Enhanced detection of pathogenic micro-organisms in the bivalve molluscs of the lower estuary of the St Lawrence

On the North Shore, shellfish harvesting is a very popular activity that provides local residents with a greatly appreciated food source and sustains a tradition connected with the St Lawrence. A number of government agencies share responsibility for monitoring the quality of shellfish in harvesting areas. The North Shore public health branch has carried out a study as part of the activities of the Human Health component of St Lawrence Vision 2000. This research was aimed at assessing the validity of the indictors that are used to estimate the level of pathogen contamination in molluscs and other shellfish.

Molluscs, which live buried in sediments or attached to rocks and reefs, feed on phytoplankton and other substances which they filter from the water. They may therefore absorb certain contaminants that could make them unfit for human consumption. Bivalve molluscs may, for example, be contaminated with pathogenic micro-organisms (bacteria, viruses and protozoans) released into the water through the discharge of human and animal faecal waste.

The consumption of molluscs contaminated with these pathogens generally leads to gastro-intestinal problems lasting from a few hours to a few days. However, more serious illnesses such as salmonellosis and Hepatitis A infection, as well as chronic infections like giardia, can also result from the ingestion of shellfish contaminated with pathogenic micro-organisms.

Monitoring of shellfish harvesting areas

Monitoring the contamination levels in shellfish and shellfish harvesting areas, including faecal contamination, is the key activity of the program administered by a number of federal and Quebec government agencies. In the 46 harvesting areas on the north shore of the St Lawrence Estuary, the bacteriological quality of the water is assessed on a regular basis in order to formulate recommendations on whether to authorize shellfish harvesting in given areas, approve this activity on a conditional basis or prohibit it. The quality of commercially harvested shellfish is assessed in seafood processing plants.

The assessment of faecal contamination of shellfish and shellfish harvesting areas centres on detecting the micro-organism Escherichia coli and measuring the concentration of faecal coliforms and total coliforms.

Validity of indicators called into question

The findings of a 1999 study by the North Shore public health branch raised questions about the reliability of the indicators that are currently used to assess faecal contamination of shellfish and harvesting areas on the north shore of the lower estuary of the St Lawrence.

A subsequent research project was undertaken to evaluate the validity of using specific micro-organisms as indicators of the quality of shellfish and harvesting areas. This was done by taking account of the impact of certain environmental parameters. The study area in this project consisted in the ZIP (Area of Prime Concern) territory on the north shore of the estuary, stretching from Tadoussac to Baie-Trinité. Nearly 4,000 soft-shell clams were harvested in nine shellfish areas between Pointe-aux-Outardes and Grandes-Bergeronnes. All of the water samples (total of 28) were taken in a single location, specifically at Baie-Laval.

The analyses performed on the water samples and the soft-shell clams that were collected were used to detect the presence and measure the abundance of six micro-organisms likely to serve as indicators of faecal contamination. Two of these micro-organisms, namely Escherichia coli and faecal coliforms, are already used in the existing monitoring programs, whereas the other four organisms represent potential indicators. The analyses also made it possible to check for the presence in soft-shell clams of four pathogens that pose a risk for public health under certain conditions.

Care is required in selecting and employing indicators

Based on this research, it appears that the frequency of detection of pathogens in soft-shell clams is high, since 90 percent of the samples turned out to be contaminated. Although contamination was identified in the clams from six sectors closed to harvesting, the contaminated clams also included samples from the three areas that were open to harvesting. This confirms the fact that shellfish contamination may represent a significant route of exposure to pathogenic micro-organisms of faecal origin.

Furthermore, Escherichia coli and faecal coliforms appeared to be poor indicators of the presence of pathogens. In fact, the detection frequency for Escherichia coli and faecal coliforms, estimated at 14 percent and 21 percent respectively, was well below the general detection frequency for pathogens. In addition, Escherichia coli and faecal coliforms were not detected in nearly 80 percent of the samples which were in fact contaminated with pathogens. "These results tend to confirm the hypothesis that Escherichia coli and faecal coliforms are not sufficiently sensitive parameters to serve as valid indicators of faecal contamination of shellfish on the North Shore," said Alexandra Valentin, project leader for this study by the North Shore public health branch. According to Mrs Valentin, an effort should be made to verify whether the use of a lower detection threshold than that used in this research, like the levels applied in some monitoring programs, would help to raise the indicators’ sensitivity to a satisfactory level.

Research into the impact of environmental parameters on the abundance of indicator micro-organisms may provide an explanation for the poor performance of Escherichia coli and faecal coliforms as indicators of the presence of pathogens. Indeed, results suggest that the salinity of the water in shellfish areas favours the rapid elimination of Escherichia coli and coliforms.

Among the four potential indicators considered in the study, somatic coliphages were identified as a promising candidate for detecting faecal contamination in shellfish. These organisms appeared to be sensitive, that is, they are readily detected in pathogen-contaminated shellfish. They also appear not to be affected by environmental conditions to a great extent.

"At the time when Escherichia coli and faecal coliforms were selected as indicators of faecal contamination in shellfish, scientists were unaware of several of the pathogenic micro-organisms that are now known to occur in harvesting areas," explained Mrs Valentin. She emphasized the importance of continuing the research aimed at assessing the quality of existing and potential indicators. The study states that the establishment of an effective tool for evaluating the risks associated with shellfish consumption must in future be based on epidemiological studies, research on micro-organisms and increased knowledge of interactions among micro-organisms and between them and the environment.

It goes without saying that this research underlines how important it is for harvesters to comply with the notices and signs indicating that a shellfish area is closed. It is also important to boil all shellfish in water for at least five minutes before eating them, as this serves to guarantee that a high enough internal temperature has been reached to destroy the majority of pathogenic micro-organisms.

For information:

Alexandra Valentin
Institut des sciences de la mer (ISMER)
Université du Québec à Rimouski
Telephone: (418) 775-0655
Email: VALENTINA@dfo-mpo.gc.ca

Jacques-François Cartier
Régie régionale de la santé et des services sociaux de la Côte-Nord
Telephone: (418) 589-9845
Email: Jacques-Francois_Cartier@ssss.gouv.qc.ca

Source:

VALENTIN, A., T. TREMBLAY, F. GAGNON et J.F. Cartier. 2000. Évaluation de la validité des indicateurs de contamination fécale des mollusques bivalves et des eaux coquillières de la rive nord de l'estuaire maritime du Saint-Laurent . Régie régionale de la santé et des services sociaux de la Côte-Nord, Direction de la santé publique, 90 p. + annexes.