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In Search of New Marine Toxins in the Magdalen Islands The Canadian Food Inspection Agency (CFIA) manages the Marine Biotoxin Monitoring Program, which is aimed at detecting the presence of toxins in molluscan shellfish harvested commercially and recreationally in Quebec. In 1998, the unexpected discovery of two toxins in molluscs in the Magdalen Islands put CFIA researchers on the alert and led to a research project with Fisheries and Oceans Canada’s Maurice Lamontagne Institute (MLI). This article outlines the main findings of the study, which was conducted under the Human Health component of St Lawrence Vision 2000.
Bivalve molluscs, which include softshell clams, surf clams, scallops, mussels, wedge clams and razor clams, are filter feeders. This means that they suck in water and microscopic algae and extract the nutrients they need. As a result, molluscs accumulate toxins produced by algae that can cause illnesses such as paralytic shellfish poisoning, amnesic shellfish poisoning and diarrhetic shellfish poisoning in humans.
Rigorous Monitoring Occasionally Leads to Unexpected Discoveries Since large numbers of the algae that cause paralytic shellfish poisoning are found in the waters off the North Shore, in the Lower St Lawrence and around Anticosti Island and the Gaspé Peninsula, the CFIA monitors levels of paralytic toxins in shellfish on a weekly basis. When the contamination level in molluscan shellfish exceeds the Canadian standards deemed to be safe for consumers’ health, shellfish harvesting areas are closed.
The CFIA also monitors levels of domoic acid, a toxin linked to amnesic shellfish poisoning. Although very large numbers of the algae known to produce this toxin are sometimes found in shellfish harvesting areas in the St Lawrence River, domoic acid had never been detected in seafood products in Quebec prior to the summer of 1998, when a routine analysis identified low concentrations of the acid in scallop gonads.
Another surprising discovery awaited Marine Biotoxin Monitoring Program officials in the summer of 1998. In the Magdalen Islands, 20 people who had eaten mussels showed symptoms resembling those of diarrhetic shellfish poisoning. An investigation revealed that the mussels contained small concentrations of one of the toxins that cause this poisoning, which affects the gastrointestinal tract. The main symptoms are diarrhea, nausea, vomiting, abdominal pain and chills. That was the first time in Quebec that a diarrhetic toxin had been detected in shellfish.
The Algae Presumed Guilty Already Known to Researchers The origin of this toxin seemed to be obvious since MLI researchers had already detected the presence of the algae that produce diarrhetic toxins in the St Lawrence, in concentrations considered dangerous in the Atlantic provinces and in other countries. Through its toxic algae monitoring program, the MLI had noted the occasional presence of Prorocentrum lima in the Magdalen Islands. On the basis of these observations, the CFIA and the MLI undertook a study to verify whether Prorocentrum lima could explain the presence of diarrhetic (DSP) toxins in Magdalen Islands mussels. They measured DSP toxin levels in the digestive glands of mussels and other molluscs in a number of areas around the Islands and in other parts of the province. There was a 9.9% contamination rate in the samples. Of this number, 56% had been sampled outside the Magdalen Islands region, which showed that the contamination observed for the first time in the summer of 1998 could affect other harvesting areas in Quebec. Mussels accounted for 88% of the cases of contamination and were the species most affected. The contamination rate among cultured mussels was shown to be almost four times greater than that of wild mussels. Samples taken indicated that this difference might be explained by the use of structures in aquaculture that provide the Prorocentrum lima with the substrate it needs to reproduce.
A Newcomer to the Group of Usual Suspects Although concentrations of the toxin produced by Prorocentrum lima were detected in mussels eaten in the Magdalen Islands in 1998, it would appear that these levels do not explain the poisoning that occurred. The guilty party may in fact be Prorocentrum mexicanum, an alga of the same family whose presence was unknown in the region. "The presence of Prorocentrum mexicanum in the Magdalen Islands was completely unexpected because this alga has until now been considered a tropical species," explained Maurice Levasseur, a researcher at the MLI.
This discovery added a new twist to the diarrhetic poisoning problem in the Magdalen Islands. Prorocentrum mexicanum also produces DSP toxins that cause the same poisoning symptoms. These toxins were not measured as part of the study conducted by the CFIA and the MLI, but they may pose a potential hazard that should be evaluated at a later date.
Toxicity of Algae Strains in the St Lawrence Studied A number of questions arose during the project led by the CFIA and the MLI. "The next step could be to grow strains of Prorocentrum lima and Prorocentrum mexicanum found in the St Lawrence in the laboratory and then compare them with those that produce toxins in other regions," explained the CFIA’s Gilbert Sauvé. The same type of study could be conducted with diatoms, which cause amnesic shellfish poisoning. These algae are sometimes very abundant in shellfish harvesting areas in the St Lawrence, but no direct link has been established between their presence and the occasionally very high concentrations of domoic acid found in molluscs. By subjecting algae strains in the St Lawrence to various conditions related to temperature, light and the availability of nutrients and observing their ability to produce toxins, researchers will be able to predict the subsequent increase in their toxicity following any change in the environmental conditions of the St Lawrence. Information: Maurice Levasseur Gilbert Sauvé Sources: SAUVÉ, G, M LEVASSEUR, J-Y COUTURE and S MICHAUD. 2000. Évaluation des biotoxines marines nouvellement identifiées dans des mollusques du Québec et identification de leurs sources. Canadian Food Inspection Agency and Fisheries and Oceans Canada, 49 p. ST-AUBIN, G. 2000. Bien cueillir… bien manger! La cueillette des mollusques et votre santé, Comité ZIP de la rive nord de l'estuaire, 12 p. |
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