Food Safety Focus (202nd Issue, May 2023)– Article 1
Shellfish Poisoning
Reported by Dr. John LUM, Scientific Officer,
Risk Assessment Section, Centre for Food Safety
The Centre for Food Safety (CFS) received a referral from the Centre for Health Protection in April 2023 of a suspected case of diarrhoetic shellfish poisoning (DSP) presented with diarrhoea, nausea, vomiting and abdominal pain within an hour after consuming venus clams from a restaurant. This article will give a brief introduction on shellfish poisoning.
What is Shellfish Poisoning?
Shellfish poisoning is caused by a group of toxins, known as shellfish toxins, produced by certain species of algae. When shellfish eat toxins producing algae, the toxins can accumulate in their tissue. Consumption of shellfish with shellfish toxins by humans can cause a variety of gastrointestinal and neurological illnesses, known as shellfish poisoning. Examples of shellfish that have been involved in shellfish poisoning include mussels, clams, oysters, scallops and geoducks.
Many shellfish toxins have been described around the world; they could cause different types of shellfish poisoning. Five important shellfish poisonings will be discussed below:
(i) Paralytic shellfish poisoning (PSP) -- caused by paralytic shellfish toxins (PSTs), which are a group of water-soluble alkaloid neurotoxins including saxitoxin (STX).
(ii) Diarrhoetic shellfish poisoning (DSP) -- caused by diarrhoetic shellfish toxins (DSTs), which are a group of lipid-soluble polyether toxins including okadaic acid (OA).
(iii) Neurotoxic shellfish poisoning (NSP) -- caused by neurotoxic shellfish toxins (NSTs), a group of lipid-soluble polyether toxins including brevetoxins (BTX).
(iv) Amnesic shellfish poisoning (ASP) -- caused by amnesic shellfish toxins (ASTs) including the water-soluble amino acid domoic acid (DA).
(v) Azaspiracid shellfish poisoning (AZP) -- caused by azapiracid shellfish toxins (AZTs), including the lipid-soluble toxin azaspiracid (AZA).
Characteristics of Different Shellfish Toxins
Different groups of shellfish toxins display a wide range of chemical diversity, which can be broadly classified into amino acids (DA), alkaloids (STX) and polyketides (OA, BTX and AZA).
It is not clear why some algae produce these toxins. These toxins are secondary metabolites with no explicit role in the algae. They are probably used by the algae as a way to compete for space, fight predation or as a defence against the overgrowth of other organisms.
In general, shellfish toxins are heat stable, odourless, tasteless and not destroyed by cooking, freezing or other food preparation procedures. Toxic shellfish cannot be distinguished from non-toxic ones visually.
Accumulation of Shellfish Toxins in Bivalve Molluscs
Shellfish toxins are produced by algae that are consumed by bivalve molluscs as part of their natural diet. After ingestion, shellfish toxins are accumulated actively by shellfish and concentrated in the internal organs such as the hepatopancreas of bivalves. (Figure 1) The adductor muscle contains only a low level of shellfish toxins in general.
Figure 1: Internal parts of a scallop
Clinical Presentation of Shellfish Poisoning
Shellfish toxins can cause a wide variety of symptoms in humans, depending on the type of toxin present and the amount of toxin ingested. Symptoms of different types of shellfish poisoning are summarised below:
| Shellfish poisoning | Onset time | Symptoms | Mortality |
|---|---|---|---|
| Paralytic Shellfish Poisoning (PSP) | Generally within 30 minutes | Predominantly neurological symptoms, such as tingling of the lips, mouth and tongue; numbness of extremities and respiratory paralysis | Can be lethal |
| Diarrhoetic Shellfish Poisoning (DSP) | From 30 minutes to 3 hours | Mild gastrointestinal disorder, such as diarrhoea, vomiting and abdominal pain | Generally not life-threatening |
| Neurotoxic Shellfish Poisoning (NSP) | From a few minutes to a few hours |
Gastrological symptoms such as diarrhoea and vomitting; neurological symptoms such as tingling of the lips, mouth and tongue |
Generally not life-threatening |
| Amnesic Shellfish Poisoning (ASP) |
Within 24 hours for gastrointestinal symptoms; |
Gastrological symptoms such as diarrhoea and vomiting; neurological symptoms such as confusion and short-term memory loss |
Fatalities occurred in elderly patients |
| Azaspiracid Shellfish Poisoning (AZA) | Within hours | Predominantly gastrointestinal symptoms, such as diarrhoea and vomiting | Generally not life-threatening |
Safety Levels of Different Shellfish Toxins
The toxicity of various shellfish toxins was evaluated by a joint expert working group of the Food and Agriculture Organization of the United Nations (FAO), the World Health Organization (WHO) and the Intergovernmental Oceanographic Commission of UNESCO (IOC) in 2004. An acute reference dose (ARfD), which is the amount of toxins that can be ingested in a period of 24 hours without appreciable health risk, has been established for these shellfish toxins. The Codex Alimentarius Commission (Codex) has also established maximum levels (MLs) for shellfish toxins in edible parts (the whole part or any part intended to be eaten separately) of live bivalve molluscs. While the ARfD is critical in assessing the safety of the amount of shellfish toxins in food, the Codex MLs are the levels recommended by Codex to be permitted in shellfish.
| Shellfish toxins | Provisional acute reference dose | Codex maximum level (per kg of mollusc flesh) |
|---|---|---|
| Saxitoxin (STX) group | 0.7 µg/kg bw | ≤0.8 mg of saxitoxin equivalent |
| Okadaic acid (OA) group | 0.33 µg/kg bw | ≤0.16 mg of okadaic equivalent |
| Domoic acid (DA) group | 100 µg/kg bw | ≤20 mg domoic acid |
| Brevetoxin (BTX) group | Insufficient data to establish ARfD | ≤200 mouse units* or equivalent |
| Azaspiracid (AZA) group | 0.04 µg/kg bw | ≤0.16 mg |
*An estimate of the toxicity of a toxin determined by a mouse bioassay.
Key Points to Note
- Shellfish toxins are heat stable, odourless, tasteless and not destroyed by cooking, freezing or other food preparation procedures.
- Toxic shellfish cannot be distinguished from non-toxic ones visually.
- In general, shellfish toxins are accumulated actively by shellfish and concentrated in the internal organs of bivalves.
Advice to Consumers
- Purchase shellfish from reliable sources.
- To reduce the health risk of shellfish poisoning, remove and discard all internal organs of shellfish where possible before consumption.
- Anyone who feels ill after eating shellfish should immediately seek medical attention.
Advice to the Trade
- Source shellfish from places where monitoring programmes for shellfish toxins have been established.
- Do not accept shellfish from dubious sources.