Shellfish Toxicity

Updated: May 11, 2022
  • Author: Thomas C Arnold, MD, FAAEM, FACMT; Chief Editor: Asim Tarabar, MD  more...
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Practice Essentials

Marine harmful algal bloom (HAB) toxins can cause a variety of illnesses in humans through shellfish ingestion. Marine HABs have occurred in the Gulf of Mexico and along the Atlantic and Pacific coasts of the United States. Two major groups of marine phytoplankton, diatoms and dinoflagellates, produce HAB toxins. Common marine HAB toxins found in shellfish include brevetoxins, azaspiracid, domoic acid, okadic acid, saxitoxin. [1]  

Toxic outbreaks can cause a red-brown discoloration of the water. This proliferation of toxic dinoflagellates, known as red tide, is favored by warmer weather. This phenomenon has led to the general teaching in North America that shellfish are safe to eat only if harvested in a month containing the letter "r."  Most states at risk for marine HABs have excellent monitoring programs in place to close harvesting when toxins are present in shellfish. 

Red tide and its resultant massive kills of various birds and marine animals have become an enormous concern in Europe, prompting numerous international congresses to address the problem. In Canada, an outbreak of diarrhea following shellfish poisoning resulted in the formation of a voluntary algal bloom network. [2]

 At least 5 distinct shellfish-poisoning syndromes have been identified, as follows [1] :

  • Paralytic shellfish poisoning (PSP)
  • Neurologic shellfish poisoning (NSP)
  • Diarrheal shellfish poisoning (DSP)
  • Amnestic shellfish poisoning (ASP)
  • Azaspiracid shellfish poisoning (AZP)

All 5 syndromes share some common features and primarily are associated with bivalve mollusks (eg, mussels, clams, oysters, scallops). These shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae in the form of dinoflagellates and diatoms. HAB toxins have also been found in gastropod mollusks (eg, abalone, whelks, moon snails), cephalopod mollusks (eg, octopi, squid, cuttlefish) and crustaceans (eg, Dungeness crabs, shrimp, lobsters). [3]  

Poisoning results in gastrointestinal and neurologic illness of varying severity. Symptoms typically appear 30–60 minutes after ingesting toxic shellfish but can be delayed for several hours. Diagnosis is usually one of exclusion and is typically made clinically in patients who have recently eaten shellfish. [3]  Care is primarily symptomatic and supportive.

For patient education resources, see Shellfish Poisoning, Gastrointestinal and Shellfish Poisoning, Paralysis.



The toxins responsible for most shellfish poisonings are water-soluble, are heat and acid-stable, and are not inactivated by ordinary cooking methods. The main toxins responsible for each of the shellfish syndromes are as follows:

  • PSP - Saxitoxin
  • NPS - Brevetoxin
  • DSP - Okadaic acid
  • ASP - Domoic acid
  • AZP - Azaspiracid

The saxitoxins act by blocking sodium ion movement through voltage-dependent sodium channels in nerve and muscle cell membranes. Conduction block occurs principally in motor neurons and muscle. The toxin is made by dinoflagellates of the Gonyaulax species (red tide).

Brevetoxins are polycyclic ethers that, like ciguatoxin, bind to and stimulate sodium flux through voltage-gated sodium channels in nerve and muscle. Brevetoxins are made by the dinoflagellate Ptychodiscus brevis.

Okadaic acid binds to intestinal epithelial cells and increases their permeability. This toxin is made by dinoflagellates of the species Dinophysis and Prorocentrum.

Domoic acid (DA) is structurally similar to the excitatory neurotransmitter glutamate. Domoic acid binds to and stimulates the kainic acid glutamate receptor, [4] which allows sodium influx and a small amount of potassium efflux—neuronal depolarization results. Domoic acid has been associated with necrosis of the glutamate-rich hippocampus and amygdala in autopsied cases. Domoic acid is produced by the diatom Nitzschia pungens.



Ingestion of raw or cooked mollusks that contain the toxin in sufficient quantities ensures the development of symptoms.

A wide range of shellfish may cause PSP, but most cases occur after eating mussels or clams. PSP occurs worldwide but is most common in temperate waters, especially off the Pacific and Atlantic Coasts of North America, including Alaska. Cases have also been reported from countries such as the Philippines, China, Chile, Scotland, Ireland, New Zealand, and Australia. [3]

NSP has been reported from the southeastern coast of the United States, the Gulf of Mexico, the Caribbean, and New Zealand.

Most cases of DSP result from eating toxic bivalve mollusks such as mussels and scallops. Occurrences have been reported in Europe, Asia, North America, South Africa, Australia, and New Zealand. [5]  

ASP has been reported from Canada, Scotland, Ireland, France, Belgium, Spain, Portugal, New Zealand, Australia, and Chile. Toxic mussels, scallops, razor clams, and crustaceans were responsible in those outbreaks. [3]  In the Pacific Northwest, Chesapeake Bay and along the coasts of western Europe and eastern Asia, peak annual levels of DA in shellfish can exceed the regulatory limit, resulting in intermittent restrictions on shellfish harvests. [6]

AZP was identified following cases of severe GI illness from the consumption of contaminated mussels from Ireland. It has since been found in the Netherlands, Belgium, Morocco, and eastern Canada. [1]  



Education, surveillance, and strict regulation by public health officials appear to be decreasing the incidence of shellfish poisoning in the United States. Additionally, enzyme-linked immunosorbent assay (ELISA) screening techniques are making detection of these toxins simple and rapid. [7]

Cases of PSP have occurred along the northeast Atlantic coast, northwest Pacific coast, or Alaska. [8] Most cases have involved recreational shellfish collectors, not commercial vendors. The 2020 Annual Report of the American Association of Poison Control Centers' National Poison Data System (AAPCC-NPDS) documented 47 single exposures to paralytic shellfish poisoning, with one death. [9]

Sporadic outbreaks have been reported in Europe, Asia, Africa, and the Pacific Islands. 

Children appear to be more sensitive than adults to the saxitoxins of PSP. To date, all the reported deaths from ASP have been in elderly persons who had more severe neurologic symptoms.



The 2020 Annual Report of the American Association of Poison Control Centers' NPDS reported 8 minor outcomes, 2 moderate outcomes, no major outcomes, and one death among 47 patients with paralytic shellfish poisoning single exposures. [9]  Although any person eating fish or shellfish containing HAB toxins may become ill, persons with some chronic diseases, such as liver disease, could potentially have more severe illnesses. [3]   

Fatality rates from PSP, the most severe of the 4 shellfish poisoning syndromes, ranges from 1-12% in isolated outbreaks. In cases of severe PSP, muscle paralysis and respiratory failure can lead to death in 2–25 hours. The risk of death from PSP is reduced if healthcare professionals have access to advanced life support capabilities. [1]

Depending upon the dose of DA and adult age, ASP may result in mild discomforts, such as gastrointestinal symptoms, but with progressively larger doses, memory loss, seizures, coma, or death occur. [10]  

The first reported human domoic acid poisoning event occurred in 1987 and affected more than 100 people after eating mussels harvested off Prince Edward Island, Canada. Gastroenteritis followed by headache and short-term memory loss occurred. In a few cases, severe cognitive dysfunction to the point of interfering with the patient's ability to perform normal daily activities was noted. Seizures, coma, hemiparesis, and ophthalmoplegia were noted in the most severe cases. The mortality rate of ASP was 3%. [4]  

Research strongly indicates that infants and fetuses are much more susceptible to the toxic effects of DA than are adults. DA can move across the placenta and concentrate in amniotic fluid, which can be swallowed during late gestation. DA also transfers to infants via breastmilk. It has been hypothesized that early exposure to DA may render the developing fetus at risk for schizophrenia and autism. Studies in rodents and California sea lions exposed to DA reported effects including seizures, diminished social regulation, and repetitive behaviors bearing a striking resemblance to the clinical features of these disorders in humans. [6, 10, 11]

To date, no deaths have been reported for NSP or DSP. There are no reports of long-term effects, but no long-term follow-up studies of those affected by HABs have been conducted. [1]