The edible oyster industry in Australia is currently valued at around $62.5 million annually of which rock oyster production accounts for approx 56%. For the industry to survive in the long-term requires the ability to service what may become a premium domestic market demanding a high quality product. The expansion of the industry is likely to be available only from international export, which in turn requires compliance with international regulations on oyster health with a transparent health audit trail. The rock oyster is potentially positioned for re-emerging export success, being a unique product with an extended shelf-life relative to other oyster species (e.g. the Pacific oyster, Crassostrea gigas) and this is an opportunity that should be exploited by the industry.
Within Australia, the Sydney Rock Oyster industry is subjected to periodic epizootics of disease induced by a range of parasitic organisms that produce significant mortality and morbidity of commercial oyster stocks. The most significant of these is the agent responsible for ‘QX disease’ (caused by the protistan parasite Marteilia sydneyi) affecting the Sydney rock oyster Saccostrea glomerata. Management of this disease has been based on quarantine of affected estuaries enforced through limitation on the movement of potentially infected stock. In this context, it was obvious that the oyster industry required a disease zoning policy based on scientifically defensible data to allow domestic best practice in oyster farming and to maximise market accessibility for the industry. This host/parasite system then formed the basis for a test of the zoning policy framework developed under the federal government’s ‘AQUAPLAN’.
A number of key issues related to zoning and surveillance for specific diseases were addressed through this project. Initially the design of field collection and the appropriate test to use for diagnosis were assessed to maximise, and allow quantification of, disease detection limits in the surveillance program.
1. The design of field sampling to identify disease infected oysters was critical in order to reach a statistically robust probability of disease detection. Global animal health standards (Office Internationale des Epizooties) recommend random sampling from a zone to detect a prevalence of 2% or greater disease in a population. This was fulfilled using a computer generated random selection of geographic co-ordinates under which individual oysters were sampled (Angus Cameron, AusVet).
2. The appropriate method for diagnosis of disease, another critical issue in disease surveillance programs, was assessed by comparing the sensitivity and specificity of: tissue imprints (cytology); or tissue sections (histology); or the presence of specific parasite DNA (by polymerase chain reaction - PCR). Our analysis showed clearly that PCR was the most sensitive diagnostic test followed by cytology then histology. PCR also detected the presence of sub-clinical infections which could not be unambiguously identified using either histology or cytology. Confirmatory diagnosis (following PCR) at sub-clinical levels was undertaken using DNA in situ hybridisation tests designed to stain the QX organism specifically in tissue section.
Combined surveillance results from 2001 (NSW estuaries only), 2002-03 (NSW and Queensland estuaries) and 2004 (Queensland estuaries only) demonstrated some significant departures from the geographic distribution expected for QX disease. In 2001 diagnosis was undertaken using cytology and no unexpected occurrences of the disease were observed, with positives recorded only from the Clarence River (1.5% of sample infected), Georges River (47% of sample infected). In 2002 the distribution of disease was significantly different to that expected. Initially using cytology for diagnosis there were no apparent unusual infections with Southern Moreton Bay (0.8% of sample infected), Richmond River (40.8% of sample infected), Clarence River (22% of sample infected) and Georges River (16% of sample infected) recording oysters positive for the disease. However, when PCR techniques were used for diagnosis in estuaries that had never recorded the presence of the disease agent it became obvious that the organism was more widespread than indicated by previous diagnostic testing or previous occurrences of disease outbreaks. In total 142 unexpected positives for Marteilia sydneyi were found in oysters scored as negative by cytological examination during surveillance in this project. Of these, 61 were identified in oysters sampled from estuaries with no prior record of Marteilia sydneyi. These represent oysters from Hastings River, Wallis Lake, Port Stephens, Bateman’s Bay, Tuross Lake, Narooma and Merimbula.
Further testing of these infections confirmed the identity of the QX organism and found it to be present in the oyster tissues at a sub-clinical level i.e. prior to reaching the oyster’s digestive gland where the parasite would normally produce spores. At this stage of development, pathology in the oyster is reduced and the condition factor of oysters is not seriously compromised.
In 2003 surveillance and diagnosis using PCR techniques showed a reduced impact of QX disease with Southern Moreton Bay (0.67% of sample infected), Brunswick River (1.3% of sample infected), Richmond River (13.3% of sample infected), Clarence River (6% of sample infected) and Georges River
(0.67% of sample infected).
This project has had a significant impact on our understanding of QX disease in rock oysters as it applies to management. Rather than the disease agent being limited geographically to those estuaries that experience periodic outbreaks, the agent has been identified in most rock oyster growing areas on the east coast of Australia. As such there is the potential for outbreaks of QX disease in all commercial growing areas (indeed such an outbreak occurred in 2004, with seasonal re-occurrence in 2005, in the Hawkesbury River) and that disease is likely to be regulated through a combination of the dynamics of the parasite lifecycle and the level of oyster fitness. Furthermore, in any aquatic system the environment will play an equally significant role in the outcomes of host/parasite interactions both through direct impact on stages (spores, infective stages) in the lifecycle of the parasite and indirectly through its impact on host fitness.
In the light of our new understanding of the distribution of the QX disease agent it could be argued that management through quarantine of identified QX-endemic estuaries is no longer appropriate. However, the biology of Marteilia sydneyi (dynamics of the life cycle of the parasite, interactions with alternate hosts) and its interaction with the host oyster’s immune system are incompletely understood and the precautionary principle should be upheld especially in the case of such a serious disease.
While estuaries which undergo periodic outbreak should remain closed to export of oysters for relaying live in water elsewhere, local management will focus on disease seasonality and stock rotation to avoid the high risk periods in mid to late summer. These periods should be identified with accuracy to maximise available growth periods in disease endemic areas of estuaries. The ongoing projects to develop QX disease resistant oysters (NSW DPI and collaboration with Macquarie University) should run parallel with a program of incremental addition to the biological knowledge of this pathogen. Specifically, an absence of our ability to maintain a laboratory based infection model hampers research on identifying those factors (pathogen-specific, oyster-specific and environment-specific) which promote disease.