Natural Marine Toxins: PSP and Domoic Acid
Paralytic shellfish poisoning (PSP) toxins and domoic acid are naturally occurring marine toxins. These toxins are produced by some single-celled marine plants (phytoplankton). Marine animals that filter their food from seawater may accumulate these toxins. The toxins do not appear to directly harm the animals, but people or some predatory animals eating toxic seafoods may become poisoned. PSP toxins and domoic acid are powerful nerve poisons. PSP toxins and domoic acid have no taste or odor. There is no visible difference between toxic and safe seafoods. Cleaning seafoods in many cases will not remove the toxins. Cooking does not destroy the toxins.
Paralytic Shellfish Poisoning Toxins
Certain one-celled organisms called dinoflagellates produce PSP toxins. Bivalve shellfish (2 shells) filter these organisms from the water. PSP toxins accumulate in the dark digestive organs or viscera of most shellfish. In Washington clams (also called Butter clams), PSP toxins also accumulate in the siphons (necks). Mussels, oysters, clams, and scallops have caused PSP outbreaks in California, but abalone, crab, shrimp and fish have never been implicated as a source of PSP.
PSP symptoms begin within a few minutes to a few hours after eating toxic shellfish. Symptoms begin with tingling and numbness of the lips, tongue and fingertips. Later symptoms are lack of balance, lack of muscle coordination, slurred speech and problems in swallowing. Complete paralysis and death can occur in severe cases.
Domoic Acid
Domoic acid was first detected in California in 1991. Certain single-celled plants called diatoms produce domoic acid. Bivalve shellfish and some finfish filter these diatoms from the water. Crustaceans can also accumulate domoic acid by feeding on bivalve shellfish or small fish that contain the toxin. Domoic acid can accumulate in the viscera as well as the flesh of these animals. Unsafe levels of domoic acid have been found in mussels, oysters, razor clams (meat and viscera), crab (viscera and meat), lobster (viscera), and sardines and anchovies (viscera and meat). Many other species have yet to be investigated.
Domoic acid poisoning symptoms begin within 30 minutes to 24 hours after eating toxic seafood. In mild cases, symptoms may include vomiting, diarrhea, stomach cramps, headache, dizziness and confusion. Difficulty breathing, seizures, coma and death can occur in severe cases. Survivors of severe cases have suffered permanent loss of short-term memory, a condition known as amnesic shellfish poisoning (ASP).
California’s Annual Mussel Quarantine
Annual Mussel Quarantine in California: May 1–October 31
The California Department of Public Health (CDPH) places a quarantine on sport harvesting of mussels for food from May 1 through October 31. This is the period when mussels are most likely to accumulate PSP toxins. The annual mussel quarantine also provides protection from domoic acid poisoning. If monitoring activities confirm unsafe levels of toxins, the quarantine may be expanded to include all bivalve shellfish in the affected area.
Local health officers enforce the quarantine. They post signs advising people of the quarantine. These signs also warn people that clams and scallops at times may contain toxins. During the quarantine period, sport harvesters should remove the viscera from clams and scallops and remove the siphons from Washington clams, and eat only the remaining white meat. Missing or destroyed signs do not change the quarantine period.
Mussels may be harvested and sold for fish bait at any time. Labels on containers of mussels for fish bait state: “MUSSELS FOR BAIT ONLY - UNFIT FOR HUMAN FOOD”
Commercially Harvested Shellfish
The annual mussel quarantine does not apply to companies licensed by the State as certified shellfish harvesters. CDPH tests and certifies the shellfish from these companies to be safe.
Oysters are the principal bivalve shellfish harvested commercially in California. Other shellfish are available in California retail food stores and restaurants throughout the year. These products come from both local and out-of-state sources. State and federal agencies regulate all commercially harvested shellfish in the United States. These regulations make certain that only safe and wholesome shellfish are available to the consumer.
Bivalve shellfish include: clams, mussels, oysters and scallops
Marine Toxin Monitoring Program
CDPH monitors marine toxins in sport and commercial seafoods year-round. This program allows CDPH to follow changes in toxin levels and to alert the public and local health agencies if necessary. When unsafe toxin levels are detected in seafoods, CDPH does not allow the affected species to be commercially harvested or sold. CDPH also issues public warnings for sport harvesters of these species.
Unsafe toxin levels sometimes occur outside the annual mussel quarantine period. CDPH declares health advisories whenever unsafe levels of toxins appear. Almost all (over 99%) of PSP cases in California have occurred during the annual quarantine months. Domoic acid poisoning has never been reported in humans in California.
Shellfish Information Line
CDPH provides a recorded current update on marine toxin activity in California. This message also reports any quarantines, health advisories, or other public warnings. The telephone number is:
Marine Biotoxin Shellfish Information Line: (800) 553-4133
CDPH updates the recorded message as needed. Callers may leave a message at this number to request more detailed information. Shellfish monitoring program reports are available on the internet at CDPH Marine Biotoxin Monitoring Reports.
Myths About Red Tides
During late spring, summer, or fall, localized patches or streamers of color appear in the ocean or bays along the California coast. These reddish areas or “red tides” appear suddenly and last from a few days to a few months before disappearing. The color of the water comes from the millions of tiny organisms in the water. Water color varies from brown to red depending on how many and what kinds of organisms are present.
Many people believe that red tides cause shellfish to be toxic. Most red tides are harmless. Toxin-producing organisms do not generally cause red tides. Although the presence of a red tide is a warning that shellfish may be toxic, the absence of a red tide does not mean they are safe to eat. Shellfish can consume enough toxic organisms to become toxic even when there is no visible red tide.
Safety Tips For Sport Harvesters
During a quarantine
- Do not gather mussels or other quarantined shellfish for food. Toxic mussels are very dangerous because people eat the entire mussel including the viscera. In toxic razor clams, the toxin is present in the meat. In toxic Washington clams, the toxin is present in the siphons.
- Remove and discard the viscera of all clams and scallops, and the siphons of Washington clams.
- When domoic acid may be present, remove the viscera of anchovies and crabs immediately after capture.
During non-quarantine periods
- Scallop viscera can remain toxic year-round. Do not eat scallop viscera at any time.
- Following a PSP outbreak, the siphons of Washington clams may retain toxins for a year or more. If this occurs, CDPH sets and publicizes special local health advisories on Washington clams.
- Razor clams retain domoic acid in their meat for over one year following a domoic acid event.
- Be familiar with the California Department of Fish and Wildlife (CDFW) sport fishing regulations. These regulations are available on the CDFW Fishing and Hunting Regulations page and in stores selling sporting goods.
- Gather shellfish only in areas free from sewage contamination. If in doubt, contact the local county environmental health agency to find out if the shellfish are safe to eat
Visit the FDA Foodborne Pathogenic Microorganisms and Natural Toxins Handbook for additional information on these shellfish toxins:
- Paralytic Shellfish Poisoning (PSP)
- Amnesic Shellfish Poisoning (ASP)
- Diarrhetic Shellfish Poisoning (DSP)
- Neurotoxic Shellfish Poisoning (NSP)
- Azaspiracid Shellfish Poisoning (AZP)
Revised by:
- Gregg W. Langlois, Senior Environmental Scientist (ret), Environmental Management Branch, California Department of Public Health
- Pamela D. Tom, Academic Coordinator Emerita, Department of Food Science and Technology, University of California, Davis
The information presented on this page is revised and expanded from the 1991 California Sea Grant publication Natural Marine Toxins by Robert J. Price, Kenneth H. Hansgen and Gregg W. Langlois
April 8, 2014
This work is sponsored in part by NOAA, National Sea Grant College Program, Department of Commerce, under grant number NA10OAR4170060, project number A/EA-1, through the California Sea Grant College Program, and in part by the California State Resources Agency.