Pathogen Dynamics

 

As filter feeders bivalve shellfish pump vast quantities of water through their bodies removing particulate food matter. When viruses, bacteria and other toxins are present in the water they are removed by the bivalves and as a result they can potentially concentrate these pathogens within their bodies.

Noroviruses (NoVs) are members of the Calicivirus family and are small; positive-polarity RNA viruses. Infection is via the oral-faecal route and infected humans can release millions of particles into the sewage system for up to 28 days after the initial infection, and as little as one viral particle is required for disease. These viruses have emerged as one of the most significant causes of gastroenteritis in humans affecting all ages and resulting in recurrent bouts of vomiting and diarrhoea lasting between 24 and 72 hours.

The burden of these illnesses on society is estimated at £30 billion annually and is heavily under reported, as less than 10% of people notify authorities. Norovirus, also known as “Winter Vomiting Bug”, shows a seasonal trend in the population, frequently between October and February, as does its detection in shellfish. The cold temperatures and reduced UV during these winter months does not deactivate the virus. Norovirus frequently evolves and research is restricted by the fact that the virus cannot be grown in the laboratory. NoVs and other entric viruses enter the coastal waters from agricultural runoff and via manmade discharges, the volume of which are significantly increased during storm events. This high microbial or viral loading can impact significantly on the shellfish beds resulting in closure and loss of revenue or declassification.

 

The environment in which shellfish grow can also become a reservoir of infectious entric bacteria which can survive for long periods of time especially when attached to particulate matter. Re suspension of sediments can release particulate matter extending the influence of the NoV further downstream.

The resilient nature of the NoV means that it is able to survive the extremes of temperature, salinity, oxygen levels and microbial degradation.  It is attracted to and absorbed onto sediment particles (organic and inorganic debris) which enable it to persist in the environment, maintaining its ability to re-infect, which has an impact on shellfish compliance and classification.

 The EU introduced guidelines to control shellfish safety (Regulations 853/2004, 854/2004 and 1021/2008) since some bivalves can be eaten raw, posing a significant human health risk. Currently microbial indicators of shellfish pollution are dependent upon Escherichia coli counts however viral contamination is not readily identified or controlled by regulation. There has been an association found between E.coli levels and the presence of entric viruses, but not so far as to be able to distinguish between infectious and non-infectious NoV. The ability of NoV to maintain its survival in the environment is of particular concern with the new directive on Bathing Water to be implemented by 2015 and the potential failure of a number of bathing waters within Wales.

In Wales there are a number of shellfish areas that appear to consistently fail or are near to failing shellfish hygiene regulations, and with the rise in NoV within the community, there is an ever increasing threat on the fisheries on a seasonal basis.

Therefore, to avoid substantial economic losses within the seafood industry, to build public confidence and prevent further human health risk, there is a need to develop quick protocols to identify levels of bacteria, viruses and biotoxin loading in shellfish in less than 48 hours.