Marine shellfish poisoning

Marine shellfish poisoning refers to illnesses caused by eating marine bivalve shellfish (e.g., clams, mussels, oysters, scallops, cockles) that contain biotoxins, which shellfish can accumulate when they feed on toxin-producing phytoplankton. Lobsters, crabs, and whelks can also accumulate these toxins if they feed on contaminated shellfish. The toxins are not destroyed by cooking or freezing.

Phytoplankton species that produce toxins often occur in blooms, known as harmful algal blooms, or HABs. Some blooms may appear as a red tide but not all HABs are red. Marine HABs and associated shellfish poisoning outbreaks are more common in warmer months, but can occur at any time of year. HABs are expected to become more frequent and widespread as climate change causes increases in sea temperature and changes patterns of nutrient cycling. This could increase the public health risks from shellfish poisoning.

Paralytic shellfish poisoning events were only added to the Canadian Notifiable Diseases database in 2018. While reported levels remain low, they can have serious and potentially fatal effects. Coastal communities that rely on shellfish as a major part of their diet may be at heightened risk of exposure, but exposure can occur in any Canadian province due to the transport of seafood nationally.

The resources listed here are intended to:

• Highlight the main types of shellfish poisoning that may occur
• Assist in identifying safe shellfish harvesting sites
• Highlight emerging areas of study, such as the impact of climate change on marine HABs.

Types of shellfish poisoning

The three main types of shellfish poisoning typically monitored are amnesic shellfish poisoning (ASP) diarrhetic shellfish poisoning (DSP), and paralytic shellfish poisoning (PSP).

Public agencies

• Diseases and conditions (BC Centre for Disease Control (BCCDC), n.d.)
  This website provides further information for health professionals on symptoms, causes, complications, and treatments for PSP, ASP and DSP.

• Illness and symptoms: marine (saltwater) algal blooms (US CDC, 2021)
  This website summarizes the illnesses and symptoms experienced in humans and animals from marine biotoxin exposure, with a summary table of six illnesses caused by eating contaminated seafood.

• Maximum levels for chemical contaminants in foods (Health Canada, 2020)
  This list of maximum levels for chemical contaminants in foods provides the maximum levels for ASP, DSP, and PSP toxins in the edible tissues of bivalves. Shellfish commercially harvested or grown via aquaculture are regularly tested for these toxins

• Marine biotoxins in bivalve shellfish: paralytic shellfish poisoning, amnesic shellfish poisoning and diarrhetic shellfish poisoning (Canadian Food Inspection Agency, 2019)
  This website provides a description of the types of illnesses caused by shellfish toxins (ASP, DSP, and PSP).

Academic resources

• Marine biotoxin concerns (BCCDC, 2022)
  This seminar recording reviews the toxins associated with PSP, DSP and ASP, emerging marine toxins, vectors, and marine organisms linked to illness.

• Three decades of Canadian marine harmful algal events: phytoplankton and phycotoxins of concern to human and ecosystem health (McKenzie et al., 2021)
  This article provides and overview of harmful algal species in Canada and examines the spatial and temporal trends of marine HAB events over the past 30 years.

• Phycotoxins in marine shellfish: origin, occurrence and effects on humans (Farabegoli et al., 2018)
  This review provides an overview of several additional marine biotoxins including their occurrence, mechanism of action, and health effects (e.g., TTX, NSP, cyclic imines, OA, AZP, pectenotoxins, PTX, and yessotoxins).

• Human poisoning from marine toxins: unknowns for optimal consumer protection (Vilariño et al., 2018)
  This review provides data on the symptoms of acute biotoxin poisoning for several toxin classes, and describes what is known about chronic toxicity.

• Outbreak of diarrhetic shellfish poisoning associated with mussels, British Columbia, Canada (Taylor et al., 2013)
  This article reports the epidemiology of the first outbreak of DSP from BC, in 2011.

Identifying safe shellfish harvesting areas

Harvesting shellfish from closed or contaminated areas is illegal and unsafe. The following resources can assist harvesters in identifying safe harvesting areas.

• Harmful algae event database (HAEDAT, 2022)
  This database, a component of the Harmful Algal Information System (HAIS), provides a searchable list of marine HAB events and locations where monitoring has detected shellfish toxins (e.g., ASP, DSP, PSP toxins).

• Biotoxin and sanitary contamination closures map for shellfish harvesting in British Columbia (BCCDC, 2022)
  This searchable interactive map provides up-to-date information on BC shellfish harvesting closures by area.

• Shellfish harvesting openings and closures (Fisheries and Oceans Canada (DFO), 2021)
  This website includes an interactive map of shellfish harvesting areas in Canada approved or closed for bivalve molluscs. Additional information on closure orders is provided by region (e.g., Pacific, Quebec, Gulf Region, Maritimes, Newfoundland and Labrador).

• “Can U Dig It” mobile app (Q’ul-lhanumutsun Aquatic Resources Society (QARS), 2021)
  This mobile app developed by QARS allows shellfish harvesters to see where it is safe to harvest and for what species.

• Shellfish harvesting and safety (DFO, 2020)
  This website provides information on harvesting area openings and closures, safe harvesting practices, contamination and illness, reporting, and the Canadian Shellfish Sanitation Program (CSSP)

Emerging areas of study, including the impact of climate change on marine HABs and shellfish poisoning

• We All Take Care of the Harvest (WATCH) (First Nations Health Authority, 2022)
  This website provides information on the WATCH project, a partnership between four BC First Nations communities and FNHA to pilot phytoplankton monitoring as a way to assess seafood safety and influence of climate change.

• Changing trends in paralytic shellfish poisonings reflect increasing sea surface temperatures and practices of indigenous and recreational harvesters in British Columbia, Canada (McIntyre et al., 2021)
  This article reports on 62 PSP investigations in BC from 1940 – 2020, with the most common month for PSP being May. The majority of events were associated with self-harvested shellfish, highlighting the importance of monitoring and risk reduction in self-harvest areas.

• Modeling harmful algal blooms in a changing climate (Ralston and Moore, 2020)
  This review article outlines approaches to predicting how climate change will affect the occurrence and severity of HABs in the future.

• Marine harmful algal blooms and phycotoxins of concern to Canada (Bates et al., 2020)
  This technical report reviews marine HABs and phycotoxins occurring in Canada up to late 2018 and provides an overview of additional toxins and organisms of concern, Canadian monitoring programs, existing knowledge gaps, and areas for future research.

• Harmful algal blooms and climate change: exploring future distribution changes (Townhill et al., 2018)
  This modeling study forecasts how climate change may change the suitability of northern waters for growth of HAB species and frequency of blooms in the future.

• Marine biotoxin workshop (BCCDC, 2017)
  This website hosts presentations and a summary report of a workshop on implications of marine biotoxins to food resources, following the 2015 closer of commercial shellfish beds in BC due to elevated biotoxins.

• An unprecedented coastwide toxic algal bloom linked to anomalous ocean conditions (McCabe et al., 2016)
  This article reports on the 2015 outbreak of the diatom Pseudo-nitzschia, which can produce domoic acid that causes ASP, on the North American west coast and contributing environmental factors.

• Marine harmful algal blooms, human health and wellbeing: challenges and opportunities in the 21st century (Berdalet et al., 2015)
  This review article provides an overview of marine HABs and biotoxins, including routes of exposure, human health effects, and recommendations for mitigating HAB impacts.

Citizen science and public participation in HAB monitoring

Citizen science and networking groups are monitoring phytoplankton blooms, climate and ocean conditions to further understanding of climate, impacts to marine mammals and birds and ecosystem changes. A few initiatives are listed here:

• Coastal First Nations Great Bear Initiative
• Olympic Region Harmful Algal Blooms
• Wildlife Algal-toxin Research and Response Network for the U.S. West Coast (WARRN-West)

This list is not intended to be exhaustive. Omission of a resource does not preclude it from having value.

[Last updated: April 2022]