Diarrhetic Shellfish Poisoning (DSP)

Diarrhetic shellfish poisoning (DSP) is a foodborne illness caused by marine biotoxins — mainly okadaic acid and related compounds — produced by microscopic algae and concentrated by filter-feeding bivalves such as mussels, oysters, scallops and pipis. The toxins are heat-stable, so cooking, freezing or smoking will not destroy them. The only reliable control is sourcing shellfish from monitored, classified harvest areas through licensed suppliers. Key takeaways: DSP is caused by heat-stable marine biotoxins (the okadaic acid group) that filter-feeding bivalves accumulate from their harvest waters — cooking, freezing, smoking and canning do NOT make affected shellfish safe.. Symptoms are mainly gastrointestinal (diarrhoea, nausea, vomiting and abdominal cramps), usually starting within hours of eating and generally settling within a few days.. Contaminated shellfish look, smell and taste completely normal, so you cannot judge safety by inspection.. The control point is the source: buy bivalves only from licensed suppliers harvesting from monitored, classified areas under a shellfish quality assurance program.. Keep supplier and batch records for traceability, and act immediately on harvest-area closures and food recalls.. What is diarrhetic shellfish poisoning? Diarrhetic shellfish poisoning (DSP) is a foodborne illness caused by a group of marine biotoxins — principally okadaic acid and closely related dinophysistoxins. These toxins are produced by microscopic marine algae (dinoflagellates) that occur naturally in coastal waters and can multiply rapidly, or "bloom", under the right conditions. Filter-feeding bivalve molluscs feed by straining large volumes of seawater, and in doing so they concentrate any toxin-producing algae that are present. The toxin builds up in the shellfish flesh long before the animal reaches a kitchen, so the hazard is effectively built in at harvest rather than created by poor handling downstream. DSP is one of several distinct shellfish biotoxin syndromes, alongside paralytic (PSP), amnesic (ASP) and neurotoxic (NSP) shellfish poisoning. Each is caused by different toxins with different effects, but they share the same underlying control point: the quality of the water the shellfish grew in. Why cooking won't make affected shellfish safe The single most important fact about DSP is that the toxins are heat-stable. Normal cooking, steaming, grilling, frying, smoking, canning and freezing do not destroy them. A batch of contaminated mussels stays hazardous no matter how thoroughly it is cooked. This makes DSP fundamentally different from bacterial hazards such as Salmonella or Vibrio, which cooking to a 75°C core temperature will control. With a preformed, heat-stable toxin there is no kitchen step that removes the risk — the same principle applies to histamine (scombroid) poisoning in fish. Contaminated shellfish also look, smell and taste completely normal. There is no bulging shell, off odour or discolouration to warn you, and the toxin is invisible. You cannot inspect, sort or process your way to safety, which is why the whole system depends on controlling where the shellfish came from. Which shellfish carry the risk DSP is associated with filter-feeding bivalve molluscs — the animals that pump seawater through their gills to feed. These are the species that can accumulate the toxin from the surrounding water. Mussels. Oysters. Scallops (particularly whole or roe-on product). Pipis and cockles. Clams. Which seafood is not the usual vehicle Finfish, crustaceans (prawns, crabs, lobsters) and cephalopods (squid, octopus) are not the typical vehicles for DSP, because they do not filter-feed and concentrate the toxin in the same way. That does not make them risk-free — other seafood hazards such as histamine, ciguatera, Vibrio and cross-contamination still apply. For bivalves, the risk cannot be linked to a particular size, colour or freshness. Any bivalve from an affected growing area can carry the toxin, whether it looks pristine or not, which is why sourcing controls rather than sensory checks do the protective work. Symptoms and who is affected DSP typically produces gastrointestinal symptoms — diarrhoea, nausea, vomiting and abdominal cramps — usually beginning within minutes to a few hours of eating affected shellfish. It is generally self-limiting, with most people recovering within a few days, and it is not usually associated with the paralysis seen in paralytic shellfish poisoning. This guide is general information, not medical advice. Anyone who becomes unwell after eating shellfish should seek medical attention, and suspected foodborne illness should be reported to your state or territory food regulator so that potential harvest-area or supply issues can be investigated. Because the toxin is preformed and heat-stable, the amount consumed depends entirely on how much toxin the shellfish accumulated at harvest — reinforcing why prevention sits with source control rather than the kitchen. How DSP is controlled at the source Commercial shellfish safety in Australia relies on managing growing waters, not on testing every plate. Shellfish-growing areas are classified and monitored under shellfish quality assurance arrangements, which watch for toxin-producing algae and biotoxins in the water and in the shellfish themselves. When biotoxin levels rise beyond acceptable limits, the relevant authority can close a harvest area so that affected shellfish do not enter the food supply. Commercially harvested bivalves are traceable back to a classified area and harvest period, which lets regulators and businesses act quickly if a problem is detected. For a food business, this means the protective system is already doing most of the work — provided you buy only shellfish that has come through it. Product that cannot be traced to a classified, open harvest area falls outside those safeguards. What food businesses must do Under the Food Standards Code (including the food safety practices in Standard 3.2.2 and the tools in Standard 3.2.2A), businesses must use safe, suitable food from approved sources and be able to trace it. For bivalve shellfish, that means buying only from licensed, reputable suppliers and keeping the records that let you identify each batch. Keep invoices, delivery dockets and supplier details so you have one-step-back traceability — you can say where a batch came from and, if needed, withdraw or discard it. Maintain the cold chain on arrival, storing shellfish at or below 5°C so you are not adding a temperature-abuse problem on top of the biotoxin risk. Buy bivalves only from licensed, approved suppliers with harvest-area traceability. Record supplier, batch and delivery details for one-step-back traceability. Keep shellfish at or below 5°C throughout receiving, storage and display. Monitor food recalls and harvest-area closures, and withdraw affected stock immediately. Recreational and wild-harvested shellfish The biggest gap in the system is shellfish gathered outside monitored programs. Recreationally collected mussels, pipis, oysters and cockles do not pass through harvest-area classification and biotoxin monitoring, so there is no assurance the growing water was safe at the time of collection. Food businesses should never accept or serve bivalve shellfish that cannot be traced to a licensed supplier and a classified harvest area. Wild or "foraged" product, gifts from customers, or informal supply arrangements bypass the exact controls that prevent DSP. Where states or territories issue advisories or temporary closures for particular waters, follow them. These notices exist precisely because biotoxin levels change with algal conditions, and the guidance is the practical front line against DSP for anyone gathering shellfish themselves. Checklist: Bivalve shellfish are bought only from licensed, approved suppliers. Every batch can be traced back to a classified harvest area and date. Delivery and supplier records are kept for one-step-back traceability. Shellfish are received and held at or below 5°C. Staff know that cooking does not remove DSP toxins. Food recalls and harvest-area closures are checked regularly. No wild-harvested or untraceable bivalves are accepted or served. Common mistakes: Assuming thorough cooking, steaming or freezing makes affected shellfish safe — the toxin is heat-stable and survives all of these.. Judging shellfish safety by look, smell or taste, when contaminated bivalves appear completely normal.. Accepting recreationally gathered or untraceable shellfish that never passed through harvest-area monitoring.. Failing to keep supplier and batch records, so affected stock cannot be identified during a recall.. Ignoring harvest-area closures and food recall notices, or acting on them too slowly..