Bivalve molluscan shellfish concentrate contaminants from the water column in which they grow. These contaminants may then cause illness to humans when the bivalves are eaten. For microbial contaminants, the risk is enhanced by the fact that these shellfish are often eaten raw (e.g. oysters) or relatively lightly cooked (e.g. mussels). Limiting the risk of illness depends partly on sourcing the shellfish from areas in which such contaminants are at relatively low levels. The risk may be reduced further by appropriate treatment following harvest.
Depuration (purification) is a process by which shellfish are held in tanks of clean seawater under conditions which maximize the natural filtering activity which results in expulsion of intestinal contents, which enhances separation of the expelled contaminants from the bivalves, and which prevents their recontamination. Depuration was originally developed as one of a number of means to address the problem of a large number of shellfish-associated outbreaks of typhoid (caused by the bacterium Salmonella typhi), which caused illness and death in many European countries and in the United States of America at the end of the nineteenth century and beginning of the twentieth century.
Depuration is effective in removing many faecal bacterial contaminants from shellfish. As currently commercially practised, it is less effective at removing viral contaminants such as norovirus and hepatitis A. It is not consistently effective, or is ineffective, in removing other contaminants such as naturally occurring marine vibrios (e.g. Vibrio parahaemolyticus and Vibrio vulnificus), marine biotoxins (such as those causing paralytic shellfish poisoning PSP, diarrhetic shellfish poisoning DSP and amnesic shellfish poisoning ASP) or heavy metals or organic chemicals.
Effective depuration requires the shellfish to be properly handled during harvest and pre-depuration transport and storage. It also requires proper design and operation of the depuration systems to meet the requirements identified above for removal and separation of contaminants. Likewise the establishments in which the system or systems are located need to be operated to good levels of food hygiene in order to prevent crosscontamination between, or recontamination of, different batches of shellfish.
This document is intended to provide a basic introduction to the public health problems that can be associated with shellfish consumption and to provide guidance as to how a depuration centre, and the associated systems, should be planned and operated. It also includes guidance on the application of Hazard Analysis Critical Control Point (HACCP) plans and associated monitoring. The document is intended to be of use to members of the shellfish industry with no or limited experience in the area and to fishery and public heath officials who may be involved in providing advice to the industry. Supplementary material may be found in the publications given in the bibliography.
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