Site Identifier (placeholder replace)
Food Safety Home
Section Link 2
Practice Personal Hygiene
Section Link 4
Avoid Cross-Contamination
Keep Foods at Safe Temperatures
Avoid Risky Foods and Water
Food Preservation
Resources in Spanish
Current Projects
Food Safety Links
Bugs that Make You Sick
Special Precautions for High-Risk Groups


Gathering Safe Shellfish in Washington

by: Val Hillers, Ph.D. , Extension Food Specialist


Hotlines for Gathering Shellfish:
Washington 1-800-562-5632 (toll-free)
British Columbia 1-604-666-3169 (not toll-free)

Gathering Safe Shellfish in Washington

Washington waters offer a delectable variety of clams, oysters, mussels, and scallops readily available to be gathered and enjoyed. At certain times, however, some shellfish become unsafe to eat because they contain poisons harmful to humans.

The poisons or toxins which are occasionally found in shellfish from the Pacific Northwest are paralytic shellfish poisoning toxin and domoic acid. These toxins are produced by microscopic marine algae. Shellfish become toxic by feeding on these algae and accumulating the toxin in their bodies.

Paralytic shellfish poisoning (PSP) and amnesic shellfish poisoning (from domoic acid) are the names of the illnesses that result from consuming the toxins.  

Paralytic Shellfish Poisoning

Paralytic shellfish poisoning (PSP) is a serious illness caused by eating shellfish that have consumed large amounts of a poison-producing microscopic organism calledAlexandrium catenella. TheAlexandrium toxins are extremely potent nerve poisons; in fact, as little as one milligram (0.000035 ounce) is enough to kill an adult. The poisons themselves, as well as the illness they cause, are referred to as PSP. The poison acts very rapidly, and no antidote has as yet been discovered.  The toxin is not affected by freezing or cooking.

Early symptoms of PSP are a tingling of the lips and tongue, which may begin within minutes of eating poisonous shellfish or may take and hour or two to develop. Depending on the amount of toxin a person has taken in , symptoms may progress to tingling of fingers and toes and then loss of control of arms and legs, followed by difficulty breathing. Some individuals have a sense of floating, while others are nauseated. If a person consumes enough poison, death can result from paralysis of the breathing mechanism in as little as 15 minutes. Worldwide, approximately 15 percent of the reported cases of PSP have resulted in death.

You should be careful to distinguish the symptoms of PSP from two other types of illness sometimes caused by eating shellfish--allergic reactions and illnesses resulting from bacteria or viruses the shellfish may pick up from the water.  (Thorough cooking of shellfish will kill most bacteria and viruses but will not break down the PSP toxin.)

It is essential to begin treating PSP immediately -- as soon as the lips or tongue begin to tingle, because the poisons can take effect rapidly and, as indicated, there is no known antidote for them. Induce vomiting and give a brisk laxative to remove the toxic shellfish from the digestive tract.Get the patient to a doctor at once. If this is not possible, prepare to administer artificial respiration, which may be required for many hours.

PSP Occurrences
The accumulation of PSP toxins in shellfish is not a new phenomenon, nor is it one confined to Washington. It has been occurring for hundreds of years in many parts of the world, primarily in temperate waters. Along the Pacific Coast, poisonous shellfish have been found from Alaska to California. The first recorded death in this region occurred in 1793 when one of Captain Vancouver's men died after eating toxic shellfish in British Columbia. Indian tribes were undoubtedly aware of the problem long before that, however.

PSP Seasons
In general, shellfish are more likely to accumulate PSP toxin in late spring, summer, and fall than in winter. However, some species -- particularly butter clams and scallops -- tend to be toxic for longer periods extending into winter or even throughout the year in some areas. In spring, longer days and warmer waters encourage faster growth of the swimming state ofA. catenella. Furthermore, the longer periods of calm weather and relatively calm seas from spring through fall allow the cells, which swim upward during the daytime and down at night, to reproduce rapidly, causing a population explosion. When enough of these cells are present in a given location, the shellfish filtering them out for food become poisonous.

Shellfish filter water at remarkable rates; for example, a large oyster can filter as much as 30 liters (about 8 gallons) per hour. The more abundant theA. catenella cells, the faster the toxin content of the shellfish increases. The warming of the water also permits the shellfish to feed faster. All of these factors combined may lead to PSP levels that require closures -- just when the first good clamming tides of spring occur, causing disappointment and frustration for eager diggers.

At times, for reasons not yet understood, some of the swimming cells ofA. catenella form non-swimming resting cells or cysts, which settle to the bottom. Scientists believe that these cysts are at least as poisonous as the swimming cells and perhaps even more so. If large numbers of the cysts are present in the sediment, clams and other shellfish may consume them at any time of the year and become or continue to be poisonous.

Shellfish Subject to PSP
Unfortunately, all species of bivalve shellfish (clams, oyster, mussels, and scallops) commonly eaten in Washington have been found to contain poisons at some time. Different species vary considerably, however, in the rates at which they become toxic, in the total amounts of toxin they take up, and in the speed with which they get rid of it.

PSP and Red Tides
There is much misunderstanding about the relationship between red tides and poisonous shellfish -- including a widespread tendency to equate the two. This misconception has led to the dangerous false assumption that shellfish are safe to eat if no red tide is visible. On the other hand, some people in Puget Sound believe that because they have eaten clams safely for years even when red tides were present, they can continue to do so.

The term "red tide" is a misnomer since red tides are not tides at all, and many of them are not even red. Furthermore, "red tide" is widely used (even in dictionaries and on some beach closure signs) to indicate the presence of poisonous shellfish, when, in fact, only a very small percentage of the visible red tides cause shellfish to be unsafe. In Washington, most outbreaks of poisonous shellfish occur when there has been no visible discoloration of the water at all.

The term "red tide" is used to refer to an area of discolored water -- usually amber, brown, purple, red, or pink -- that is formed by accumulations of very large numbers of microscopic plants or animals, hundreds of thousands per liter of water. Often the organisms forming a particular red tide belong to one species. A discolored area or red tide may be confined to relatively small patches, or it may cover several acres or even many square miles of the sea.

In the Pacific Northwest, there are many species of plankton (small plants or animals carried along by currents) that cause red tides. Of the many species that form red tides in Washington water,A. catenella is the only one known to cause PSP.

A. catenellacan discolor water, with the color varying from the color of weak tea to rusty-red. However, visible red tides caused byA. catenella are unusual in Washington water. Since shellfish filter great volumes of waterA. catenella does not need to be very dense for the shellfish to collect enough poison to require that beaches be closed to harvesting. In fact, most shellfish PSP from Washington to Alaska occurs whenA. catenella is relatively sparse, not nearly dense enough to discolor the water.

Keep in mind, then, that, although a red tide may indicate that shellfish are toxic, it is dangerous to assume that lack of a red tide means that shellfish are safe to eat. Determining which shellfish gathering areas in Washington are safe or unsafe is the responsibility of the state and/or counties and their recommendations should be followed.

Amnesic Shellfish Poisoning

The human illness known as "amnesic shellfish poisoning" (ASP) is caused by eating shellfish containing domoic acid. Domoic acid is a naturally occurring toxin produced by microscopic marine algae calledPseudo-nitzschia. Shellfish and possibly other marine organisms can become toxic to wildlife and humans by feeding onPseudo-nitzschia algae or by feeding on organisms that have already accumulated the toxin.

Gastrointestinal problems such as vomiting, nausea, diarrhea and abdominal cramps may occur with 24 hours of ingestion of domoic acid. In more severe cases, neurological symptoms may develop within 48 hours and include severe headache, dizziness, confusion, difficulty in breathing, loss of short-term memory, and seizures. Severe cases of domoic acid toxicity can cause permanent brain damage or death. There is no antidote for the poison.

The toxin is not known to be affected by freezing or cooking.

ASP Occurrences
The first documented outbreak of ASP occurred off Prince Edward Island in eastern Canada in 1987. Over 100 illnesses and 4 fatalities were attributed to the consumption of cultured blue mussels contaminated with domoic acid. Since that time, the coast of Maine and of Prince Edwards Island has been monitored for domoic acid. Domoic acid has been found every year in Canada. Low levels have been found at times in some species in Maine. No further outbreaks of ASP have resulted in eastern Canada.

Domoic acid was first detected on the West Coast in California in September 1991 when pelicans and cormorants were poisoned after feeding on contaminated anchovies.  Later that year, razor clams on the Oregon and Washington coast became contaminated with domoic acid.

Whenever the Washington Department of Health detects domoic acid in shellfish, the affected areas are closed to shellfish gathering.

Monitoring Programs in Washington

The Washington Department of Health began testing shellfish for PSP in the 1930's, when there were many PSP illnesses and deaths in California. Testing for domoic acid started in 1991. Warnings about poisonous shellfish on Washington's beaches are provided by the Marine Biotoxin Monitoring Program. The Washington Department of Health is responsible for all testing of shellfish for both recreational harvest and commercial harvest.  The state works with local health jurisdictions, tribes and volunteer organizations to collect samples on a routine basis.  The state closes beaches to recreational digging when high levels of toxin are found in shellfish.  Commercial harvesting is also closed when high levels of toxin are found in the shellfish of commercial growing areas.  These closures apply for both PSP and domoic acid.

Safety Precautions
Should you give up the enjoyment of gathering your own shellfish and just hope that the problems will eventually vanish? Such a course is not necessary. Avoid all do-it-yourself methods of trying to determine whether shellfish are poisonous and instead use the state and county warning system to find times and places to gather safe shellfish.

Please remember -- You can't rely on the color of the water to indicate the presence of PSP or domoic acid since shellfish may be unsafe even if there is no discoloration of the water.

You can't tell if shellfish are toxic
  • By examining them, since poisonous shellfish do not look, taste, or smell any different from non-toxic ones.

  • By cooking them with a clove of garlic or a silver spoon to see if the garlic or spoon turns black

  • By using a field or home testing kit since no reliable kit has yet been developed

  • By using a "sample and see" method since a single shellfish will occasionally contain enough poison to kill an adult, and even if a single one does not cause symptoms, a whole meal could contain a potentially lethal dose

So please --
  • Do remember that the PSP and domoic acid programs carried on by the state and counties offer the best information about areas where shellfish should not be harvested.

  • Do call the hotline or check the website before gathering shellfish. (Contact information is listed below)

  • Do observe all beach closures.

The State of Washington maintains a hotline with current information on closures for recreational gathering of shellfish as set by counties. The recording are updated whenever changes in closures are made. You are urged to take advantage of this service and call a hotline numbereach time before gathering shellfish. The hotline numbers for Washington and British Columbia are listed below.  Information from the Washington Department of Health is available
Washington 1-800-562-5632 (toll-free)
British Columbia 1-604-666-3169 (not toll-free)



Contact 509-335-3843 |Accessibility |Copyright |Policies
646376 Department of FSHN,Washington State University, Pullman, WA 99164-6376 USA