Shellfish Poisoning Syndromes Part 2

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Nov 14, 2018
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Seawitch submitted a new Article:

Shellfish Poisoning Syndromes Part 2


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This is Part 2 of our article on Shellfish Poisoning Syndromes. We'll pick up with the fourth subtype in our list.

4. Diarrhetic Shellfish Poisoning

It’s a bit odd that this type of shellfish poisoning is labeled “diarrhetic” when ASP, PSP, and NSP all usually present with diarrhea. In any case, DSP may be caused by more than one toxin.

There’s okadaic acid (OA) and dinophysistoxin-1 and -2 and their esterified forms, referred to as the “OA group” or “OA equivalents” or “DTX equivalents.”

Okadaic acid is naturally produced by dinoflagellate (alga) species in the genus Dinophysis. This dinoflagellate has one more bit of bad news attached: these algae not only produce okadaic acid and equivalent poisons, but they also produce another poison, the pectenotoxin, of which at least 15 forms have been described.

Currently, the OA group of toxins includes pectenotoxins, but now, the European Union suggests that the pectenotoxin group (PTX) should be classified separately even though there are no definitive cases on record of PTX poisoning in humans.

I asked Dr. Rachel Grillot, an evolutionary psychologist, why a dinoflagellate would have evolved to have two toxins instead of one. “The dinoflagellate might have evolved to have a second toxin if it was beneficial to have a diversified defense strategy or have different toxins targeted against different predators. If one toxin isn’t completely effective against one predator then an animal might develop another toxin,” she said.

The current rule-of-thumb is that shellfish should not contain more than 45 µg OA-equivalents/kg shellfish so as not to exceed the ARfD of 0.33 µg/kg for a person, assuming the person weighs around 60kg. If the PTX group is examined separately, then the recommended cut off is 120 µg/kg (shellfish) PTX2 equivalents.

DSP presents with the same general constellation of symptoms as the syndromes previously mentioned: headache, diarrhea, vomiting, dizziness. The OA group causes diarrhea mainly, but the PTX group causes liver damage at least in animal models.

Note that both the OA and PTX groups of toxins are lipophilic and have high molecular weight. In other words, it won’t be easy to rid your system of them.

Yet, there are other toxins that also fall under the DSP category.

Yessotoxin is one of these. Yessotoxin is produced by the dinoflagellates Protoceratium reticulatum and Lingulodinium polyedrum. So far, 90 analogues have been identified, and this group of toxins is referred to as the YTX group or YTX equivalents. The European food safety authority has suggested shellfish should not exceed a concentration of 3.75 mg YTX-equivalents/kg shellfish, to be safe.

Yessotoxin is another lipophilic toxin of high molecular weight. Since YTX equivalents and PTX equivalents do not cause diarrhea, some researchers believe they should be classified separately. In this category it is the OA group and DTX equivalents that cause diarrhea.

Because of its effect on intracellular pathways, yessotoxin is currently being studied in Spain for its pharmacological and therapeutic potential.

Oysters on the half shell.

This is a royalty-free image from Pixabay.

5. Azaspiracid Shellfish Poisoning (AZP)

Azaspiracid Shellfish Poisoning (AZP) came on to the world stage in 1995 when eight people in Holland became ill after eating mussels imported from Ireland. At least 32 different similar or analogue toxins have been identified, so scientists refer to “AZA-equivalents” or the “AZA group.”

Azaspiracid poisoning has similar symptoms to the other syndromes noted above: headache, dizziness, vomiting, diarrhea, etc. This toxin is not as well understood as the previously mentioned ones because of its relatively new arrival on the scene.

We do know that the toxin is produced naturally by the teeny tiny dinoflagellate, Azadinium spinosum, which is smaller (12–16 µm) than any other toxin-producing algae known so far. And, again, this toxin is lipophilic, with a high molecular weight and heat stable. Current EU legislation states that the total amount of AZAs should not exceed 160 µg/kg (shellfish) AZA1-equivalents.

In animal studies, AZP does not appear to be as lethal as some of the other shellfish poisoning syndromes. However, whether some of the toxins have a synergistic effect when consumed together remains unknown.


This is a royalty-free image from Pixabay.

6. The Cyclic Imines including spirolides and gymnodimines

The cyclic imine toxins known as the SPX group are not well very known or understood. I can tell you that they are produced by at least one dinoflagellate species under the genus Alexandrium. Note that this is the same genus of dinoflagellates that produces saxitoxin, which causes Paralytic Shellfish Poisoning. At this time, I could not find information available on what symptoms these toxins cause, any treatment recommended or an Acute Reference Dose. I did find that the toxins seemed quite deadly in animal (mouse) models. So, here we have our second example of a dinoflagellate that produces two different toxins.


I had difficulty finding world statistics on shellfish poisoning. I found one study that suggests an incidence of 100 cases of shellfish poisoning per year in the UK. The UK has a population of about 66 million people.

Let’s take a look at a map of the UK. Note that the UK is surrounded by water, so eating fish and shellfish is probably a big part of the UK diet.

Map of UK

This is a royalty-free image to which I have added the names of the bodies of water surrounding the UK.

The statistics of illnesses from shellfish are not necessarily reliable for several reasons. First, self-limiting illnesses tend to go unreported or underreported. That’s when you’re sick for a few days, but stay home and treat (or not treat) yourself.

Second, if you’re sick and go to the doctor, even if the doctor thinks it may be shellfish poisoning, and thinks it looks like a duck and quacks like a duck, if there are no laboratory tests confirming the diagnosis, then this illness does not go into the public records tracking these cases.

Third, the same thing applies at the hospital: even if 67 people show up with the same symptoms and backstory, and the doctors are 99% sure they know what they’re dealing with, unless there is some of the uneaten shellfish available for testing (which there usually is not,) there is no confirmation.

The only shellfish poisoning that gets documented is when the patient is sick enough to go to the doctor or hospital and lives somewhere where a doctor or hospital is even available and the doctors specifically test for and identify one these toxins mentioned from of portion of the shellfish previously consumed.

Really, the only thing you can do if you’re worried about this, is to save in a little jar a little piece of whatever shellfish you’ve been eating, and take it home with you. Then tell your spouse or friends, if I get sick, take this little jar to the hospital with me. “I’ll keep it in the fridge for a week.”

All things considered, shellfish poisoning is relatively rare. It’s not like the common cold. I just thought the topic was interesting, so I wrote about it. Many countries including the US, Canada, the UK, and Holland, routinely test the waters for the toxins I’ve discussed and shut down fishing when the levels are high.

If I use myself as an example, at age 56, I’ve lived in three countries and traveled throughout the world. I only know of one person, one case, of an individual who got sick from eating shellfish. And I mean very sick, and she still has some neurological deficits. So, I don’t think this is a common occurrence.

If you have ever eaten shellfish, you have likely heard the old admonition to eat it only during months that end in “er.” This advice dates back at least to a cookbook from 1599. But is that advice useful?

Well, my guess is that refrigeration methods were not in 1599 what they are today, and eating any kind of meat in summer (non-“er” months) was iffy.

Also, Harmful Algal Blooms (HAB’s) also tend to happen in the summer, and avoiding them might help you to avoid eating the shellfish that sucked back lots of toxin-containing dinoflagellates. Maybe.

Another reason for this advice is that many bivalve mollusks reproduce during the summer and their flesh may have a slightly different texture and taste. You could also argue that it’s kinder for the mollusks to let them reproduce in peace.

So, to answer my own question, yeah, it might be better to eat shellfish in months ending in “er,” but that does not mean you will be forever-after protected against contracting shellfish poisoning.

It seems that there’s really nothing you can do to prevent getting this illness aside from not eating shellfish—which I am not recommending. The toxins all appear to be colorless and odorless. They are heat stable and remain stable even in freezing conditions. So, you can’t cook or freeze your shellfish and be in the clear.

There does appear to be some evidence that the toxins concentrate in the "business" portions of the mollusk, so if you take a scalpel and magnifying glass and cut away the internal organs of the critter eating only the other flesh, then you mightreduce your risk of poisoning.

We often hear about “harmful algal blooms” or HAB’s like a “red tide” and the suggestion is that the algae is more harmful to whatever shellfish eat it and then us by association. But that’s not necessarily true. Not all algae produce toxins, and there are plenty of cases of shellfish poisoning when there was no obvious HAB.

I would like to mention that a medical doctor—a gastroenterologist friend of mine--once told me that when people get a sudden bad stomach, like diarrhea and vomiting, they usually are quick to say they got food poisoning. But, he told me that it’s rarely food poisoning. He said that 90%+ of the time it’s a virus. He did, however, add that it’s better not to take anything to stop the diarrhea and vomiting at least for the first day or two unless you’re getting dehydrated because whether it’s actually a toxin or a virus or bacteria or a toxin produced by a virus or bacteria, you’re better off letting your body rid itself of what is making it sick. So, that’s my free advice on the subject.

Obviously, someone who has diarrhea and vomiting to the point of dehydration or who is experiencing numbness or dizziness or confusion or difficulty breathing or memory problems or any changes to their mental state should be seen in emergency without delay.

My last point is that when we examine shellfish poisoning, we typically think of sitting down to one bad meal of something like oysters, for example, and that you eat a big portion of them. But there are plenty of chronic cases as well. According to the literature, there is a big population of people who eat a little of these toxins daily or at least on a regular basis and teeter continuously on the brink of poisoning. These people can suffer chronic and perhaps irreparable damage.

Bon appétit.

European spider crab. Maja squinado.

This is a royalty-free image from Pixabay.


Rachel Grillot, Ph.D.

Gerssen, Arjen et al. “Marine toxins: chemistry, toxicity, occurrence and detection, with special reference to the Dutch situation” Toxins vol. 2,4 (2010): 878-904.

Farabegoli, Federica et al. “Phycotoxins in Marine Shellfish: Origin, Occurrence and Effects on Humans” Marine drugs vol. 16,6 188. 29 May. 2018, doi:10.3390/md16060188.

Meyer-Rochow, Victor Benno. “Food taboos: their origins and purposes” Journal of ethnobiology and ethnomedicine vol. 5 18. 29 Jun. 2009, doi:10.1186/1746-4269-5-18.

H Sicherer, Scott & Muñoz-Furlong, Anne & Sampson, Hugh. (2004). Prevalence of seafood allergy in the United States determined by a random telephone survey. The Journal of allergy and clinical immunology. 114. 159-65. 10.1016/j.jaci.2004.04.018.

Acute and chronic dietary exposure to domoic acid in recreational harvesters: A survey of shellfish consumption behavior. Ferriss B.E., Marcinek D.J., Ayres D., Borchert J., Lefebvre K.A. (2017) Environment International, 101 , pp. 70-79.

V Morse, E. (1978). Paralytic shellfish poisoning: A review. Journal of the American Veterinary Medical Association. 171. 1178-80.

Visciano, Pierina et al. “Marine Biotoxins: Occurrence, Toxicity, Regulatory Limits and Reference Methods” Frontiers in microbiology vol. 7 1051. 6 Jul. 2016, doi:10.3389/fmicb.2016.01051


Twiner, Michael J et al. “Azaspiracid shellfish poisoning: a review on the chemistry, ecology, and toxicology with an emphasis on human health impacts”Marine drugs vol. 6,2 39-72. 7 May. 2008, doi:10.3390/md20080004


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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.


30jan20 Rip skeeter you where wonderful dog
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Jun 19, 2016
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Springfield il
I don't eat shellfish. Personal taste and texture thing. I do use them for fish food. Are these toxins able to make one sick just by consuming them or can they absorb through the skin. Also any dangers to our fish that eat them.


Water, water everywhere,
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Nov 3, 2018
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Vancouver Island
I don't eat shellfish. Personal taste and texture thing. I do use them for fish food. Are these toxins able to make one sick just by consuming them or can they absorb through the skin. Also any dangers to our fish that eat them.
It's not clear. But since diatoms are at the source of it, this is a good reason to wear gloves and protect your face. These particular toxins from diatoms don't seem to hurt the fish, and in fact, they become more and more concentrated as it moves up the food web with bigger fishes eating smaller fishes.

Greg Gdowski

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Oct 16, 2017
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Rochester, NY
Nice set of articles with a lot of background research. That was a very nice article.
I grew up in RI in a big shellfishing area. Quahogging was the thing to do in high school. HABs are a big deal. The DEM in RI watches this very carefully. The harvesting zones are constantly opening and closing to fisherman.
There is a lot of good information on the RI DEM site.
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Dec 28, 2016
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its a great article - the abbreviations - though are too much
- -
It’s a bit odd that this type of shellfish poisoning is labeled “diarrhetic” when ASP, PSP, and NSP all usually present with diarrhea. In any case, DSP may be caused by more than one toxin.

you might have defined these in part 1 - but IMHO way too many.

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