Norovirus in shellfish: An overview of post-harvest treatments and their challenges

Filter feeding bivalve molluscan shellfish such as oysters, mussels and clams can readily accumulate norovirus present in growing water contaminated by human faecal material from point and non-point sources. While pre-harvest preventative interventions are preferable, post-harvest interventions such as depuration, relaying and thermal treatment have been used to mitigate the risk of norovirus infection associated with shellfish consumption. However, even with available pre- and post-harvest interventions, norovirus outbreaks associated with shellfish consumption are still reported worldwide. Analyses of faecal indicator bacteria, i.e. total coliforms, Escherichia coli or Enterococci spp. are commonly used to classify shellfish growing waters. Such classifications are used as a pre-harvest intervention, to identify when harvesting should be restricted and/or determine the necessity of post-harvest treatments. Post-harvest depuration treatment does not necessarily remove norovirus from shellfish tissue, and freezing has little or no effect on norovirus infectivity in shellfish. Thermal treatments can inactivate norovirus but they also change the organoleptic characteristics of shellfish which makes them unacceptable to some consumers. High pressure processing is an alternative post-harvest intervention that has potential to inactivate norovirus effectively with a reduction of 2.8-4.0 logio genome copies at 300-450 MPa. However, a human challenge showed that less than 600 MPa are not sufficient to prevent norovirus infection when people consume artificially-contaminated shellfish. This study critically evaluates the applicability of, and the challenges associated with, these interventions to improve shellfish safety.