Paralytic Shellfish Poisoning Causes
What causes paralytic shellfish poisoning?
Paralytic shellfish poisoning (PSP) is caused by saxitoxin (STX)-group toxins. Microscopic marine algae called dinoflagellates produce them, mainly species of Alexandrium. Filter-feeding bivalves such as oysters, mussels, scallops and clams take the toxins in as they feed, and people are poisoned when they eat the contaminated shellfish [1].
The toxins
- STX-group toxins are closely related, water-soluble tetrahydropurines. More than 30 analogues have been identified. EFSA's CONTAM Panel names saxitoxin, neosaxitoxin, gonyautoxin-1 (GTX1) and decarbamoyl saxitoxin as apparently the most toxic [2][3].
- They harm the nervous system. They bind to voltage-gated sodium channels in nerve cells and block the flow of ions through them, which the Panel considers the main way they cause damage [2].
The organisms that make them
- The main producers are Alexandrium species: A. tamarensis, A. minutum (syn. A. excavata), A. catenella, A. fraterculus, A. fundyense and A. cohorticula. Pyrodinium bahamense and Gymnodinium catenatum also produce them [3].
- Some cyanobacteria in fresh and brackish waters also produce STX-group toxins [3].
- The mix of toxins differs by algal species and by region [3].
How shellfish become toxic
- Shellfish feeding on these algae build up the toxins but are themselves fairly resistant to them [3]. A later EFSA opinion says the toxins concentrate in the shellfish's digestive gland (hepatopancreas) [4].
- The algae survive as resting cysts on the seabed. When conditions suit, the cysts germinate and the cells can bloom. Triggers include changes in salinity, water temperature, nutrients and sunlight, plus water conditions such as a thermocline (a warm upper layer that does not mix with the water below). Blooms cannot be predicted [3].
- The balance of nutrients affects how toxic a bloom is. Shortage of phosphorus slowed algal growth but increased toxin production; shortage of nitrogen reduced both [3].
- The producing algae occur worldwide. In Europe they are found along the Atlantic coast and North Sea from Norway to Portugal, and in the Mediterranean [3].
Effects in people Symptoms get worse with dose. They start as tingling or numbness around the lips, then slurred speech, poor coordination and weakness, then muscle paralysis and difficulty breathing. In fatal cases breathing stops 2 to 12 hours after the shellfish is eaten [2][1].
Monitoring example Northern Ireland's official monitoring programme (AFBI annual reports) tests oysters and mussels for PSP by liquid chromatography. It found no samples above the regulatory limit in 2019 (444 tests), 2020 (256) or 2022 (200) [5][6][7].
Approach: I used EFSA's 2009 CONTAM Panel opinion on saxitoxin-group toxins in shellfish and its 2012 opinion on clean seawater, plus the AFBI Northern Ireland marine biotoxin monitoring reports.
What shellfish toxin limits apply in the UK?
The UK uses five legal limits for marine biotoxins, all set in Regulation (EC) No 853/2004, Annex III, Section VII, Chapter V. In Great Britain this is now the assimilated version of that Regulation. Food Standards Scotland's April 2025 guidance gives the same five figures as the EU consolidated text dated 7 May 2026.
| Toxin group | Limit |
|---|---|
| Paralytic shellfish poison (PSP) | 800 µg saxitoxin equivalents diHCl per kg |
| Amnesic shellfish poison (ASP) | 20 mg domoic acid per kg |
| Okadaic acid and dinophysistoxins together | 160 µg okadaic acid equivalents per kg |
| Yessotoxins | 3.75 mg yessotoxin equivalent per kg |
| Azaspiracids | 160 µg azaspiracid equivalents per kg |
How the limits apply
- Measurement basis: live bivalve molluscs "must not contain marine biotoxins in total quantities (measured in the whole body or any part edible separately)" above these limits [8]. In this Regulation, "marine biotoxins" means poisonous substances that bivalves build up, in particular by feeding on plankton containing toxins [9].
- Other species: scallops (pectinidae), marine gastropods and echinoderms that are not filter feeders and are harvested outside classified production areas "must not be placed on the market" unless they meet the Chapter V standards. Their food business operators must show this through their own checks [8].
Great Britain
- Food Standards Scotland's April 2025 scallop guidance (pp. 5 and 7) says that since 1 January 2024, under the Retained EU Law (Revocation and Reform) Act 2023, references to EU Regulations mean assimilated EU law. In Scotland these rules are enforced through the Food Hygiene (Scotland) Regulations 2006.
- The guidance lists the five "maximum permitted levels" in the same figures as the table. Its okadaic acid line is worded more loosely ("OA / DTX / palytoxin (PTX)"). The Regulation itself says "okadaic acid and dinophysistoxins together".
- The guidance adds that the limits apply to every batch sold. Food business operators selling whole scallops "must" have end-product testing in place.
- I could not open the GB text of Annex III on legislation.gov.uk or the FSA's biotoxin monitoring page, so the figures above have not been checked against the GB statute directly.
Northern Ireland
- The 2024 AFBI annual report for the Northern Ireland official control programme, approved by the FSA in Northern Ireland, tests for domoic acid, PSP and lipophilic toxins. It reported 0 samples above the regulatory limits out of 602 tests [10].
- The report does not name the legal instrument behind those limits. I found no official page this time stating which version of Regulation 853/2004 governs Northern Ireland.
- The England and Wales official control results use the same analytes and units: µg STX eq/kg, µg OA eq/kg, µg AZA1 eq/kg, mg YTX eq/kg and mg/kg ASP [11].
Approach: I used the EU consolidated Regulation 853/2004, the AFBI Northern Ireland and FSA England and Wales biotoxin monitoring records, and Food Standards Scotland's official guidance.