The unfortunate link between bivalve shellfish consumption and illness is a result of a number of factors including:

• Bivalve shellfish are filter feeders extracting microscopic particles including bacteria, viruses and phytoplankton from the surrounding water.
• If harvested from polluted waters these micro-organisms may include pathogenic bacteria (eg Salmonella) or viruses (eg hepatitis A).
• Some microscopic algae produce substances toxic to humans and these biotoxins can accumulate in shellfish during algal blooms. These toxins are not removed during cooking.
• Some marine bacteria (eg some Vibrio species) are pathogenic to humans and these are able to multiply rapidly in shellfish if proper temperature controls are not implemented after harvest.

Bivalve shellfish are often eaten raw or only partially cooked allowing pathogens to survive and infect the consumer.

Shellfish producing countries around the world have adopted a range of strategies to minimise the risk of shellfish borne illnesses affecting consumers. A “clean waters” approach is implemented in Australia. This strategy is best described as ensuring that shellfish are only harvested from waters that are shown to be free of harmful contaminants. If shellfish are grown in clean, unpolluted waters they should not contain unacceptable levels of pollutants, potentially harmful pathogens or biotoxins.