Saltwater algal blooms can give rise to harmful toxins that accumulate in filter-feeding mollusks, like mussels, clams, and oyster. While these toxins may be harmless to our shellfish friends, they can be particularly harmful, even deadly, to the humans that eat them. It’s taken a while, but we’re finally at the last installment of shellfish poisoning. To recap, here are the four shellfish poisoning syndromes, with links to the previous posts:
- Amnesic shellfish poisoning (ASP)
- One of the best! Domoic acid not only causes anterograde amnesia – the ability to form new memories after exposure, but can be lethal. Alfred Hitchcock observed the bizarre behavior of a flock of domoic acid poisoned birds and used it as inspiration for his classic movie “The Birds.”
- Diarrheal shellfish poisoning (DSP)
- Okadaic acid: spend 3 days on the toilet after ingesting algae bloom affected oysters. As little as 0.048 milligrams of okadaic acid is all that is needed to bring about nausea, vomiting, GI distress, and all day toilet marathons.
- Paralytic shellfish poisoning (PSP)
- I grew up with red tides in the Pacific Northwest and its accompanying PSP caused by saxitoxin. Symptoms start with a tingling of the lips but can progress to total paralysis and even death…from one clam. Saxitoxin is one of the more potent, and lethal toxins known.
- Neurotoxic shellfish poisoning (NSP)
- You’re on this page, so if you want to read about how Mother Nature has weaponized toxins, keep reading.
Not all red tides are created equal. In the Pacific Northwest, red tides are primarily due to the explosive growth of the dinoflagellate genus Alexandrium, which produce saxitoxin, the toxin responsible for paralytic shellfish poisoning (PSP). On the kitty-corner side of the country, red tides in the Gulf of Mexico and up the southeastern coast of the United States are chiefly due to the dinoflagellate Karenia brevis.
Blooms of K. brevis occur every year off the west coast of Florida in the late summer and early fall months. These noxious blooms kill off large numbers of fish and birds, and appear to be more extreme and occurring more frequently. Whether climate and ocean temperature change are responsible is debatable, as are human interventions and pollution, but it is important to note that red tides in Florida have been recorded as early as 1844, long before notable human development and pollutions. Regardless of the cause, K. brevis blooms pose a significant human health risk in the form of neurotoxic shellfish poisoning (NSP). (1)
Most dinoflagellates have a thecal plate made of cellulose that is essentially armor plating. Those dinoflagellates are hardcore. K. brevis? They swim around naked. They’re fragile. As the waves break upon the shores, our delicate dinoflagellates rupture, spewing their toxin. These toxins are harmful to fish, leading to massive fish die-offs, but are inert to shellfish. Filter-feeding mollusks like clams and oysters continuously take up these toxins and store them within their tissues. When they are harvested and eaten by humans and other animals, they become intoxicated and feel its effects. Neither freezing or cooking has any effect on the toxin.
But ingestion is only a small part of the problem. Of particular risk to humans is the aerosolization of toxins. As K. brevis breaks up on the shore, its toxin is taken up by the salt air and winds, spreading it throughout. In this way, Mother Nature has weaponized a toxin in her arsenal and subjected it to unsuspecting humans. And the toxin? Brevetoxin.*
*The total synthesis of brevetoxin was seen as the “Holy Grail” of natural product synthesis, and the success by K.C. Nicolaou – or rather dozens of graduate students and postdocs – was widely celebrated. It’s cool and all I guess, but at over 100 steps with a total yield of about 0.0005%, it’s certainly not practical. (2)Brevetoxins bind to voltage-gated sodium channels – membrane proteins that allow the flow of sodium ions through the cell’s plasma membrane, generating an action potential, which is essentially an electrical current across the cell – and keeps them activated. The constantly open sodium channel allows sodium ions to flow freely through the cell membrane, allowing nerves and muscles to spontaneously fire, as if in a constantly excited state. This is in contrast to saxitoxin, of PSP fame, in which saxitoxin blocks sodium channels, keeping them in a persistently closed state, leading to paralysis.
Signs of brevetoxin intoxication include a range of both neurological and gastrointestinal symptoms. Neurologically, victims report the loss of body control (ataxia), slurred speech, and disorientation. On the gastrointestinal side, there is the trifecta of nausea, vomiting, and diarrhea. Symptoms usually occur within hours after the consumption of contaminated shellfish and may last several days. On the plus side, no one is known to have died from brevetoxin exposure, and people recover completely in a few days. Treatment is general supportive care, including fluid repacement and sedatives as necessary. Exposure due to aerosolized brevetoxin presents a little differently, with symptoms primarily consisting of eye and throat irritation, nasal congestion, and headaches. But these too, like with ingestions, will go away on their own without any lingering effects. (3, 4)
Aerosolized brevetoxin is what I find to be so cool about NSP, it’s like Mother Nature is using chemical warfare against us. Imagine taking a vacation to Florida and walking along the sandy beach, only to start having itchy, watery eyes and a massive headache! Mother Nature is certainly out to get us, so be on you guard.
** Featured image of Dead Fish after Red Tide by Judy Baxter (CC BY-NC-SA 2.0) **
- Kirkpatrick, Barbara, et al. “Literature Review of Florida Red Tide: Implications for Human Health Effects.” Harmful Algae 3.2 (2004): 99-115.
- Nicolaou, K. C., et al. “Total Synthesis of Brevetoxin A.” Nature 392.6673 (1998): 264-69.
- Watkins, Sharon M. “Neurotoxic Shellfish Poisoning.” Marine Drugs 6.3 (2008): 430-55.
- Backer, Lorraine C., et al. “Occupational Exposure to Aerosolized Brevetoxins during Florida Red Tide Events: Effects on a Healthy Worker Population.” Environmental Health Perspectives 113.5 (2005): 644-49.