Aetiology
Marine toxins are found in a variety of marine species.
Saxitoxin
Saxitoxin comprises a group of neurotoxins that produce symptoms through the blockade of voltage-gated fast sodium channels and inhibition of signal propagation in neural tissue.
Produced by dinoflagellates, a type of phytoplankton, poisoning is caused by ingesting hinged-shelled shellfish that ingest the phytoplankton, especially dinoflagellates of the genus Alexandrium, and concentrate saxitoxin in their system.[13] Saxitoxin can also be produced by cyanobacteria and can be found in the flesh of certain xanthid crabs.
Saxitoxin commonly accumulates in the flesh of shellfish during harmful algal blooms and cannot be destroyed through cooking. Consumption of saxitoxin-contaminated shellfish produces the paralytic shellfish poisoning syndrome.
Saxitoxin is a potent neurotoxin 3 orders of magnitude more toxic than sodium cyanide (Lethal Dose 50: saxitoxin 10 microgram/kg, cyanide 10 mg/kg).
Tetrodotoxin
Tetrodotoxin exerts its effects by blocking voltage-gated fast sodium channels, halting nerve conduction and muscle action potentials. Poisoning can lead to progressive paralysis and death from respiratory failure.
It is a nonprotein, alkaloid toxin that is heat and acid stable. It is found in many species of the puffer fish family (Tetraodontidae) as well as in certain gobies, amphibians, invertebrates, and the blue-ringed octopus. Consumption of puffer fish requires clear identification of the appropriate species of fish and meticulous preparation of the meal to be safe.
Most commonly reported following consumption of puffer fish or contact with the blue-ringed octopus. Over 20 species of puffer fish have been found to harbour tetrodotoxin.[14] The amount of tetrodotoxin accumulated in the puffer fish depends on the species and season, but it is most concentrated in the liver, ovary, and intestine.[15] Envenomation from the blue-ringed octopus comes from the saliva via a painless bite.[16] Tetrodotoxin has also been detected in gastropods, newts, crabs, and ribbon worms.
Conotoxin
Conotoxins are a group of toxins in the venom of cone snails.[8]
Conotoxin venom is produced from predatory cone snails of the genus Conus. This genus is composed of over 500 species of snails, each with its own distinct venom containing more than 100 venom components. Envenomation occurs when venom is injected through a single hollow tooth during a defensive strike.[17] There is a linear correlation between the size of the cone snail shell and the quantity of venom injected.
Analyses of Conus geographus venom identified numerous conotoxins showing potent paralytic effects involving a wide array of ion channels, including voltage-gated sodium channels, presynaptic calcium channels, postsynaptic sodium channels, and muscle-type nicotinic receptors.[18] There are an estimated 50,000 different active components in the various venoms of cone snails.[17] Conotoxins have high target specificity for specific ion channels, providing therapeutic potential for pain control and cardioprotection, among other human conditions.
Pathophysiology
Intoxication from marine toxins results from ingestion of or direct contact with contaminated seafood or venomous creatures.
Both tetrodotoxin and saxitoxin are ingested and rapidly absorbed through the oral mucosa and gastrointestinal system.[19] Once in the blood stream, tetrodotoxin and saxitoxin block sodium channels in excitable cells of the heart, muscle, and peripheral nervous system.[14] These toxins inhibit sodium influx into nerve axons and prevent nerve impulse propagation, resulting in progressive paralysis.[20]
Conotoxins are a group of toxins in the venom of cone snails.[8] Conotoxin contains more than 100 components that act on a wide variety of receptors. Following envenomation, these components frequently result in a syndrome of severe pain due to activity with potassium and calcium channels and nicotinic acetylcholine receptors.[21] Similar to tetrodotoxin, conotoxin contains active compounds that bind to voltage-gated sodium channels and can result in progressive paralysis.
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