A mussel farm in Croatia.

This is a royalty-free image from Pixabay.


I’ve always been a bit leery about eating shellfish. In the 1980’s, when I lived in Europe, I ate more than my fair share of raw oysters, cooked mussels, and crab. I knew that shellfish can make people sick, and I was aware of some religious convictions that specifically forbade eating shellfish. But it wasn’t until I came across an article by some Dutch researchers, that I learned exactly why shellfish makes some people sick. And since what I learned was news to me, I decided to share the information.

I have always naively thought that there was just something inherently bad about shellfish—that perhaps shellfish spoiled faster than other proteins or that shellfish contained garbage because it was often harvested close to shore where there is a higher percentage of pollutants or maybe the problem lay in eating “dead” shellfish. I knew that in France, much is made of eating “live” raw oysters and not dead ones. As you’ll see, I was completely wrong.

An oyster farm.

This is a royalty-free image from Pixabay.


There are plenty of food taboos throughout the world. Scholars suggest that across cultures there are a lot of different reasons why food taboos exist, and one of those reasons may be that there is some real practical basis for the taboo, such as people can get sick from eating certain foods.

I’m Jewish, and my grandmother was very observant during her lifetime, so I’m familiar with the laws of Kashruth or “keeping Kosher.” One of those rules follows the quotes from Leviticus below and is, namely, that you can eat fish with fins and scales, but no other creatures from the sea (including shellfish.)

Leviticus 11:9 English Standard Version (ESV)

“These you may eat, of all that are in the waters. Everything in the waters that has fins and scales, whether in the seas or in the rivers, you may eat.”

Leviticus 11:10 ESV

“But anything in the seas or the rivers that has not fins and scales, of the swarming creatures in the waters and of the living creatures that are in the waters, is detestable to you.”

Some divisions in Islam also prohibit eating shellfish such as the Hanafi school of thought among Sunni Muslims and the Ja’fari school of thought among Shia Muslims. And you may know that observant Buddhists are vegetarian altogether. But I digress.

There are six generally recognized marine shellfish toxins and five shellfish poisoning syndromes. There are other types of shellfish poisoning, but since these other cases tend to occur less frequently and are not as well understood, I’ll leave those out.

I also want to mention in passing that the incidence of allergy to shellfish is estimated to be around 2% of the general population in the United States. Self-reported statistics run higher, but incidence of allergy verified by a physician are in the 2% range. Unfortunately, we don’t know whether this percentage of the public is allergic to crustaceans or mollusks or both because “shellfish” denotes both crustaceans and mollusks.

Shellfish poisoning occurs most often with the bivalve molluks, including, oysters, clams, mussels, geoducks, scallops, cockles, and any other 2-shell mollusks with a hinge. Bivalvia is a class within the phylum Mollusca. And while most of what we are discussing here relates to marine bivalves, there are also freshwater bivalves within this class and which can also cause Shellfish Poisoning Syndromes. And these syndromes also occur from eating other non-mollusk crustaceans.

Cockles in Thailand.

This is a royalty-free image from Pixabay.

Shellfish Poisoning Syndromes

1. Amnesic Shellfish Poisoning (ASP)

ASP comes typically from eating bivalve mollusks like oysters, clams, or mussels, but there have been documented cases from other crustaceans. So, you’re not in the clear even if you stick to crab legs.

It’s caused by a toxin, domoic acid or DA, produced by certain diatoms. There are 10 known diatoms that produce DA, but Pseudo-nitzschia pungens seems to be the most common species associated with ASP. Diatoms naturally produce this toxin, and in high population areas where shellfish is routinely collected for consumption, scientists regularly test the water for the presence and levels of domoic acid. And for the uninformed, diatoms are algae, a tiny one-celled alga.

The Dutch research paper cited below said that “The European Union (EU) has established a permitted level of 20 mg DA/kg shellfish.” The Acute Reference Dose (ARfD) is 0.075 mg DA/kg bodyweight per day. Or that’s how much domoic acid you need in your body--the threshhold--to get sick.

While ASP may have existed since time immemorial, it did not reach the public stage until 1987 when a lot of people got sick in eastern Canada on Prince Edward Island (one of the provinces). A hundred people were hospitalized and three people died during that episode after eating blue mussels, Mytilus edulis. Now that this syndrome has a name and made the news, occurrences of ASP have been reported worldwide.

Domoic acid is a neurotoxin that seems to primarily affect the hippocampus in humans. So, while those afflicted with the syndrome may exhibit a dazzling array of awful symptoms such as diarrhea, vomiting, headaches, and disorientation, what makes this syndrome stand apart is the effect of domoic acid on memory. After an episode of ASP, those poisoned may have permanent short-term memory loss.

There is no antidote for domoic acid poisoning, and the poison is heat-resistant and very stable. Domoic acid can also cause kidney damage at lower thresholds than is required for brain damage. So, hospital care is mainly supportive, for example, to keep the patient from getting dehydrated, protect the airway, and manage seizure activity if present. Animal studies suggest that treatment with valproate (an anti-seizure drug) and pyridoxine (vitamin B6) may decrease the effect of domoic acid and are worth a try in severe cases.

If there is any good news to report on ASP, it’s that domoic acid is of low molecular weight—below 500 daltons—and hydrophilic, or water soluble. For that reason, the poison is likely to be excreted faster than the lipophilic poisons we’ll address later.

As an anecdote, Alfred Hitchock’s 1963 film, The Birds, is believed to have been inspired by an incident in southern California in August of 1961. Thousands of “crazed” birds were flying into walls and acting weird. It is believed that the birds were poisoned by high levels of domoic acid in North Monterey Bay and suffering from a bird version of ASP.


This is a royalty-free image from Pixabay.

2. Paralytic Shellfish Poisoning (PSP)

The next subtype of shellfish poisoning is PSP caused by saxitoxins from dinoflagellates of the Alexandrium genus and others. Saxitoxins are also produced by some freshwater cyanobacteria.

There are several saxitoxins that closely resemble each other chemically, so for the purposes of dealing with poisonings, scientists refer to “saxitoxin equivalents.”

The European Union has established a permitted level of 800 µg saxitoxin 2-HCl equivalents/kg shellfish, although some research suggests a much lower level of 75 µg saxitoxin 2-HCl equivalents/kg shellfish would be more appropriate. The Acute Reference Dose (ARfD) is 0.5 mg µg saxitoxin 2-HCl (or STX)/kg bodyweight per day.

Saxitoxins are again neurotoxins. Poisoning presents with all the horrid symptoms noted in ASP except for the memory loss, but with added muscle weakness, tingling, numbness and paralysis. The medical treatment is similarly supportive in nature with Benzedrine being helpful in some cases.

Interestingly, in the Philippines, in remote areas, PSP is treated with a drink made of coconut and brown sugar, and a study done in 1992 seems to show that this cocktail may, in fact, have some detoxification value (Viviani, 1992).

Saxitoxin poisoning or PSP is the best known of the four types of shellfish poisonings and has been written about and discussed since the 1920’s. Because of its very low LD-50 (lethal dose required to kill half—or 50%--of the tested group in an experiment) saxitoxin has been weaponized in the past. Supposedly it isn’t being used for that anymore. Supposedly is the operative word.

Saxitoxin equivalents are also of low molecular weight, water soluble and heat-resistant.


This is a royalty-free image from Pixabay.

3. Neurologic Shellfish Poisoning (NSP)

NSP is caused by brevetoxin, another naturally occurring neurotoxin produced by Karenia dinoflagellate (algae) species and possibly others. It is known mostly in the United States and New Zealand. There are, again, several closely related brevetoxins, so scientist refer to “brevetoxin equivalents.”

The current regulatory limit is 800 µg brevetoxin-2 (PbTx-2) equivalents/kg shellfish. The Acute Reference Dose (ARfD) is estimated to be 2.8-4.8 μg BTX/kg b.w.

There is not as much data available for NSP because it’s not as common as some of the other syndromes.

NSP and the brevetoxin equivalents have a few issues that make them somewhat different and worse than ASP and PSP. First, brevetoxins are lipophilic. That means they are not soluble in water and they have a higher molecular weight (above 600 daltons). For practical purposes, this means that the poison is stored in fatty tissues and is not excreted as fast as the water-soluble ones. It’s harder to get it out of your system.

Let me use the analogy of vitamins. Vitamin C is water soluble. It isn’t stored by the body, so whatever you don’t use during the day is excreted. Vitamin A, in contrast, is fat soluble. That means that what you don’t use, you store. And if you store way too much, it’s there for a long time and not so easy to get rid of and can make you really sick.

As an aside, in 1913, when Douglas Mawson and Xavier Mertz were exploring Antarctica, they were starving and ate their sled dogs. Dog liver is high in vitamin A, and when Mawson and Mertz became extremely ill, some suggested it was the overdose of vitamin A. New research debates that, however. Mertz did not survive the expedition. Mawson did survive and lived until 1958.

My point is that a poison that is lipophilic is bad news.

The second issue is that you can get poisoned from brevetoxin just from breathing it in. You don’t have to eat 400 clams to get sick. All you have to do is stand on the beach where the waves are breaking and breathe in that delightful sea breeze. And if that breeze is full of brevetoxin, you can become very ill. Or die.

Brevetoxin poisoning presents not unlike the previous three I’ve covered: headache, vomiting, diarrhea, dizziness, etc. What’s different about brevetoxin equivalents is difficulty breathing, especially if you’ve breathed it in, and changes in heart rhythm/function/pattern. Those with NSP often need respiratory support faster than ASP and PSP.

This is the end of Part 1. Stay tuned for Part 2 and the conclusion of Shellfish Poisoning Syndromes, which we will post tomorrow. Exciting stuff!

A very large, perhaps, record-breaking, 10-pound (bivalve) geoduck clam from the Puget Sound. The geoduck has the longest siphon known (to date) in clams.

This photos is courtesy of Jmabel. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License.

References will be noted at the end of Part 2.


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Author Profile: Cynthia White

Cynthia received her BA in English from NYU a long long time ago. She has been a freelance writer and editor for over 20 years. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch.