On a summer morning in June 1990, a group of fishermen aboard a vessel in the Georges Bank area off the Nantucket coast sat down to a meal of blue mussels they had harvested from deep water about 115 miles from shore. Within one to two hours, all six men developed symptoms: numbness of the mouth, vomiting, tingling in their extremities, and lower back pain that persisted for days. Two required hospitalization, and one had briefly lost consciousness. Laboratory analysis of the uneaten mussels later revealed saxitoxin concentrations of 24,400 micrograms per 100 grams in the raw shellfish, more than 300 times the safe level of 80 micrograms per 100 grams. What these fishermen experienced was paralytic shellfish poisoning (PSP), a potentially fatal illness caused by one of the most potent neurotoxins known to science.
PSP is not a new phenomenon, but it is a growing concern. As ocean temperatures rise and harmful algal blooms become more frequent and widespread, the geographic range of PSP toxins is expanding into regions where they were previously unknown. Understanding the science behind this poisoning, its symptoms, its victims, and its history is essential for anyone who consumes shellfish, whether from a commercial supplier or harvested for personal consumption.
The Science of the Toxin: How Saxitoxin Paralyzes
Paralytic shellfish poisoning is caused by the ingestion of a group of neurotoxins collectively known as saxitoxins, which are produced by certain species of marine dinoflagellates (microscopic algae) and, in some cases, freshwater cyanobacteria. These toxins are among the most potent non-protein poisons known. As little as one milligram, roughly 0.000035 ounces, is enough to kill an adult human.
The mechanism of action is precise and devastating. Saxitoxin binds to receptor site 1 of the voltage-gated sodium channels located in the pore-forming region of nerve and muscle cell membranes. By blocking the influx of sodium ions into these excitable cells, the toxin interrupts the conduction of action potentials, the electrical signals that enable nerves to communicate with muscles. The result is a progressive, descending paralysis that can ultimately affect the muscles required for breathing. In essence, saxitoxin does not destroy tissue; it simply prevents it from functioning. The toxin is heat-stable and is not destroyed by cooking or freezing, meaning that even thoroughly boiled or frozen shellfish can remain dangerous.
How Shellfish Become Poisonous
The pathway from algae to human illness is a classic example of bioconcentration in the food chain. Shellfish such as clams, mussels, oysters, geoduck, and scallops are filter feeders. They pump water through their systems, filtering out microscopic algae and other food particles. When the water contains toxin-producing dinoflagellates, often during algal blooms that occur in warmer months from June to October, the shellfish consume these algae and accumulate the toxin in their tissues.
The toxin does not harm the shellfish, but it can persist in their tissues for extended periods. Butter clams, for instance, may retain the toxin for up to a year after an algal bloom has subsided. The toxin accumulates in specific tissues, such as the siphon, neck, and gills of butter clams, which is why health authorities recommend removing and discarding these parts before eating. Other affected species include whelks, moon snails, whole scallops, and the hepatopancreas (the “tomalley” or digestive gland) of crabs and lobsters. While crab meat itself is not known to contain the toxin, crabs feed on shellfish, and the toxin can accumulate in their guts and “butter” (the white-yellow fat inside the back of the shell). Thorough cleaning and evisceration of crab is therefore recommended.
Crucially, shellfish containing toxic levels of saxitoxin do not look, smell, or taste any different from safe shellfish. Laboratory testing of shellfish meat is the only reliable method of detection. This is why commercial shellfish from certified growers are subject to frequent mandatory testing, while shellfish harvested recreationally pose a much higher risk.
The Clinical Picture: Symptoms and Progression
The symptoms of PSP are distinctive and typically begin rapidly. The median time between ingestion and the onset of symptoms is approximately one hour, though it can range from as little as 10 to 30 minutes to as long as three hours. The progression and severity depend on the amount of toxin ingested.
The hallmark early symptom is a tingling or numbness of the lips, tongue, and fingertips. As the toxin takes effect, the numbness can spread to the face, legs, arms, and neck, accompanied by a loss of muscular coordination (ataxia), dizziness, a feeling of floating, and difficulty swallowing. Some individuals experience nausea, vomiting, and diarrhea. In severe cases, the paralysis progresses to the muscles of the chest and abdomen, leading to respiratory distress and complete muscular paralysis. Death can occur in as little as two hours from respiratory failure. The amount of poison required to cause illness can be as little as 1 milligram, and death may result from as little as 1 to 4 milligrams.
The prognosis depends on the dose and the speed of medical intervention. Symptoms usually resolve completely within a few hours to days after ingestion if the patient survives the acute phase. However, there is no known antidote for saxitoxin poisoning. Treatment is entirely supportive: patients with respiratory distress require mechanical ventilation and oxygen support until the toxin is cleared from the body. This is why any person exhibiting symptoms after eating shellfish should seek emergency medical attention immediately.
Who Is Most at Risk?
Anyone who consumes shellfish contaminated with saxitoxin is at risk of PSP. However, certain groups are more vulnerable to severe complications. Children, the elderly, and individuals with chronic illnesses or compromised immune systems are more susceptible to the severe effects of the toxin and should exercise particular caution when consuming shellfish. Children may be more susceptible to PSP, likely due to their lower body weight relative to the dose of toxin ingested.
For pregnant women and nursing mothers, the risks extend beyond the mother. While specific data on the effects of saxitoxin on fetal development are limited, the general precautionary principle applies: because there is no antidote and the consequences of severe poisoning are catastrophic, pregnant women should avoid consuming shellfish from unverified sources.
There is also an elevated risk for certain cultural and subsistence communities. Indigenous peoples in Alaska and the Pacific Northwest, who have traditionally relied on shellfish harvesting as a food source, are disproportionately affected by PSP. Between 1993 and 2021, 132 people in Alaska were reported sickened with PSP, and between 1994 and 2020, five people died after eating saxitoxin-tainted food. In British Columbia, changing trends in PSP reflect the practices of Indigenous and recreational harvesters as sea surface temperatures rise.
A History of Recurring Outbreaks
PSP has been documented for centuries, but systematic surveillance in North America began in the mid-20th century. The first documented PSP in Maine occurred in 1958. In 1957, an epidemic of shellfish poisoning was reported in New Brunswick, Canada, highlighting the risks along the Bay of Fundy and the estuary of the St. Lawrence River.
Between 1973 and 1987, state health departments reported 19 PSP outbreaks with a mean size of eight persons to the CDC’s Foodborne Disease Outbreak Surveillance System. From 1976 through 1989, 42 PSP outbreaks were documented. A broader review from 1973 to 1992 documented 54 outbreaks involving 117 ill persons, with one death, four cases requiring intubation, and 29 requiring emergency medical evacuation.
The 1990 outbreaks in Massachusetts and Alaska, documented by the CDC, illustrate the diversity of PSP scenarios. In Massachusetts, fishermen fell ill after eating mussels harvested from deep water in an area that had been closed to harvesting. The fishermen had not received the closure notice, underscoring the importance of effective communication in prevention. In Alaska, a Native Alaskan man died after consuming butter clams harvested from a beach on the Alaska Peninsula. He developed symptoms within an hour and suffered a fatal cardiopulmonary arrest two hours later.
In 1991, a severe outbreak occurred in Manzanillo and Guayacán on the northwestern coast of Margarita Island, Venezuela. That 1991 outbreak resulted in at least one death, a two-year-old child.
Recent Outbreaks: A Growing Threat
The 2020s have seen a series of significant PSP events that underscore the growing threat. In May 2024, the Oregon Health Authoritywarned the public that multiple people with suspected PSP had consumed self-harvested mussels. The ensuing investigation identified 42 cases with illness onset dates during May 23 to June 6, 2024, leading to seven hospitalizations and no deaths. This was Oregon’s largest recorded PSP outbreak. Leftover mussels from three patients yielded PST levels above the closure threshold, with one sample producing the highest level ever detected in Oregon shellfish: 5,500 micrograms per 100 grams of tissue.
The same year, in August 2024, dead northern fur seals were discovered on St. Paul Island, AK in the Bering Sea. Testing revealed that the seals had been killed by saxitoxin, the first-ever conclusive case of marine mammals killed by the toxin that causes PSP. This event was a harbinger of a broader pattern: as waters around Alaska warm, algal toxins are turning up in new places in the food web, affecting not just shellfish but also the marine mammals and seabirds that depend on them.
Throughout 2025 and into 2026, PSP advisories and closures have become routine along the U.S. West Coast. In June 2025, the California Department of Public Health (CDPH)warned consumers not to eat sport-harvested bivalve shellfish from Marin County due to dangerous levels of PSP toxins. In August 2025, a similar warning was issued for San Luis Obispo County. In Washington State, PSP was detected at unsafe levels in samples collected along Colvos Passage on the west side of Vashon Island in July and August 2025. In Alaska, the razor clam fishery in Del Norte County reopened under a continuing health advisory for PSP.
Analysis & Next Steps
What is new in the understanding of PSP is the recognition of its expanding geographic range and the link between climate change and the spread of saxitoxin-producing algae. The 2024 Oregon outbreak, which was the largest in the state’s history, and the first-ever confirmed deaths of marine mammals from saxitoxin in the Bering Sea, are stark indicators that this is not a static problem. Warmer ocean temperatures are creating favorable conditions for algal blooms in regions where they were previously rare, and the toxin is now appearing in parts of the food web that had not been a concern for local communities. The CDC’s One Health Harmful Algal Bloom System (OHHABS)reported the first human death associated with PSP in its system, and the number of reported PSP cases, while still low relative to other foodborne illnesses, is trending upward.
Why this matters is because PSP is one of the most lethal foodborne illnesses, with a case-fatality rate that can approach 10 to 20 percent in severe outbreaks. There is no antidote, and treatment relies entirely on mechanical ventilation. Unlike bacterial foodborne illnesses, which can be treated with antibiotics, PSP is a neurotoxin that can kill within hours. The fact that cooking and freezing do not destroy the toxin means that consumers cannot rely on standard food safety practices to protect themselves.
Who is affected includes anyone who consumes shellfish, but the burden falls disproportionately on recreational and subsistence harvesters. Commercial shellfish are subject to rigorous testing and monitoring programs, but self-harvested shellfish are not. Indigenous communities in Alaska and the Pacific Northwest, who have historically relied on shellfish for subsistence, are particularly vulnerable, as are children and the elderly. The expanding geographic range of the toxin means that populations in areas that were previously considered low-risk, such as the Pribilof Islands, are now facing new threats.
What to do now requires action at multiple levels. For consumers, the most important step is to understand that PSP cannot be detected by sight, smell, or taste. Shellfish should be purchased only from certified commercial sources that are subject to mandatory testing. Anyone who harvests shellfish recreationally should check with local health authorities for current advisories and closures before harvesting. The California Department of Public Health provides a toll-free Shellfish Information Line (800-553-4133) and an interactive advisory map for real-time updates. In Canada, the Canadian Shellfish Sanitation Program (CSSP) monitors water quality and marine toxins, and harvest areas are opened and closed based on testing. Self-harvesters should never consume shellfish from untested beaches, and crab should always be cleaned and eviscerated before cooking.
For public health agencies, continued investment in monitoring and rapid detection methods is essential. The 2024 Oregon outbreak highlighted the challenges of timely detection and closure of harvest areas. Rapid PST detection methods to supplement routine monitoring could increase the timeliness of risk detection. Collaboration among public health officials, poison control centers, health care professionals, and shellfish program staff is essential to illness prevention and outbreak response.
For researchers, the link between climate change and the spread of algal toxins demands continued investigation. The movement of saxitoxin into new parts of the marine food web, including marine mammals, has implications for ecosystem health and for the communities that depend on those ecosystems. Understanding the conditions that trigger algal blooms and the factors that influence toxin persistence in shellfish is critical for developing predictive models that can inform public health warnings. The “I-don’t-know moment,” as one NOAA biologist described it, is a call for more research and even greater caution.
