Nodavirus in wild barramundi populations

* There is a need to address concerns about the effect of stocking hatchery-reared barramundi on the level of unapparent nodavirus infections (that is, the prevalence) in wild barramundi.
* The first step is to determine the prevalence of nodavirus in wild populations of barramundi (that is to say the natural level of nodavirus-carrier status – an infection without disease).
* The baseline nodavirus prevalence data will permit:
- comparison of barramundi populations in areas where stocking has or has not occurred,
- assessment of changes in prevalence of nodavirus in future years,
- effective decisions about appropriate sources of replacement broodstock for breeding programs.

Nodavirus in freshwater fishes

* There is a need to address concerns about the risk of possible lethal transmission of barramundi nodavirus to freshwater fishes.
* Recent investigations have shown a possible susceptibility of freshwater fishes to barramundi nodavirus and that nodaviruses naturally occur in species other than barramundi in Australia, including the freshwater species, sleepy cod.
* There is a need to determine if there are nodaviruses in freshwater fishes as a risk analysis for translocation should include disease-status information in the receiving population.

Are the nodaviruses found in freshwater fishes related to barramundi nodavirus?

* If nodaviruses are detected in freshwater fish an analysis of relatedness (sequence analysis of PCR products) could indicate an association to previous stocking in that area of hatchery-reared barramundi. This information would support effective risk analysis for future translocation considerations.

A testing protocol for hatchery production of nodavirus-free fish fry.

* Broodstock screening protocols to identify nodavirus-free broodstock have been described from overseas but the detection tests used then are not as sensitive as the two-step or nested RT-PCR, and the protocols include a requirement for egg/water disinfection and repeat testing of larvae.
* There is a need to evaluate and validate the sensitivity of the two-step or nested RT-PCR to identify nodavirus-free broodstock and to determine if one or more tests are required to confirm the nodavirus-free status.
* There is a need to confirm in barramundi that larvae/fry become infected by nodavirus through the vertical transmission route (ie., from their parent(s)).
* There is a need to determine if fry can become infected via nodavirus-contaminated water once they are stocked into nursery systems.
* If the vertical infection route is the same for all fish species, the information on the testing protocol required to produce nodavirus-free barramundi fry will be a model testing protocol applicable to all fish species in breeding programs in Australia.