Are there risks to consuming mollusks?
	What do we know about mollusk contamination?

What do we know about mollusk contamination?

Contamination via micro-organisms

Mollusks filter water to eat and are therefore susceptible to absorbing and accumulating various types of micro-organisms which exist in water, whether they are pathogens or not. Little data exists about the presence of micro-organisms in mollusks harvested for non-commercial purposes. A study in the summer of 1999 of nine soft-shell clam harvesting areas between Grandes-Bergeronnes and Pointe- aux-Outardes on the North Shore revealed new information about microbial contamination and mollusks. The most notable results indicated an increased frequency of detection of Cryptosporidium sp. and Giardia sp. pathogens in soft shell clams as the most frequently detected organisms, followed by Campylobacter sp. However, high levels of contamination indicators (Escherichia coli and fecal coliform) were only detected in a small proportion of these contaminated samples. Other studies are necessary to validate results obtained. Indeed these results may be explained by the low sensitivity of the method used for counting bacteria and the effect of water salinity on the common indicators.28

Poisoning caused by mollusk consumption29

A surveillance study of mollusk-related illnesses was conducted in Eastern Quebec from spring 1999 to winter 2001. A total of 48 poisoning episodes affecting 66 people were inventoried. An exhaustive analysis of the information collected for each case (symptoms, type of food consumed, blood and stool analyses, investigation report, etc.) was conducted to determine whether there was a link between the illness and mollusk consumption. Among the recorded episodes, 3 were classified as “confirmed”, 30 “potential”, 11 “unlikely”, and 4 “rejected”. Two of the three “confirmed” episodes were of microbial origin and involved a bacterial pathogen (Bacillus cereus and Clostridium perfringens). The other episode concerned
marine toxins (PSP). Close to 70% of the episodes classified as “confirmed” or “potential” were associated with consuming seafood from a restaurant, grocery store or fish market. Non-commercial harvesting was responsible for six episodes of poisoning (18%). Mussels are without a doubt the most frequent cause of these episodes (55% of cases), followed by whelk (15%) and soft-shelled clams (15%). In a 1997 study in the North Shore region, close to 8% of respondents said that they had become ill from eating mollusks.

Biotoxin contamination

Mollusks are also capable of absorbing microscopic algae, some of which produce biotoxins, especially planktonic algae, Alexandrium tamarense.30 These algae, found mostly in the lower estuary in summer and along the Gulf of St. Lawrence shoreline, multiply and concentrate according to various environmental conditions (temperature, variations in salt content, abundance of nutrient matter, currents).31, 33

Once contaminated, mollusks can maintain their toxicity from several weeks to several months. These toxins accumulate in the mollusk’s internal organs and can cause various types of poisoning in those who eat them. The three most likely to occur in Quebec are paralytic shellfish poisoning (PSP), amnesiac shell fish poisoning (ASP) and diarrhetic shellfish poisoning (DSP).32

Mollusks from the St. Lawrence are filtering organisms which may contain pathogenic micro-organisms and biotoxins.

PSP is the most frequently experienced poisoning among those who eat mollusks from the St. Lawrence – the region of Canada that sees the largest number of such cases. Since 1984, at least 16 confirmed cases of PSP have been recorded. The last cases of ASP recorded in Quebec were in 1987 and were related to consumption of mollusks from Prince Edward Island. No cases of DSP have been reported in Quebec.32

In 1998 the unexpected discovery of new toxins in mollusks from the Magdalen Islands, particularly those that cause diarrhetic poisoning, alerted researchers from the Canadian Food Inspection Agency to measure the toxin in question in the digestive glands of mussels and other mollusks from various shellfish harvesting areas. Approximately 10% of samples were contaminated with diarrhetic toxins. Of these, 56% were taken from outside the Magdalen Islands, demonstrating that the contaminant first seen in the summer of 1998 existed in other harvest regions of Quebec. The occurrence of these toxins may have been caused by Prorocentrum lima algae, as was the case in mussels analyzed from elsewhere in Eastern Canada. Researchers also discovered Prorocentrum mexicanum from the same algae family. They had been previously unaware of its presence in the region, as it is normally found in more southern latitudes. They noticed that under certain environmental conditions these algae could be found in the digestive glands of cultivated mussels.33

Chemical contamination

A health risk evaluation regarding the occurrence of chemicals in mollusks was conducted in 1997 on the North Shore between Tadoussac and Baie-Trinité. The majority of known chemical contaminants were detected in the soft-shell clam samples. Exposure to chemical contaminants when consuming soft-shell clams was evaluated according to various consumption scenarios. The results indicated that chemical substances capable of causing threshold toxic effects, such as non carcinogenic systemic effects, were not present in sufficient concentrations to create a risk for an individual eating a normal portion of soft-shell clams, even among regular consumers.34

Several cases of poisoning are recorded each year among mollusk harvesters.

Some preventive measures25, 35, 36

Precautions against pathogenic micro-organisms:

Precautions against toxins:

Increasing awareness among mollusk harvesters

ZIP committees from the north shore of the estuary, Bay of Chaleurs and the Magdalen Islands developed unique communication methods adapted to their individual environments to ensure that mollusk harvesters were well
informed. Given the possibility of mollusk contamination via pathogenic micro-organisms and biotoxins, their message included the importance of following the preventive measures.