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Filter feeding bivalve molluscan shellfish concentrate and retain human microbial pathogens derived from sewage contamination of their growing waters. They can then present a health risk when consumed raw or lightly cooked. In the UK the main pathogens of concern are the unculturable Norwalk-like viruses (NLVs). Past difficulties in detecting these viruses in shellfish has impeded development of control measures. Methods such as single- round PCR had inadequate sensitivity for routine detection of low levels of NLVs present in shellfish, and thus the overall aim of this project was to develop more sensitive methodology and to apply this to the investigation of viral contamination of harvesting areas and shellfish related illness. This objective required the development of post-PCR virus genome sequencing strategies to identify and categorize NLV strains polluting shellfish. The new methods were successfully used to detect NLVs in shellfish both pre and post depuration, to characterise the virus strains occurring in a polluted harvesting area and to show how NLVs behaved during the depuration process. Finally, these tools were used to investigate a large shellfish associated outbreak of viral gastro-enteritis. This identified the same NLV strain in both faecal material and shellfish samples and was the first definitive linkage of clinical material and shellfish. Moreover during this outbreak enteroviruses were also identified in both shellfish and faecal samples and this is the first report of an epidemiological link between enterovirus contamination in shellfish and enterovirus infection in shellfish consumers Virus monitoring of shellfish harvesting areas using RT-PCR is a novel approach to combat the transmission of NLVs by molluscan shellfish, and could lead to better protection for consumers of bivalve shellfish. These developments also open up a number of avenues for future research work.