The NSW oyster industry and the Australian shellfish industry at large can ill afford further food poisoning episodes either from seage borne viral contamination or potential deaths from organisms such vibro vulnificus. Other diseases such as hepatitis A and salmonellaosis can also pose a continuing underlying risk to growers and consumers of shellfish. As urban development continues along the NSW coast there is also a concomitant increase in proposals for sewage treatment plants to discharge treated effluent into rivers and clear STP performance standards, guidelines and recommendations from a shellfish farming perspective are needed.
The development of a broader understanding of the limitations of purification and the possible development of technology to enhance and render the current purification process a more active technique, which does more than just rely on the shellfish to "self clean" would make an important contribution to public health as well as the the image, confidence and ultimate financial well being of the industry.
Current purification techniques alone are now seen to be incomplete in ensuring that oysters are safe for human consumption, especially in regard to viral infection. It is now recognised that the current UV purification process has to be linked to water quality monitoring at the time of harvest and monitoring at the time of harvest and monitoring of meat samples. However, there has been considerable work done around the world related to R&D in purification technology since the legislation was first introduced in NSW. A number of purification related R&D project proposals have been referred to the NSW Oyster Research Advisory Committee for support for funding by the FRDC. Although the NSW Shellfish Quality Assurance Committee does not believe that there is likely to be a "one step cure all", in purification technology, an appraisal of the current state of play, the practical limitations of purification and recommendations on areas worth pursuing in shellfish purification technology development by a suitably qualified consultant is timely if not overdue. The form of this investigation would be a desk top review and such a review has potential application for emerging shellfish farming industries in other states. This review was identified as one of the urgent priorities in the recently completed NSW Oyster Industry R&D Strategic Plan.
Final report
Depuration in the context of this report can be defined as the process by which harvested shellfish are placed in land based plants containing clean estuarine water to permit the purging of their gastrointestinal contents under controlled conditions. Depuration does not include the practice of relaying shellfish to clean estuarine waters for long periods to promote self cleansing.
Shellfish (defined as bivalve molluscs for the purpose of this review) filter large volumes of water and trap particulate matter and dissolved substances suspended in the water as a source of food. Consequently, if the water in which they are grown is polluted, then the shellfish may concentrate microbes or chemicals which may be injurious to the consumer. Because shellfish are often consumed raw (or slightly cooked) and whole (including their gastrointestinal tract), they are generally classified as a high-risk food group by health authorities worldwide.
Shellfish are depurated in order to reduce the likelihood of transmitting infectious agents to consumers. Depuration has been demonstrated to successfully reduce to low levels the number of bacterial and some viral agents in moderately polluted shellfish. The effectiveness of the depuration process is dependent on a number of variables including the health status of the shellfish, environmental parameters within the depuration plant (salinity, temperature, turbidity), the type of pathogen, and level of contamination.
Depuration has been practiced around the world since early this century. In 1978 the practice was formally introduced in NSW as a response to a food poisoning outbreak over 2000 cases of viral gastroenteritis, which was attributed to oysters farmed in the Georges River. Depuration of all oysters harvested in NSW became a statutory requirement in 1983.
After reviewing the literature and consulting industry and relevant experts, two areas of contention have been identified with the practice of depuration in NSW and perhaps elsewhere in the world. The first issue relates to operational parameters and regulation of the process, the second appears to be a simple failure by sections of industry and regulatory authorities to appreciate that depuration alone does not ensure shellfish food safety.
Keywords: depuration, shellfish, quality, viruses, bacteria, HACCP.
