PSP Biotoxins
Eating contaminated Shellfish can be life
threatening:
take a look at the
areas
that are closed due to PSP (Red Tide)!
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).
Properties of the Agent Relative to the Food and 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. On the other hand, boiling or
steaming lobsters for 20 minutes per pound has been found to reduce PSP content of the
tomalley by approximately half.
Assay and Reported Units:
The mouse bioassay is the most commonly used method for
routine analysis of PSP in shellfish throughout the world and is the accepted method for
regulatory purposes in Canada. The assay is standardized using purified saxitoxin. Results
are given as micrograms of saxitoxin equivalents per 100 grams of shellfish meats
(µg/100g)
Action Levels:
Molluscan shellfish with a PSP content of less than 80
µg/100 g meat are permitted to be harvested, processed and sold.
For ‘roe-on’ scallops, PSP must be less than 80
µg/100 g roe.
Harvesting of molluscan shellfish for canning purposes is
permitted in areas where scores are between 80 and 160 µg/100 g and the canned packs are
tested before release for sale. On the Pacific Coast, butter clam canning has been
permitted up to 300 to 500 µg/100g, where it is done under permit and the neck has been
removed and discarded.
There is not a regulatory level for PSP in lobster
tomalley although a health advisory issued by Health and Welfare Canada suggested
consumption of lobster tomalley should be limited to the amount corresponding to two
lobsters per day.
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, induce vomiting, take a laxative and
drink water with baking powder or baking soda, then get medical attention as
soon as possible. There is no known
antidote for PSP but persons suffering from it should be given artificial respiration.
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. Lobsters have
also become part of the toxin monitoring program. Where PSP is a perennial problem, such
as in the Bay of Fundy, permanent PSP closures for certain species (e.g. mussels) are put
into effect.
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