Outbreaks of QX disease in Port Stephens in 2022 and 2023 mark the continued spread of this disease into Sydney rock oyster (SRO)-producing estuaries in NSW and QLD over the last 50 years. In severe years, QX can cause up to 90% mortalities in affected stock, therefore this disease poses a major threat to an industry that is of substantial economic, historic and cultural value.
Despite the apparent presence of the causative agent (M. sydneyi) in nearly all estuaries undertaking SRO production, disease only occurs in some, and biosecurity protocols further complicate SRO farming as stocks from high-risk estuaries cannot be moved into estuaries that have a lower QX disease risk profile.

Reasons behind the expression of QX disease in some estuaries but not others currently remain unknown. In France, a related parasite, M. refringens, was originally thought to be a widespread cause of marteiliosis in the European flat oyster (Ostrea edulis) as well as mussels belonging to the genus Mytilus. M. refringens O (oyster) type and M (mussel) type, which were originally described due their differential pathogenicities in the respective hosts, have more recently been found to constitute separate species, with M type being renamed as Marteilia pararefringens. A similar situation may exist in Australia with M. sydneyi constituting more than one species and with the more pathogenic strains being responsible for QX disease outbreaks. Historically these questions could not be meaningfully answered due a lack of genetic information about M. sydneyi; however NSW DPI has recently undertaken a genome sequencing project on M. sydneyi that can facilitate strain comparison. Therefore, one aim of this project is to characterise Marteilia strains from estuaries where disease occurs, and compare with those from estuaries where disease does not occur, to better inform biosecurity policies. If the M. sydneyi strains are identical across estuaries, then this may enable biosecurity policy to be modified to allow oyster translocations across so called “high” and “low” risk estuaries. However, if strains do differ across high and low risk estuaries, then any biosecurity policy will be aimed at protecting estuaries not currently experiencing QX outbreaks from the introduction of high pathogenicity strains.

QX disease remains as the primary known threat for SRO production. Due to significant knowledge gaps in how this disease is transmitted, the use of selectively bred QX resistant oysters is the main management tool used to enable cultivation to continue in estuaries where the disease is enzootic. QX survival is a quantitative and a responsive trait where applied breeding offers a good solution to increase QX survival with significant economic benefits for industry. QX survival breeding is reliant on field challenges however, this method works well to increase resistance. Best results for improving QX survival are achieved through a combination of breeding and management practices to minimise impacts. It is recommended to use oysters selected for QX survival as a risk management strategy to reduce stock losses before a QX disease outbreak occurs in an estuary. When oysters selected for QX survival are used in estuaries affected by QX, it is important to deploy spat when M. sydneyi infections have ceased and harvest these oysters prior to a second disease exposure. This relies on specific timing of commercial hatchery production and fast oyster growth which is a trait under selection in combination with QX survival. Field exposures that run over two seasons of QX disease are now used to increase survival following consecutive outbreaks. Other diseases or factors that compromise SRO health prior to or during M. sydneyi infections also reduce the effectiveness of breeding.

Increasing genetic gains for QX survival has been the primary objective of the breeding program since its inception. A genomics project is currently underway which aims to identify genetic markers for QX disease resistance to increase genetic progress for this trait. Batches of Richmond River Rock oyster (RRRO) produced by NSW DPI have shown high levels of of QX disease survival. Prior studies on RRROs suggest that genetically they are classified as SROs but they appear to have developed significant resistance, presumably due to years of exposure to QX in the Richmond River estuary where the disease in enzootic. Preliminary experimental evidence suggests that RRROs display enhanced survival when exposed to QX disease, justifying their inclusion in the selective breeding program. Therefore, the second aim of this project is to assess QX survival of current RRRO families across multiple years of QX exposure and compare these results to other QX-resistant families in the breeding program. This information will be used in this project to formulate a breeding plan to create additional families using batches of RRROs that have been assessed for QX survival.