Marine Shellfish Poisoning

Marine shellfish poisoning refers to illnesses in humans caused by consumption of marine bivalve shellfish (e.g., clams, mussels, oysters, scallops, cockles) that contain biotoxins (e.g., domoic acid, okadaic acid, saxitoxin). Shellfish feed by filtering microscopic marine plants, called phytoplankton, from the water. Some species of phytoplankton naturally produce toxins. These toxins can cause illness when they bioaccumulate to concentrations harmful to humans. Phytoplankton species often occur in blooms, known as harmful algal blooms (HABs) when they contain toxin-producing species.

The species causing HABs in the marine environment are different to those causing HABs in freshwater bodies. In the marine environment, HABs, and consequently shellfish poisoning outbreaks, are more common in warmer months. With climate change causing longer, warmer summers, and altering sea temperature, HABs are expected to become more frequent, of longer duration, and with a greater distribution (James et al., 2010; Wells et al., 2015), increasing the public health risk from shellfish poisoning. Although rates of shellfish poisonings remain low in Canada (PHAC, 2016), they can have serious and potentially fatal effects.

• Three types of shellfish poisoning have been documented in Canada – amnesic shellfish poisoning (ASP, caused by domoic acid), diarrhetic shellfish poisoning (DSP, caused by okadaic acid, pectenotoxins, dinophysis toxins), and paralytic shellfish poisoning (PSP, caused by saxitoxin) (CFIA, 2019).
• Shellfish poisoning can occur in any Canadian province due to the ability to transport produce.
• The toxins causing ASP, DSP, and PSP are not destroyed by cooking or freezing.
• Although shellfish poisoning outbreaks are more common in warmer months, toxin levels can be high enough to cause illness at any time of year.
• To minimize the risk of shellfish poisoning, always check for marine harvesting closures before collecting shellfish (BCCDC, 2016; DFO, 2019). (BCCDC link is an interactive map to BC coastal harvesting closures; DFO has information for BC, Quebec, Gulf, Maritimes, Newfoundland and Labrador harvesting closures).
• Shellfish commercially harvested or grown via aquaculture are regularly tested for these toxins (DFO, 2018).
• In October 2016, a workshop on marine biotoxins was hosted by the BCCDC in Vancouver (BCCDC, 2016), examining the use of environmental data for public health purposes. Slides from presentations are available, and a workshop report will be published early March 2017.
• If you become ill after eating shellfish, contact your local health care provider and poison control centre.

NCCEH Resources

• Paralytic Shellfish Poisoning
  A case study providing an example of investigation questions and answers on an outbreak of PSP in BC in 2010.

Selected External Resources

• Amnesic Shellfish Poisoning (BCCDC, 2016)
  Diarrhetic Shellfish Poisoning (BCCDC, 2016)
  Paralytic Shellfish Poisoning (BCCDC, 2016)
  These webpages provide information on symptoms, causes, complications, and treatments for ASP, DSP, and PSP, respectively.
   
• Maximum Levels for Chemical Contaminants in Foods (Health Canada, 2016)
  Health Canada’s list of maximum levels for chemical contaminants in foods, including the maximum levels for ASP, DSP, and PSP in the edible tissues of bivalves.
   
• Harvest shellfish safely (DFO, 2013)
  A guide on how to harvest shellfish safely to decrease the risk of becoming ill.
   
• Amnesic Shellfish Poisoning (FAO, 2004)
  Diarrhoeic Shellfish Poisoning (FAO, 2004)
  Paralytic Shellfish Poisoning (FAO, 2004)
  These webpages provide detailed information on symptoms, chemical structure of the toxins, methods of analysis, source organisms, occurrence and accumulation in seafood, toxicity, prevention, global cases and outbreaks, and global regulations and monitoring on ASP, DSP and PSP.

• An outbreak of toxic encephalopathy caused by eating mussels contaminated with domoic acid (Perl et al., 1990)
  This article reports the epidemiology of the first outbreak of ASP that occurred in Canada in 1987.
   
• Paralytic shellfish poisoning: Seafood safety and human health perspectives (Etheridge, 2010)
  This article gives an overview of PSP and saxitoxin, and highlights PSP seafood safety concerns, and monitoring and management strategies from a global perspective.
   
• 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, Canada in 2011.

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