Paralytic Shellfish Poisoning (PSP)

Paralytic shellfish poisoning (PSP) is a serious illness caused by eating shellfish that have concentrated saxitoxin and related toxins from harmful algal blooms. The toxins block nerve signals, causing tingling, numbness and, in severe cases, muscle paralysis. Cooking does not destroy them. In Australia, the main defence is harvest-area monitoring: shellfish waters are tested and closed when toxin levels rise, so businesses must source only from open, monitored areas. Key takeaways: PSP is caused by saxitoxin and related biotoxins that filter-feeding shellfish concentrate from harmful algal blooms.. The toxins are heat-stable, so cooking, freezing, smoking or marinating shellfish does not make contaminated stock safe.. Symptoms start with tingling and numbness around the mouth and can progress to muscle weakness and breathing difficulty; PSP is a medical emergency.. The primary control is harvest-area monitoring: authorities test shellfish-growing waters and close them when toxin levels rise.. Businesses protect customers by buying only from approved, monitored suppliers, keeping supplier and batch records, and acting quickly on recalls and closures.. What is paralytic shellfish poisoning? Paralytic shellfish poisoning is an illness people get from eating bivalve shellfish that contain a group of natural toxins known as saxitoxins. These are among the most potent naturally occurring toxins known, and they affect the nervous system by blocking the sodium channels that nerves use to send signals. The result can range from mild tingling to life-threatening paralysis. The shellfish themselves are not usually harmed. They act as a vehicle: filter-feeding species such as oysters, mussels, pipis, clams and scallops draw the toxins in from the water and hold them in their tissues. A shellfish can look, smell and taste completely normal and still carry a dangerous dose, which is why you cannot judge safety by appearance. How do algal toxins concentrate in shellfish? PSP toxins are produced by certain species of microscopic marine algae, or phytoplankton. Under the right conditions of temperature, sunlight, nutrients and water movement, these algae can multiply rapidly into a harmful algal bloom, sometimes visible as a discoloured patch of water. During a bloom, the toxin-producing algae become abundant in the water column. Bivalve shellfish feed by pumping large volumes of seawater across their gills and straining out the plankton. As they filter, they accumulate the algae and the toxins those algae contain, concentrating them to levels far higher than in the surrounding water. This is called bioaccumulation. Because toxin levels rise and fall with the bloom, a growing area can be perfectly safe one week and unsafe the next. Shellfish also clear the toxins from their tissues at different rates once a bloom passes, so an area stays closed until testing confirms levels have dropped back to safe limits. Toxin-producing algae multiply during a harmful algal bloom.. Filter-feeding shellfish pump seawater and accumulate the algae and their toxins.. Bioaccumulation concentrates toxins in shellfish tissue to hazardous levels.. Toxin levels change over time, so safety depends on when and where the shellfish were harvested.. What are the symptoms of PSP? PSP symptoms typically appear quickly, often within minutes to a few hours of eating contaminated shellfish. The first signs are usually a tingling or numbness around the lips, mouth and tongue, which may spread to the face, neck, fingertips and toes. Mild cases can also involve headache, dizziness and nausea. In more serious cases the toxin's effect on the nervous system progresses to muscle weakness, loss of coordination, difficulty speaking or swallowing and, at high doses, paralysis of the muscles used for breathing. Severe PSP is a medical emergency because respiratory failure can occur. There is no antidote; treatment is supportive medical care, which is why rapid medical attention matters. Anyone who develops these symptoms after eating shellfish should seek urgent medical help and mention the shellfish meal. Because cooking does not destroy the toxin, everyone who shared the same shellfish may be at risk. Early: tingling and numbness of the lips, mouth, tongue and extremities.. Progressing: headache, dizziness, nausea, weakness and loss of coordination.. Severe: difficulty speaking or swallowing and paralysis affecting breathing.. PSP is a medical emergency with no antidote; seek urgent care immediately.. Why cooking does not make contaminated shellfish safe Many food hazards are controlled by heat, but PSP toxins are not one of them. Saxitoxin and its relatives are heat-stable, so normal cooking, steaming, boiling, grilling, frying, smoking, marinating in acid or freezing will not break them down to safe levels. Shellfish that were unsafe when harvested remain unsafe after preparation. This makes PSP different from most bacterial hazards a kitchen manages with the danger zone, the 2-hour/4-hour rule and cooking to a 75C core temperature. Those controls are essential for pathogens, but they do nothing against a preformed chemical toxin already in the flesh. The practical consequence is that safety has to be built in before the shellfish reach your kitchen. You cannot cook your way out of contaminated stock; you can only prevent contaminated stock from entering the supply chain in the first place, through monitoring and sourcing controls. How harvest-area monitoring protects consumers In Australia, the main safeguard against PSP is a shellfish quality-assurance and monitoring system operated around commercial growing areas. Growing waters are classified and routinely monitored for harmful algae and for biotoxin levels in shellfish flesh. When algal counts or toxin readings rise toward action limits, the affected harvest area is closed and harvesting stops. This is a preventive control at the source. It means that shellfish reaching the legitimate market from approved areas have been harvested from waters that were open and within safe limits at the time. Areas reopen only after follow-up testing confirms toxin levels have fallen back within acceptable limits set under the Food Standards Code framework. For a food business, the monitoring system only protects your customers if you stay inside it. That means buying commercially harvested shellfish from approved, licensed suppliers who can trace their product back to a monitored, open harvest area, and never using shellfish of unknown origin. Growing waters are classified and monitored for harmful algae and biotoxins.. Harvest areas are closed when toxin levels approach action limits.. Areas reopen only after testing confirms levels have returned to safe limits.. Businesses must source only from approved suppliers linked to monitored areas.. What food businesses should do The most important thing a restaurant, retailer or wholesaler can do is control sourcing and traceability. Buy shellfish only from licensed, reputable suppliers who participate in the shellfish monitoring system, and keep records that let you trace each batch back to its supplier and harvest area. Good records also let you respond fast if a recall or area closure is announced. Stay alert to official communications. Harvest-area closures, public health warnings and food recalls are how you learn that a particular batch or region is affected. Check recall notices, act on them immediately by isolating and withdrawing affected stock, and do not sell product you cannot verify as safe. Recreationally harvested shellfish deserve special caution. Shellfish gathered by the public from beaches or estuaries are not part of the commercial monitoring system and should never be brought into a commercial kitchen or sold. Their safety cannot be verified, and this is a common route to PSP. Source only from licensed suppliers within the shellfish monitoring system.. Keep supplier and batch records so every lot traces to a harvest area.. Monitor and act promptly on recalls, health warnings and area closures.. Never accept, sell or serve recreationally or unknown-origin shellfish.. Checklist: Buy shellfish only from licensed suppliers in the shellfish monitoring system. Keep supplier and batch records that trace every lot to a harvest area. Check for harvest-area closures and food recalls regularly. Isolate and withdraw affected stock as soon as a recall or closure is announced. Never accept, prepare or sell recreationally or unknown-origin shellfish. Remember that cooking, freezing and marinating do not remove PSP toxins. Know the early symptoms so staff can respond and seek help fast. Common mistakes: Assuming cooking or freezing makes contaminated shellfish safe.. Judging shellfish safety by smell, taste or appearance.. Using recreationally harvested shellfish in a commercial kitchen.. Buying from unverified suppliers with no link to a monitored harvest area.. Failing to keep traceability records, making recall response slow or impossible.. Ignoring or delaying action on harvest-area closures and recall notices..