ANALYSIS OF CHEMICAL AND MICROBIOLOGICAL RISKS RELATED TO THE CONSUMPTION OF HAND-PICKED SHELLFISH IN THE REGION OF BAIE-COMEAU
Photo: Thierry Tremblay Source: TREMBLAY, Thierry, Jacques-François CARTIER and Fabien GAGNON. 1999. Analyse du risque chimique et microbiologique lié à la consommation de mollusques cueillis de façon artisanale dans la ZIP de Baie-Comeau (Analysis of the chemical and microbiological risks associated with consumption of recreationally harvested shellfish from the Baie Comeau Area of Prime Concern), Public Health Branch, North Shore Regional Health and Social Services Board, 150 pages + appendices For information: Jacques-François Cartier Health and Environmental Advisor North Shore Regional Health and Social Services Board Telephone: (418) 589-9845 Fax: (418) 589-8574 E-mail: Jacques-Francois_Cartier@ssss.gouv.qc.ca
ABSTRACT
In December 1996, the Public Health Department of the Côte-Nord Region initiated a study which aimed to describe shellfish harvesting, a traditional activity in this region, and to assess the health risks associated with the consumption of molluscan shellfish living on the North Shore of the St.Lawrence Lower Estuary. There were three main parts to this study, financed by the Health Component of St.Lawrence Vision 2000. These three parts were the following : • a field survey of recreative molluscan shellfish harvesters; • a microbiological risk assessment associated with molluscan shellfish consumption (bivalves); • a chemical risk assessment associated with molluscan shellfish consumption (bivalves). The field survey focused on four themes: 1) the harvesting habits of recreational shellfish harvesters; 2) the shellfish consumption habits of these harvesters; 3) the harvesters’ knowledge and perception of the benefits and risks associated with shellfish consumption; and 4) the harvesters’ knowledge and perception of the closure of shellfish growing areas. In the three-month period ranging from March 28th to June 28th, 18 growing areas on the North Shore of the Lower Estuary were visited twice on an average by a team of two interviewers (some areas were only visited once while others were visited up to five times). Over 200 harvesters were met and 162 were formally interviewed during this period. The interviews typically lasted 15 minutes and were conducted in French. Even though the questions asked were standard and had been established beforehand, the interviews were generally conducted like a friendly discussion rather than a formal questioning. The questions were asked orally by one of the interviewers and the harvesters’ answers were noted in a field notebook. In the study area, those engaged in shellfish harvesting were mainly 20 to 50 years old men. The results of the survey show that there are two main types of recreational harvesters in the area: 1) the occasional recreational harvester; and 2) the regular recreational harvester. The main characteristics of these two types of recreational harvesters are the following : The occasional recreational shellfish harvester • collects shellfish less than three times a year, in the spring only; • goes harvesting mainly during the weekend; • eats shellfish less than five times a year; • eats shellfish only at certain times of the year, mainly in the spring. The regular recreational shellfish harvester • collects shellfish many times per mo nth; • goes harvesting any day of the week; • collects shellfish mainly during the spring but at other times as well; • eats shellfish at least eleven times a year; • eats shellfish any time of the year.
On average, the recreational shellfish harvester eats fifteen meals of 410 grams of molluscan shellfish meat in a year (an average of 47 individuals soft-shell clams are required to obtain a meal size of 410 grams). About a third of the harvesters eat shellfish more than twenty times a year. The meal size ranges from less than 80 to 2700 grams of meat. Shellfish are mainly consumed after being steamed for a few minutes, until the valves open up. Most harvesters believe that molluscan shellfish consumption has little or no health benefit; they eat shellfish only because they like the taste. Even though they are aware of the closure of shellfish growing areas in the region, more than half of the harvesters believe that there is no particular health risk associated with molluscan shellfish consumption. Many species of molluscan shellfish live in the intertidal flats of the Lower Estuary. The soft-shell clam (Mya arenaria) is by far the species most exploited and most eaten by the recreational harvesters in the area of study. Other species harvested and consumed in the area are the blue mussel (Mytilus edulis) and the gastropod Buccinum glaciale (northern buccinum). Both the microbiological and the chemical risks associated with the consumption of the soft-shell clam contaminants were assessed in this study. The main objectives of these assessments were : • to determine whether molluscan shellfish were a source of exposure to chemical contaminants and pathogenic microorganisms introduced in the Lower Estuary; • to determine if molluscan shellfish consumption was associated with a significant health risk. Prior to the microbiological risk assessment, soft-shell clams were collected in seven growing areas during the three-month period ranging from April 21st to July 29th 1997. Pathogenic microorganisms were detected in more than 23 % of the shellfish homogenates, which tends to indicate that the consumption of this particular species of shellfish is a source of exposure to pathogenic microorganisms introduced in the Lower Estuary. The microbiologicalrisk was assessed by comparing the number of microorganisms associated with a typical shellfish meal to the infective doses reported in the literature. Based on meal sizes of 410 and 2002 grams, the number of microorganisms associated with soft-shell clam consumption were generally much lower than the reported infective doses. However, the number of Salmonella spp. and Giardia lamblia would have been high enough to cause an infection, if the shellfish had been eaten raw. Soft-shell clams were also collected in eight growing areas and analyzed for the following chemicals: aluminum, arsenic, cadmium, chlordane, chromium, copper, hexachlorobenzene, lead, lindane, manganese, mercury, mirex, PAHs, PCBs, pp’-DDE, pp’-DDT, nickel, selenium, tin and zinc. Blue mussels and Mesodesma arcatum specimens were also collected in two growing areas. Most of the chemicals mentioned above were found in the shellfish homogenates. Shellfish consumers’ exposure to these chemicals was estimated by using four consumption scenarios ranging from 17 to 95 grams per day. Exposure to chemical contaminants was evaluated by multiplying these consumption levels with the 95th percentile of the contamination distribution. Exposure levels associated with all the scenarios were lower than daily exposure limits established by various agencies (ATSDR, EPA, Health Canada, WHO), which tends to indicate that this consumption is not associated with a significant non-cancer health risk. The cancer risk related to soft-shell clam consumption was determined by using an oral slope factor established by USEPA (IRIS Database). Even if the excess risk associated with most organochlorinated
compounds were lower than 10-6, exposure levels to inorganic arsenic and PCBs were associated with an excess risk of cancer greater than the 10-5-10-6, levels generally deemed acceptable for an environmental exposure; these levels were exceeded for all consumption scenarios. These results should be considered since an association between the consumption of food contaminated by inorganic arsenic and various types of cancer has yet to be observed. Aside from being a source of exposure to pathogens and chemical contaminants, soft shell clams and blue mussels are considered excellent sources of iron and vitamin B12 and good sources of niacine, phosphorous and zinc.