Marine Biotoxins in Shellfish: an Overview

Marine biotoxins are natural poisons produced by microscopic algae that filter-feeding shellfish concentrate in their flesh. In Australia the main groups cause paralytic, diarrhetic, amnesic and neurotoxic shellfish poisoning. Cooking and freezing do not destroy these toxins, so safety depends on harvesting from monitored, classified waters. Food businesses manage the risk by buying oysters, mussels and other bivalves only from approved, traceable sources. Key takeaways: Marine biotoxins come from harmful algal blooms; filter-feeding bivalves such as oysters, mussels, pipis, cockles and scallops concentrate them in their flesh.. There are four main groups, causing paralytic (PSP), diarrhetic (DSP), amnesic (ASP) and neurotoxic (NSP) shellfish poisoning.. Cooking, steaming and freezing do not destroy these toxins, so the 75C cook rule does not make affected shellfish safe to eat.. Safety relies on shellfish quality assurance programs that monitor waters and shellfish flesh, classify harvest areas and close them when biotoxins rise.. Food businesses control the risk by buying only from approved, licensed suppliers and keeping records that let a batch be traced back to its harvest area.. What are marine biotoxins and where do they come from? Marine biotoxins are naturally occurring poisons made by tiny marine algae (phytoplankton). At certain times of year these algae multiply rapidly in what is often called a harmful algal bloom. Bivalve shellfish such as oysters, mussels, pipis, cockles, clams and scallops feed by filtering large volumes of seawater, and in doing so they concentrate any toxins the algae contain. The shellfish themselves are usually unharmed, but their flesh can become dangerous to the people who eat them. Because the toxins are natural and come from the environment rather than from poor handling, contaminated shellfish look, smell and taste completely normal. There is no way to tell by sight or smell whether a batch is affected. That is why the risk is managed at the source, through water and flesh testing, rather than in the kitchen. Biotoxins are produced by algae, not by bacteria or spoilage.. Filter-feeding bivalves accumulate the toxins as they feed.. Affected shellfish look, smell and taste normal.. Risk rises during harmful algal blooms, which vary by season and location.. The main shellfish biotoxin groups Australian shellfish safety programs focus on four main groups of biotoxins, each linked to a different illness. They differ in the algae that produce them, the symptoms they cause and how quickly those symptoms appear. Paralytic and amnesic toxins are generally the most serious, because in high doses they can affect the nervous system. You do not need to memorise the chemistry, but it helps to understand that these are distinct hazards managed together under one monitoring system. The Australia New Zealand Food Standards Code sets maximum levels for these biotoxins in shellfish, and harvest areas are tested against those limits. Paralytic Shellfish Toxins (saxitoxins) cause Paralytic Shellfish Poisoning (PSP), which affects the nervous system and can, in severe cases, impair breathing.. Diarrhetic Shellfish Toxins (including okadaic acid) cause Diarrhetic Shellfish Poisoning (DSP), a gastrointestinal illness with nausea, vomiting and diarrhoea.. Amnesic Shellfish Toxins (domoic acid) cause Amnesic Shellfish Poisoning (ASP), which can cause gastrointestinal upset and, rarely, memory and neurological effects.. Neurotoxic Shellfish Toxins (brevetoxins) cause Neurotoxic Shellfish Poisoning (NSP), with a mix of neurological and gastrointestinal symptoms.. Why cooking and freezing do not make shellfish safe Most food-poisoning bacteria are destroyed by cooking food so the centre reaches 75C, and many parasites are controlled by freezing. Marine biotoxins are different. They are stable, heat-resistant chemicals, and normal cooking, steaming, boiling and freezing do not break them down or reduce them to safe levels. Shellfish harvested from affected waters remain unsafe no matter how thoroughly they are prepared. This is the single most important point for a kitchen to understand. Your usual cooking controls, which handle biological hazards so well, do not apply here. Because you cannot cook the risk away and cannot detect it by taste or smell, the only reliable control is making sure the shellfish were harvested from waters that were open and being monitored at the time. Biotoxins are heat-stable, so the 75C cook rule does not neutralise them.. Boiling, steaming and freezing do not make affected shellfish safe.. Cooking liquid can also carry toxins leached from the shellfish.. The control point is safe harvest, not safe cooking.. How safe-harvest systems manage the risk Because the hazard begins in the water, Australia manages it through shellfish quality assurance programs run by state authorities. Growing waters are classified based on their water quality, and both the water and the shellfish flesh are tested regularly for biotoxins and the algae that produce them. This monitoring is the backbone of shellfish safety. When biotoxin levels or algal counts rise toward unsafe levels, the relevant harvest area can be closed so that no shellfish are harvested until testing confirms the water is safe again. Commercial harvesters must operate under licence within this system, and each batch of shellfish is tagged or documented so it can be traced back to a specific, approved harvest area. This is why shellfish from a reputable, licensed supply chain is generally safe. Growing waters are classified and monitored for biotoxins and harmful algae.. Harvest areas are closed when toxin or algal levels rise, and reopened only after testing clears them.. Commercial harvesters work under licence within the quality assurance program.. Batches are tagged so they can be traced to an approved harvest area.. What food businesses should do For most cafes, restaurants and retailers, the practical duty is simple: only buy shellfish from approved, licensed suppliers who operate within a shellfish quality assurance program, and never accept shellfish that cannot be traced back to a classified harvest area. Recreationally harvested shellfish, or shellfish of unknown origin, should not be sold or served, because they sit outside the monitoring system entirely. Traceability records do the heavy lifting. Keep supplier invoices, batch tags and harvest-area details so that, if a recall or public health alert is issued for a growing area, you can quickly identify and remove any affected stock. Recalls for marine biotoxins do happen when monitoring detects a problem, and being able to act on them fast is part of a working food safety program. Source shellfish only from licensed suppliers within an approved quality assurance program.. Reject shellfish that cannot be traced to a classified harvest area.. Do not sell or serve recreationally harvested shellfish of unknown origin.. Keep supplier records and batch tags so you can respond quickly to a recall or growing-area alert.. Recognising shellfish poisoning Symptoms depend on the toxin group but generally begin within minutes to a few hours of eating affected shellfish. Diarrhetic poisoning tends to cause gastrointestinal upset such as nausea, vomiting and diarrhoea. Paralytic and neurotoxic poisoning can cause tingling or numbness around the mouth and in the fingers, along with other neurological effects, and severe paralytic cases can affect breathing. Amnesic poisoning can, rarely, cause confusion or memory effects. Marine biotoxin poisoning is a medical matter. Anyone with symptoms after eating shellfish, particularly numbness, tingling or difficulty breathing, should seek medical care promptly. From a business point of view, stop serving the suspect batch, preserve any remaining stock and packaging, record the supplier and batch details, and be ready to cooperate with the relevant food safety authority. Onset is usually rapid, from minutes to a few hours after eating.. Tingling or numbness around the mouth and fingers is a warning sign of paralytic or neurotoxic poisoning.. Severe paralytic poisoning can affect breathing and needs urgent medical care.. If poisoning is suspected, stop service of the batch, keep samples and packaging, and record supplier details.. Checklist: Shellfish are bought only from approved, licensed suppliers within a quality assurance program.. Every batch can be traced back to a classified harvest area via tags or invoices.. Recreationally harvested or unknown-origin shellfish are never sold or served.. Staff understand that cooking and freezing do not destroy marine biotoxins.. Supplier records and batch details are kept and easy to retrieve.. The business monitors and acts on recalls and growing-area alerts affecting its suppliers.. Common mistakes: Assuming that thorough cooking or steaming makes shellfish from an affected area safe.. Judging shellfish by look and smell, when contaminated shellfish appear completely normal.. Serving recreationally caught or unknown-origin shellfish that sit outside the monitoring system.. Failing to keep batch tags and supplier records, so affected stock cannot be traced during a recall.. Ignoring public health alerts or harvest-area closures for a growing region you buy from..