Scientists recommend new methods to reduce risk of shellfish contamination

Shellfish, such as oysters, mussels and clams, can accumulate norovirus from contaminated waters. Norovirus is a major cause of gastroenteritis outbreaks and is globally responsible for 125 million cases of foodborne illness. Gastroenteritis-like symptoms associated with the consumption of contaminated shellfish are reported in many countries.

Given the growing human population and likely increase in the demand for shellfish, this issue is projected to escalate, with potentially negative effects on the shellfish industry in New Zealand.

Despite pre- and post-harvest intervention treatments used to reduce the risk of norovirus infection in shellfish, outbreaks associated with shellfish consumption are still reported worldwide. Therefore better tools are needed to ensure the safety of shellfish.

A recent Institute of Environmental Science and Research (ESR) study involving Plant & Food Research scientists critically evaluated the usefulness and challenges around commonly-used treatments to improve shellfish safety.

The scientists in this study reported that standard guidelines, based on faecal indicator bacteria, do not accurately predict a risk of norovirus contamination in water and shellfish. Instead, better microbial source tracking indicators should be used to reduce the risk of norovirus contamination. Additionally, microbial source tracking indicators should ideally include those specific to human faecal contamination and persist in the environment in a similar way to human norovirus. 

The study also found that while pre-harvest preventative measures are preferable, post-harvest interventions can be used to mitigate risk. Current post-harvest methods – such as depuration, relaying, thermal and high-pressure processing – all have limitations therefore the choice of method employed should be based on environmental, economic and social factors.

Finally, the study recommended that contamination risks can be managed for instance by preventing inadequately treated wastewater from being discharged into the environment, making sure harvesting does not take place for some time after contamination events and locating shellfish farms more than 10 km from possible sources of contamination.

Journal Reference:

Gyawali, P, Fletcher, G, McCoubrey, D, Hewitt, J 2019.  Norovirus in shellfish: An overview of post-harvest treatments and their challenges. Norovirus in shellfish: An overview of post-harvest treatments and their challenges Food Control 99 https://doi.org/10.1016/j.foodcont.2018.12.049