Oyster depuration: a re-assessment of depuration conditions and the role of bacterial and viral indicators in determining depuration effectiveness
Kenneth A. Buckle
The NSW oyster industry has suffered severe economic losses during the past 20 years as the result of major outbreaks of gastroenteritis and other oyster-borne diseases. Further outbreaks will cripple the industry from immediate losses and resulting litigation. There is an urgent need to re-examine the conditions for depuration of NSW oysters harvested in very different geographic and environmental conditions with particular reference to water temperature, salinity and turbidity during depuration. It is important to determine what limitations exist in the current technology and how they might affect purification efficiency and commercial acceptance by oyster farmers. In light of such information, there will be a need to modify the Code of Practice for Oyster Depuration. Equally important is the need to educate and train oyster farmers in proper quality assurance of oyster quality and safety, which will include optimal application of purification technology. Current safety of oysters is assessed by the presence of E. coli as indicators of bacterial pathogens, yet the majority of oyster-borne disease is of viral origin, principally NV and more recently, HAV. Unfortunately, direct testing for human viruses in oysters is time consuming and very expensive, and will remain so for the foreseeable future. From the perspective of implementing a quality assurance program that will effectively protect public health, it is essential to have more accurate and more reliable indicator tests for the presence of human viruses in oysters. Consequently, there is a clear need to examine the relationship between the presence of bacterial indicators (E. coli), coliphages and human infectious viruses in oysters before, during and after purification. Such a study will not only clarify the behaviour of viruses during purification but could lead to a rapid coliphage assay as a more accurate indicator of human virus presence in oysters.
1. To optimize the process of oyster depuration with respect to conditions of: initial load of contamination; water temperature, salinity, turbidity and oxygen content; oyster: water ratio; time of depuration; geographical location of oysters; and oyster eating quality.
2. Based on the findings from objective 1, and in consultation with the NSW Department of Health, NSW Fisheries and representatives of the oyster industry, develop and write modifications to the Code of Practice for the Depuration of Oysters in NSW.
3. In conjunction with the NSW Shellfish Quality Assurance Program, conduct a series of workshops aimed at education of farmers in management of the quality and safety of oysters.
4. Investigate the possibility of using bacteriophages as an additional indicator of oyster safety and the performance of oyster depuration, with specific reference to the elimination of human viruses such as Norwalk and Hepatitis A viruses. Correlate bacteriophage occurrence and behaviour in oysters during purification with traditional E. coli standards and human virus behaviour as measured by an inactivated strain of polio virus.
Principal Investigator: Sandra von Olnhausen, Iona Reid, Graham Fleet, Ken Buckle & Julian Cox