safeguarding our sydney rock oyster industry against qx disease
BUDGET EXPENDITURE: $200,000
PRINCIPLE INVESTIGATOR: Dr Cheryl Jenkins
ORGANISATION: NSW Department of Primary Industries
PROJECT CODE: 2022-191
PROJECT STATUS: Completed
WHY WAS THIS RESEARCH UNDERTAKEN?
QX disease continues to pose a major threat to the Sydney rock oyster industry, with severe outbreaks capable of causing up to 90% mortalities in affected stock. Although the QX parasite (Marteilia sydneyi) has been reported in many estuaries, disease only occurs in some. This inconsistency has created uncertainty for growers and complicated biosecurity rules governing oyster movements between high, medium and low‑risk estuaries.
Two major knowledge gaps prompted this research:
-
Unclear parasite behaviour across estuaries: Earlier studies suggested M. sydneyi might be present in low‑risk estuaries, but these detections were based on older PCR methods and had never been confirmed using modern molecular tools. It was also unknown whether genetically different strains of M. sydneyi might explain why some estuaries experience disease while others do not.
-
Need to optimise selective breeding for QX resistance: Richmond River Rock Oysters (RRRO) have undergone decades of natural selection under heavy QX pressure and show enhanced survival. However, their performance varies, and industry needed robust, multi‑year evidence to guide how RRRO genetics should be incorporated into the breeding program.
Together, these gaps limited growers’ ability to manage risk and restricted movement of oysters between estuaries. This project was designed to generate the evidence needed to refine biosecurity policy and strengthen long‑term breeding strategies.
OBJECTIVES:
The project set out to address both pathogen behaviour and host resistance. Specifically, it aimed to:
-
Develop a multilocus sequence typing (MLST) framework for M. sydneyi using newly available genomic data, enabling detailed comparison of parasite populations across estuaries.
-
Resample low‑risk estuaries using modern qPCR diagnostics to confirm whether M. sydneyi is present, absent, or previously misidentified.
-
Compare any low‑risk parasite sequences with those from high‑risk estuaries to determine whether strain differences influence disease expression.
-
Create new selectively bred families incorporating RRRO genetics and evaluate their performance as spat and adults across multiple QX exposure sites and seasons.
-
Generate Estimated Breeding Values (EBVs) for QX survival, growth, and meat condition to quantify the contribution of RRRO genetics to the breeding program.
-
Compare RRRO‑derived families with commercial SRO breeding lines over two consecutive QX seasons.
These objectives were designed to produce evidence that directly informs biosecurity policy, breeding program design, and grower decision‑making.
WHAT WERE THE MAJOR FINDINGS / OUTCOMES?
A. No M. sydneyi detected in low‑risk estuaries
Across three years and 1,809 oysters, no parasite was detected using a highly sensitive qPCR assay. All 1,809 oysters tested negative for M. sydneyi. This strongly suggests that earlier detections may have been false positives or misidentifications. The absence of the parasite supports maintaining current movement restrictions to protect these estuaries.
B. Parasite strains appear genetically uniform across high‑risk estuaries
Genomic sequencing revealed very limited genetic variation among M. sydneyi samples from high‑ and medium‑risk estuaries. Core ribosomal genes were identical across all samples. Variation observed in the ITS regions occurred within individual oysters, not between estuaries, indicating natural within‑host diversity rather than distinct pathogenic strains. The implication here is that differences in disease expression between estuaries are unlikely to be driven by parasite strain differences. Environmental factors, host susceptibility, and transmission pathways are more probable drivers.
C. RRRO genetics improve QX survival, but require careful use
RRRO‑derived families consistently ranked among the highest‑surviving groups in QX exposure trials at both spat and adult stages. However, performance was highly variable, with some RRRO families ranking among the lowest.
This confirms RRRO material is valuable but must be integrated through structured crossing and rigorous field testing.
D. Strong gains in QX resistance across the breeding program
Across the 2022 and 2023 year classes:
-
QX survival continued to improve year‑on‑year.
-
Adult survival EBVs reached 88% in the 2023 year class.
-
Growth EBVs averaged 22–24% above wild oysters.
-
Meat condition remained stable or improved.
These results demonstrate that selective breeding remains the most effective long‑term tool for managing QX disease.
E. RRRO families now entering commercial production
Of the RRRO families produced, 22 have been approved for commercial hatchery use, giving growers access to new high‑resistance lines.
RESOURCES GENERATED:
