What Is Paralytic Shellfish Poisoning?

The FDA says most shellfish toxins are produced by naturally occurring marine algae that molluscan shellfish eat, which causes the toxins to accumulate in the shellfish flesh, and that paralytic shellfish poisoning, or PSP, is caused by neurotoxins called saxitoxins or paralytic shellfish toxins. The CDC Yellow Book calls PSP the most common and most severe form of shellfish poisoning, says the toxins are produced by various dinoflagellates, and says a wide range of shellfish can cause it but most cases occur after people eat clams or mussels. The FDA says food containing these toxins may look, smell and taste normal, and that the toxins cannot be removed by cooking or freezing.

Symptoms come on fast. The FDA says most people with PSP begin to develop symptoms within 30 minutes of eating contaminated seafood, and that effects can range from tingling of the lips, mouth and tongue to respiratory paralysis, with other symptoms such as numbness of the arms and legs, a pins and needles sensation, weakness, loss of muscle coordination, nausea, shortness of breath, dizziness, vomiting and headache. The CDC Yellow Book says symptoms usually appear 30 to 60 minutes after eating toxic shellfish, and that severe cases are associated with large doses of toxin and features such as unsteady movement, difficulty swallowing, paralysis, mental status changes and respiratory failure.

States monitor shellfish and close harvest areas. The Oregon Department of Fish and Wildlife says the Oregon Department of Agriculture tests shellfish for biotoxins every two weeks as tides and weather permit, and that harvest areas close when paralytic shellfish toxin exceeds 80 micrograms per 100 grams of tissue. In April 2026, the California Department of Public Health announced that its annual quarantine on recreationally harvested mussels would begin May 1 along the California coast, in addition to advisories against eating sport-harvested mussels, clams, scallops and oysters from Sonoma, Marin and San Mateo counties because of elevated PSP toxin levels in shellfish samples.