4 distinct shellfish-poisoning syndromes have been identified: Paralytic shellfish poisoning (PSP) Neurologic shellfish poisoning (NSP) Diarrheal shellfish poisoning (DSP) Amnestic shellfish poisoning (ASP) All 4 syndromes share some common features and primarily are associated with bivalve mollusks (eg, mussels, clams, oysters, scallops). These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae in the form of dinoflagellates and diatoms.

Pathophysiology: The toxins responsible for most shellfish poisonings are water-soluble, heat and acid-stable, and are not inactivated by ordinary cooking methods. The main toxin responsible for PSP is saxitoxin.

The saxitoxins act by blocking sodium ion movement through voltage-dependent sodium channels in nerve and muscle cell membranes. Conduction block occurs principally in motor neurons and muscle. The toxin is made by dinoflagellates of the Gonyaulax species which create the conditions known as "Red Tide". Almost all bivalve molluscs such as clams, mussels, oysters, snails, scallops, and barnacles ingest these organisms while feeding, and the poison is stored in their bodies. Most shellfish only store this toxin for six weeks after a "Red Tide" passes, but some such as butterclams are known to store the toxin for up to two years.

PSP can be fatal in extreme cases (particularly those already immuno-suppressed).

PSP affects those who come into contact with the affected shellfish (by ingesting). With 10-30 minutes after ingestion symptoms can include nausea, vomiting, diarrhea, abdominal pain, and tingling or burning lips, gums, tongue, face, neck, arms, legs, and toes. Shortness of breath, dry mouth, a choking feeling, confused or slurred speech, and lack of coordination are also possible outcomes.

The easiest way to steer clear of PSP is to check the fisheries agency in the area you intend to collect shellfish to ensure there is no Red Tide warning in effect. If you or someone does begin to exhibit the signs of PSP, induce vomiting and if necessary (should the condition not improve, or should the person be unconscious) get immediate medical attention.

There are urban legands in upper Australia and Indonesia reporting victims of paralytic shellfish poisoning being mistaken for dead, and being hastily interred or luckily recovering sufficiently to give a signal during the funeral.

Related Websites

• Toxicity, Shellfish
• SeaGrant Alaska's Information on PSP