Paralytic Shellfish Poisoning (PSP) Toxins
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.
Public Health Significance
- Acute toxicity is the main concern for PSP poisoning.
- The level at which PSP intoxications occur in humans varies considerably. Symptoms have been reported in some cases following ingestion of a dose of 120 μg PSP toxins per person.
- Children, elderly and individuals with poor health status are particularly susceptible.
- Visceral organs of the contaminated shellfish contain the highest level of PSP toxins.
- The cooking fluid of toxic shellfish may contain high levels of the toxins.
- In Hong Kong, as stipulated in the Public Health and Municipal Services Ordinance Cap 132, all food available for sale on the market must be fit for human consumption.
Advice to the Public
To avoid Paralytic Shellfish Poisoning, members of the public are advised to observe the following:
- Buy shellfish from reliable and licensed seafood shops;
- Remove the viscera, gonads and roe before cooking;
- Eat a smaller amount of shellfish in any one meal and avoid consuming the cooking liquid;
- Children, patients and the elderly may be susceptible to poisoning and should be cautious in consuming shellfish; and
- When symptoms occur after consuming shellfish, seek medical advice immediately.
Advice to the Trade
- Source food supplies from reliable suppliers. Do not accept food from dubious sources.
Risk Assessment Section
Centre for Food Safety