Paralytic shellfish poisoning

PSP and cyanobacteria

PSP toxins (of which saxitoxin is the most ubiquitous) are produced in eukaryotic dinoflagellates and prokaryotic cyanobacteria (usually referred to as blue-green algae). Within the freshwater marine ecosystem, the largest contribution in the accumulation of PSP toxins derives from saxitoxin produced by cyanobacteria. The biosynthesis of saxitoxin is well-defined in cyanobacteria, while within dinoflagellates it remains mostly unknown. Cyanobacterial saxitoxin biosynthesis has been studied in radioisotope tracing experiments, and turns out to be highly complex, involving many steps, enzymes and chemical reactions. The starting reagent, L-arginine, goes through several chemical reactions (among which is a rare chemical reaction known as a Claisen condensation), going through four intermediates before resulting in saxitoxin.

The Australian freshwater mussel Alathyria condola is highly susceptible to neurotoxin accumulation. After two to three days of exposure to the cyanobacterium Anabaena circinalis it may contain upwards of 80 micrograms of neurotoxins per 100 grams of mussel, a level high enough to cause significant health risks to humans.**

Pathophysiology

PSP affects those who come into contact with the affected shellfish by ingestion. The toxins responsible for most shellfish poisonings—mainly saxitoxin, although several other toxins have been found, such as neosaxitoxin and gonyautoxins I to IV—are water-insoluble, and heat- and acid-stable. Therefore, ordinary cooking methods will not eliminate the toxins.

Symptoms typically appear within ten to 30 minutes after ingestion, and may include nausea, vomiting, diarrhea, abdominal pain, and tingling or burning lips, gums, tongue, face, neck, arms, legs, and toes. PSP can be fatal in extreme cases, particularly in immunocompromised individuals; children are known to be more susceptible.

Most shellfish can store saxitoxin for several weeks after a harmful algal bloom passes, but some, such as butter clams, can store the toxin for up to two years.

PSP in wild marine mammals

PSP has been implicated as a possible cause of sea otter mortality and morbidity in Alaska, as one of its primary prey items, the butter clam (Saxidomus gigantea) bioaccumulates saxitoxin as a chemical defense mechanism. In addition, ingestion of saxitoxin-containing mackerel has been implicated in the death of humpback whales.

Additional cases where PSP was suspected as the cause of death in Mediterranean monk seals (Monachus monachus) in the Mediterranean Sea have been questioned due to lack of additional testing to rule out other causes of mortality.

Detection and treatment

Several detection methods can be used in order to determine the concentration of saxitoxin within an organism (be it shellfish or human), both in vivo and in vitro. The most commonly used in vivo method is the mouse bioassay, which provides quantitative and qualitative data in case of a (suspected) PSP neurotoxin exposure; in vitro receptor binding assays provide equivalent data, while being animal-friendly. PSP neurotoxins can also be detected by high-performance liquid chromatography (HPLC), amongst other forms of chromatography. Shellfish containing 80 or more micrograms of saxitoxin per 100g of edible shellfish tissue are deemed to be unsafe for human consumption. Currently, there is no antidote for PSP neurotoxins. Most PSP patients suffer only minor symptoms, these lasting until the toxin is eliminated from the body. With minor exposure, spontaneous recovery can thus be expected. In the relatively rare case of clinically significant respiratory paralysis, symptomatic treatment in the form of oxygen supplementation and/or mechanical ventilation should be employed until symptoms subside.

References

References

1. (1999). "A review of selected seafood poisonings". Undersea & Hyperbaric Medicine.
2. (2003). "Manual on Harmful Marine Microalgae". UNESCO.
3. (1998). "Physiological Ecology of Harmful Algal Blooms". Springer.
4. (2001). "Are ''Pyrodinium'' Blooms in the Southeast Asian Region Recurring and Spreading? A View at the End of the Millennium". Ambio: A Journal of the Human Environment.
5. (2008). "Occurrence of paralytic shellfish toxins in Cambodian Mekong pufferfish Tetraodon turgidus: Selective toxin accumulation in the skin". Toxicon.
6. (2016-02-04). "Biosynthetic route towards saxitoxin and shunt pathway". Scientific Reports.
7. (1995-05-01). "Bioaccumulation of paralytic shellfish poisoning (PSP) toxins from the cyanobacterium ''Anabaena circinalis'' by the freshwater mussel ''Alathyria condola''". Toxicon.
8. "Paralytic Shellfish Poisoning".
9. (2013-03-27). "An Overview on the Marine Neurotoxin, Saxitoxin: Genetics, Molecular Targets, Methods of Detection and Ecological Functions". Marine Drugs.
10. (November 1989). "Sea Otter Mortality at Kodiak Island, Alaska, during Summer 1987". Journal of Mammalogy.
11. (1989). "Humpback Whales (''Megaptera novaeangliae'') Fatally Poisoned by Dinoflagellate Toxin". Canadian Journal of Fisheries and Aquatic Sciences.
12. (1998). "Did algal toxins cause monk seal mortality?". Nature.
13. (2005). "Marine Mammal Research: Conservation Beyond Crisis". Johns Hopkins University Press.
14. (2016-03-01). "Paralytic shellfish toxin biosynthesis in cyanobacteria and dinoflagellates: A molecular overview". Journal of Proteomics.
15. "Paralytic Shellfish Poisoning — Southeast Alaska, May–June 2011".