Fish and Shellfish Poisoning

Paralytic Shellfish Poisoning

Paralytic shellfish poisoning (PSP) in humans is caused by ingestion of shellfish containing PSP toxins. These PSP toxins are accumulated by shellfish grazing on algae producing these toxins. Symptoms of human PSP intoxication vary from a slight tingling or numbness to complete respiratory paralysis. In fatal cases, respiratory paralysis occurs within 2 to 12 hours of consumption of the PSP contaminated food.

Pathophysiology

The toxins responsible for most shellfish poisonings are water-soluble, heat and acid-stable, and ordinary cooking methods do not eliminate the toxins. The principal toxin responsible for PSP is saxitoxin. Some shellfish can store this toxin for several weeks after a harmful algal bloom passes, but others, such as butter clams, are known to store the toxin for up to two years. Additional toxins are found, such as neosaxiton and gonyautoxins I to IV. All of them act primarily on the nervous system.

PSP can be fatal in extreme cases, particularly in immunocompromised individuals. Children are more susceptible. PSP affects those who come into contact with the affected shellfish by ingestion. Symptoms can appear ten to 30 minutes after ingestion, and include nausea, vomiting, diarrhea, abdominal pain, 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 loss of coordination are also possible.

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What are Paralytic Shellfish Poisoning (PSP) toxins?

  • PSP toxins are a group of natural toxins which are usually found in bivalve shellfish. They are water-soluble, heat and acid stable. The various toxins differ in toxicity with saxitoxin being the most toxic.
  • Poisoning can result from ingestion of shellfish contaminated with PSP toxins.
  • PSP toxins are produced by free-living micro-algae, upon which the shellfish feed. Bivalve shellfish such as clams, mussels, oysters, fan shells and scallops are common vehicles responsible for shellfish poisoning. This is because they are filter-feeders and naturally ingest the toxic algae in water.
  • When the algae are toxin-producing, the toxin will be concentrated in the shellfish tissue, particularly in the viscera. The amount of toxin in the shellfish depends on the number of toxic algae ingested.
  • Harmful algal bloom, commonly known as red tide, occurs when the micro-algae grow very fast and densely accumulate in water to the extent that visible colour patches appear. During a red tide, the concentration of toxin in shellfish living in the affected seawater rapidly increases.

Symptoms of Paralytic Shellfish Poisoning

  • The symptoms of Paralytic Shellfish Poisoning are predominantly neurological and the onset is usually within minutes to hours after ingestion of the shellfish. Initial symptoms include tingling, numbness of the mouth and extremities and gastrointestinal discomfort such as vomiting and diarrhoea. Symptoms usually resolve completely within hours to days.
  • In severe cases, difficulty in swallowing and speech, paralysis with respiratory arrest and even death may occur.
  • There is no antidote for PSP toxins. The management for poisoned individuals is therefore supportive.

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More Information

For more information see the Q & A section for Paralytic Shellfish Poisoning

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