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Paralytic Shellfish Poisoning

Paralytic shellfish poisoning occurs from ingesting bivalve shellfish (such as mussels, oysters, and clams) that contain toxins. These toxins can cause severe and life-threatening neurological effects.   

Shellfish harvested in BC coastal waters can sometimes be contaminated with this toxin. Self-harvesters of shellfish should check to see if the area they are harvesting from is open.

More on shellfish harvesting  Go to the shellfish harvesting map

Information for Health Professionals


Confirmed Case

  • Clinical illness [1] within 24 hours of eating at risk shellfish [2] or contaminated seafoods [3];
  • Detection of saxitoxin or related toxins in samples of shellfish or contaminated seafoods that were consumed by an individual meeting the clinical case definition, in edible tissues in excess of 0.8 mg/kg (0.8 ppm);
  • Detection of saxitoxin in urine or feces collected within 24 hours of exposure and illness;
  • Detection of high levels of dinoflagellates (Alexandrium  spp.) associated with shellfish poisoning in water from which epidemiologically related shellfish [2] or seafood [3] were gathered.

Probable Case

  • Clinical illness [1] within 24 hours of consumption of at risk shellfish [2] or contaminated seafoods [3] and in the absence of other known causes.


  1. Clinical illness defined as: neurological symptoms such as paresthesia and/or paralysis involving the mouth and extremities, which may be accompanied by gastrointestinal symptoms. 
  2. At risk shellfish include filter feeding molluscan bivalve shellfish: clams, mussels, scallops (digestive tissues), oysters, cockles, and whelks.
  3. Seafood at risk of being contaminated are any species that feeds on plankton, including crabs, prawns, squid and planktiverous fish eaten whole (e.g., sardines, anchovies). Rarely, edible algae may be contaminated via contact with toxic phytoplankton. 

  • Tingling (pins and needles feeling or paresthesia), and
  • Numbness, spreading from lips and mouth to face, neck and extremities
  • Dizziness
  • Arm and leg weakness, paralysis
  • Respiratory failure and in severe cases, death
  • Headache
  • Nausea
  • Vomiting

Symptoms start quickly, median time between ingestion and onset is 1 hour (between 30 minutes to 3 hours). Progression and intensity of symptoms vary with the intensity of the toxin poisoning. In severe cases, muscle paralysis, respiratory failure and death can occur within 12 hours.  


Paralytic shellfish poisoning is caused from the ingestion of toxin-contaminated bivalve shellfish and crustaceans.  Algal blooms of dinoflagellates, usually during the warmer months of June to October, result in toxin accumulation in filter feeders such as bivalves. The most common fish species affected are clams; mussels; whelks, moon-shells and dogwinkles; oysters; whole scallops; crabs and lobster hepatopancreas (tomalley). Toxin bioaccumulates in specific tissues, and can persist for long periods in certain species of shellfish.  Butter clams for instance, may retain toxin for up to one year after a toxin producing algal bloom (the latin name for butter clams is Saxidomus, the origin of the name saxitoxin). Toxin accumulates in the siphon, neck and gills in butterclams – it is recommended these be removed and discarded before eating. DO NOT feed these parts to your pets.  Sea otter deaths have been linked to butter clams in Alaska.   


Symptoms usually resolve completely within a few hours to days after shellfish ingestion.  

The detection of toxin in epidemiologically linked food confirms the diagnosis.   

There is no antidote. Respiratory support is recommended.  


In Canada, a monitoring and prevention program for saxitoxin (and other shellfish toxins such as those causing amnesic and diarrhetic shellfish poisoning) exists. Levels of saxitoxin should not exceed 80 micrograms of saxitoxin per 100 grams of shellfish. When this level is exceeded, beaches are closed to harvesting, and shellfish are not permitted for retail sale. All shellfish in BC must be inspected by federally registered shellfish processing plants before going to the commercial market – this is part of the Canadian Shellfish Sanitation Program (CSSP), the federal monitoring and prevention program in Canada. The CSSP classifies harvesting areas and controls the commercial and recreational harvesting and processing of shellfish for the consumer market. The CSSP is run by 3 federal government agencies (1) Environment Canada - responsible to monitor water quality in shellfish areas, (2) Canadian Food Inspection Agency - responsible for monitoring marine toxins in shellfish areas and for registering and inspecting shellfish processing plants, and, (3) Fisheries and Oceans Canada - responsible for opening and closing harvest areas, and prohibiting shellfish harvesting when bacteriological or toxin levels are unsafe.  More on Shellfish Contamination.

What can you do to prevent risk of PSP?

  • Harvest shellfish from open beaches - check before you harvest.
  • Purchase shellfish from reputable suppliers - all shellfish should have a tag verifying federal inspection.


SOURCE: Paralytic Shellfish Poisoning ( )
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