaks of VNN.
- Knowledge of the persistence of barramundi nodavirus in the environment will allow fisheries managers to decide on effective conditions for barramundi farming licences that minimise the risk of transferring virus outside the culture facility.

Sensitivity of the cell culture isolation system.

- While sensitive detection tests are available, the barramundi cell line offers a more practical diagnostic method that can be used by any laboratory with cell culture/ virology capability.
- An evaluation of the cell culture isolation system’s ability to detect virus in carrier (no disease) fish, and standardization of the cell culture presentation, is required before the method can be recommended.