Paralytic Shellfish Poisoning

Eating contaminated Shellfish can be life threatening: Always check for Marine Biotoxin and Sanitary Contamination Closures if you plan on harvesting and consuming any shellfish.

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  • Agent: Paralytic Shellfish Poison (PSP)
  • Classification: Marine Biotoxin
  • Fish Products Most Affected
    • Clams
    • Mussels
    • Whelks
    • Moon-Shells and Dogwinkles
    • Oysters
    • Whole Scallops
    • Crabs and Lobster hepatopancreas (tomalley)

The majority of the toxin within shellfish is normally found within the digestive gland.

Properties of Agents Relevant to Fish Products or Illness

Paralytic shellfish poisoning (PSP) results from eating shellfish which have fed upon toxic dinoflagellates, a type of one celled microscopic organism which forms an important part of the ocean plankton. During ‘blooms’, that is rises in the number and concentration of phytoplankton, filter-feeding shellfish such as clams and mussels accumulate the poisons from the dinoflagellates when feeding on them. The uptake and clearance rates of the toxins vary widely with species and are influenced by a number of factors. However, it should be noted that some species (e.g. butter clams) can retain the toxins for long periods after the bloom has finished.

PSP has occurred world-wide. The dominant species of dinoflagellate associated with PSP in Canada are in the Alexandrium family, previously called Gonyaulax.

PSP is caused by a group of related toxins. The best known of these is saxitoxin (SXT). In all, there are a total of 18 to 24 known toxins comprised of the parent compound, STX and its derivatives. The relative abundance of each poison varies with the species and strain of dinoflagellate. While the gonyautoxins (GTX’s) appear to predominate in the Bay of Fundy, saxitoxin tends to be higher in the Gaspe Region of Quebec. Saxitoxin is the dominant toxin in some British Columbia species.

The PSP toxins are relatively heat stable and normal cooking or canning processes cannot be relied upon to render a contaminated product as safe. Normal cooking does not reduce the PSP content of bivalve molluscs such as clams and mussels although retorting with do so to a certain extent.

Characteristics of the Illness

People eating bivalve molluscs (such as clams, oysters, scallops and mussels) should be aware of the symptoms of PSP. The first indication of poisoning is numbness or tingling of the lips and tongue, which spreads to the fingers and toes. These symptoms are followed by a loss of muscular coordination, terminating in paralysis as well as inability to breathe.

The first evidence of PSP intoxication usually begins with a tingling sensation or numbness around lips within 5-30 minutes of ingestion, gradually spreading to face and neck, a prickly sensation in fingertips and toes as well as headache and dizziness. With moderate to severe intoxication, symptoms include incoherent speech, progression of prickly sensation to arms and legs within 4-6 hours, stiffness and non-co-ordination of limbs, general weakness with slight respiratory difficulty and a rapid pulse. Other symptoms sometimes observed include a sensation of lightness (‘floating in the air’), salivation, intense thirst and temporary blindness. Gastrointestinal symptoms of nausea, vomiting, diarrhea and abdominal pain are less common. Most victims are calm and conscious of their condition throughout their illness. Following high intake, paralysis of respiratory muscles may progress to respiratory arrest and death within 2-12 hours after consumption.

At the first sign of such symptoms, contact your Poison Control Centre at 1-800-567-8911 for first aid advice and seek medical attention immediately.

Among the bivalve species, butter clams and scallops retain PSP for long durations, sometimes more than a year. The poison is likely to be concentrated in the siphon (neck) and gills of the butter clam. As a precaution, when butter clams are steamed open, one should discard the siphon, the gills and the liquid released during the steaming process.

Control Measures

The single most effective control measure is responsible harvesting, i.e. that molluscan bivalves come only from open harvest areas.

Under the Canadian Shellfish Sanitation Program, molluscan bivalves in all harvest areas are tested regularly for the presence of PSP.

For more information, please visit the Canadian Shellfish Sanitation Program page (Canadian Food Inspection Agency site)