Food Safety Focus (215th Issue, June 2024) – Article 1

Shellfish Toxins - Nemesis of Shellfish Lovers

Reported by Dr. John LUM, Scientific Officer,
Risk Assessment Section, Centre for Food Safety

The Centre for Food Safety (CFS) received referrals from the Centre for Health Protection of two suspected cases of paralytic shellfish poisoning (PSP) and neurotoxic shellfish poisoning (NSP) in April and May 2024 respectively.   Patients in both cases developed symptoms within one hour after consuming a variety of shellfish at home, including conches, scallops, common oriental clams, sea snails and areolate babylons. An overview of shellfish toxins including their sources, modes of transmission, impact on human health and control measures in food will be provided in this article.

What are Shellfish Toxins?

Shellfish toxins are a group of natural toxins produced by certain species of algae known as dinoflagellate. Suspected or confirmed episodes of human poisoning caused by shellfish toxins have been reported worldwide and the more important ones included paralytic shellfish toxins (PSTs), diarrhoetic shellfish toxins (DSTs), neurotoxic shellfish toxins (NSTs), amnesic shellfish toxins (ASTs) and azapiracid shellfish toxins (AZTs).  Details on these shellfish toxins and the symptoms of different shellfish poisoning have been discussed previously.

How do Shellfish become Contaminated with Shellfish Toxins?

Shellfish toxins mostly originate from harmful algal blooms (HABs), commonly known as "red tides", which are caused by certain species of phytoplankton. Since shellfish are filter feeders, they pump water through their systems, filtering out and eating algae and other food particles. When shellfish eat large amounts of these toxic algae during red tides, the toxins will accumulate in the shellfish and sicken people who eat them (Figure 1). Examples of shellfish that have been involved in shellfish poisoning include mussels, clams, oysters, scallops, conches, sea snails and geoducks.

Risk-based Sampling and Test Selection

A risk-based approach is adopted for the selection of food samples and testing parameters, prioritising high-risk foods, previous unsatisfactory samples, reports of local or overseas food incidents, intelligence and public concerns. An internal review is conducted regularly to reflect new legislation and standards.  The testing scope includes chemical, microbiological and radiological hazards, ranging from food additives and contaminants to pathogens and radiological parameters, as well as antimicrobial resistance bacteria (Figure). 

Besides, the FSP is also reviewed by the Expert Committee on Food Safety before implementation. Additionally, a working group focuses on the unique challenges of foods purchased via online platforms, adapting surveillance methods to this modern buying trend. The outcomes of these surveillance activities and the progress of the working group are reported regularly to the Panel on Food Safety and Environmental Hygiene of the Legislative Council, facilitating transparency and continuous improvements in food safety practices.

Figure 1: Shellfish filter feed on toxin-producing algae and accumulate the toxins in their bodies, which may eventually cause shellfish poisoning in humans

Controlling Measures on Shellfish Toxins

The possible presence of shellfish toxins in shellfish, as well as the health consequences to consumers, make shellfish toxins one of the most important hazards associated with shellfish. The amounts of toxins taken up by shellfish and the times that these toxins remain in the shellfish tissues are different for different species of shellfish and depend on growth environment conditions.  It has been reported that some shellfish species can remain toxic for an extended period of time, up to several months, after being exposed to toxins.  Currently, there are no practical and reliable methods to detoxify shellfish that are contaminated with shellfish toxins.

In the international arena, the control of shellfish toxins is mainly achieved through the implementation of algal toxin monitoring programmes, which monitor the presence of toxin producing algae in shellfish harvesting or culture zones in the sea.  The testing of the levels of shellfish toxins in the meat of shellfish is also widely practised in different places. In case toxic algal blooms are noted or shellfish toxins are detected at hazardous levels, the affected sea areas will be closed and suspended from shellfish harvesting and production until toxicological investigation has made clear that the shellfish is free from hazardous amounts of shellfish toxins. Hong Kong has also adopted a similar monitoring programme for shellfish toxins producing algae.

With the advancements in food technology and the globalisation of the food trade, shellfish products can be shipped around the world. Therefore, in addition to adopting monitoring programmes in shellfish harvesting areas, many places have also implemented a food surveillance programme to cover the testing of shellfish toxins on food products available locally.

Myth and Facts

Can I tell if shellfish contain shellfish toxins by how they look?

No. Shellfish containing dangerous levels of shellfish toxins do not look or taste any different from shellfish that are safe to eat. Therefore, toxic shellfish cannot be distinguished from non-toxic ones visually. Laboratory testing of shellfish is the only method of detecting shellfish toxins.

Does cooking shellfish destroy shellfish toxins and make them safe to eat?

Shellfish toxins are heat stable and cannot be destroyed by cooking, freezing or other food preparation procedures. However, as shellfish toxins tend to concentrate in the internal organs of shellfish, the risk of shellfish poisoning can be reduced by removing and discarding all internal organs of shellfish where possible before consumption.

Key Points to Note

• Shellfish toxins are produced by algae known as dinoflagellate, and cannot be destroyed by cooking, freezing or other food preparation methods.
• Toxic shellfish cannot be distinguished from non-toxic ones visually.
• In general, shellfish toxins are concentrated in the internal organs of shellfish. The risk of shellfish poisoning can be reduced by removing and discarding all internal organs of shellfish before consumption.

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, and save any leftovers for investigation and laboratory testing.

Advice to Trade

• Maintain proper trade records to facilitate source tracing when necessary.
• Do not accept shellfish from dubious sources.