Introduction to Seafood Toxins and Fish & Shellfish Poisonings
Seafood toxins and poisonings occur due to the consumption of marine fish, bivalve shellfish, and freshwater fish that contain harmful toxins responsible for causing foodborne illnesses worldwide.
Fish are highly nutritious and play an important role in maintaining a healthy life as they are rich in omega-3 fatty acids, which are essential for heart health and improving brain and eye functions.
Despite their nutritional benefits, seafood can act as a vector for various foodborne illnesses, including:
Amnesic Shellfish Poisoning (ASP)
Ciguatera Fish Poisoning (CFP)
Diarrhetic Shellfish Poisoning (DSP)
Azaspiracid Shellfish Poisoning (AZP)
Neurotoxic Shellfish Poisoning (NSP)
Paralytic Shellfish Poisoning (PSP)
Puffer Fish Poisoning
Pfiesteria Toxin Poisoning
Cyanobacterial Intoxication
Scombroid (Histamine) Fish Poisoning
Other finfish and shellfish toxins
Seafood poisoning is also linked with bacterial intoxications such as staphylococcal poisoning, Vibrio infections, and botulism, along with certain viral infections.
Over the past five decades, seafood consumption and supply have been increasing at an annual rate of 3.2%, making aquaculture one of the fastest-growing animal food-producing sectors globally.
China and other Asia–Pacific regions are among the most affected areas, where seafood toxins cause significant gastrointestinal and neurological illnesses.
Due to increased importation of seafood from endemic to nonendemic areas, more cases of seafood poisoning are now being reported in previously unaffected regions.
Several contributing factors are responsible for the growing incidence of seafood poisonings, including adventurous eating habits, climate change, coral reef destruction, and the widespread occurrence of toxic algal blooms.
Causes and sources of seafood toxicity
Certain dinoflagellate species are responsible for producing shellfish toxins that contaminate bivalve shellfish such as mussels, clams, and oysters. These shellfish can accumulate toxins without being harmed themselves, as they possess special mechanisms that protect them from toxic effects, but they become poisonous when consumed by humans.
Shellfish and some finfish species can also become toxic due to contamination by spoilage-causing bacteria or by feeding on toxic algae, which leads to human foodborne illnesses upon consumption.
Blue-green algae (cyanobacteria) and Pfiesteria are known to occasionally cause waterborne poisoning in both animals and humans.
Marine biotoxins accumulate through the food chain when small fish consume toxigenic algae, leading to toxin buildup in their skin, viscera, blood, and organs. Larger fish then feed on these smaller ones, perpetuating the accumulation of toxins through successive levels of the food web.
Brevetoxins, responsible for neurotoxic shellfish poisoning, cannot be removed or reduced by rinsing, cleaning, cooking, or freezing, nor can they be detected by taste or smell. This increases the risk for consumers who unknowingly eat contaminated seafood.
Harmful Algal Blooms (HABs) facilitate the growth of bacteria that cause seafood poisoning and have been rapidly increasing over the past five decades, posing threats to animals, humans, and the environment.
The rise in global seafood imports has heightened the risk of seafood poisoning among humans. Consequently, countries such as the United States, Canada, and others have developed improved strategies to assess risks and characterize hazards associated with seafood toxicity.
Shellfish Toxins
Amnesic Shellfish Poisoning (Domoic Acid):
Caused by consumption of shellfish contaminated with domoic acid, leading to gastroenteritis and nervous system disorders.
Domoic acid is produced by diatoms (Pseudo-nitzschia spp.) and is found in mussels, razor clams, and crustaceans.
Geographical distribution includes Northeast Canada, the U.S. Northeast and West coasts, Europe, Australia, and New Zealand.
Symptoms appear about 24 hours after ingestion and include nausea, vomiting, abdominal cramps, and diarrhea.
Neurological symptoms include headaches, seizures, hemiparesis, ophthalmoplegia, and memory loss, which may progress to coma.
Gastrointestinal symptoms resolve within 1–2 days, but neurological deficits such as memory loss and motor neuropathy may persist.
Treatment is supportive, focusing on seizure control and reducing brain lesions.
Ciguatera Fish Poisoning (CFP):
Caused by ciguatoxin and maitotoxin produced by the dinoflagellate Gambierdiscus toxicus.
Found in large carnivorous tropical and subtropical reef fish such as barracuda, grouper, moray eel, snapper, jack, and seabass.
It is one of the most common seafood-associated illnesses, with around 20,000 cases reported annually worldwide.
Outbreaks occur in Hawaii, Puerto Rico, the Virgin Islands, the U.S., and tropical/subtropical regions of the Pacific, Indian, and Caribbean seas involving about 400 fish species.
Symptoms appear 12–18 hours after ingestion and involve gastrointestinal (diarrhea, nausea, vomiting, abdominal pain), neurological (dizziness, numbness, tingling, metallic taste, blurred vision, reversal of temperature sensation), and cardiovascular effects.
Severe cases may cause paralysis or death, though the fatality rate is usually <1%.
Treatment involves supportive care for respiratory and cardiovascular functions; mannitol IV may help with acute symptoms, while amitriptyline or tocainide may relieve chronic symptoms.
Diarrhetic Shellfish Poisoning (DSP):
Occurs after consuming toxic mussels, scallops, or clams contaminated with okadaic acid produced by Dinophysis acuminata.
Reported mainly in Japan, Northern Europe, South America, South Africa, Southeast Asia, and New Zealand.
Causes gastroenteritis with severe diarrhea, nausea, vomiting, abdominal cramps, and fever.
Onset occurs within 30 minutes to a few hours after ingestion.
Treatment is symptomatic, and full recovery typically occurs within three days.
Paralytic Shellfish Poisoning (PSP):
A life-threatening illness caused by saxitoxin accumulation in bivalve mollusks (clams, mussels, oysters), gastropods, chitons, starfish, and crustaceans.
Saxitoxin is produced by Gonyaulax (a dinoflagellate genus) and blocks sodium ion channels in nerve and muscle cells.
Outbreaks occur in North America, Europe, Japan, South Africa, Indonesia, New Zealand, and South America.
Symptoms appear within 30 minutes, starting with prickling sensations on lips, tongue, and fingertips, followed by numbness, floating sensations, and paralysis.
Respiratory system paralysis can occur, leading to death from respiratory failure in severe cases.
Fatality rate is around 8.5%, with a lethal dose of 2–4 mg for humans.
Survivors recover completely without chronic effects.
Treatment includes artificial respiration and supportive medical care, as no antidote exists; immediate hospitalization is necessary.
Neurotoxic Shellfish Poisoning (NSP):
Caused by brevetoxin produced by the dinoflagellate Ptychodiscus brevis (associated with red tides).
Transmitted through consumption of contaminated bivalve mollusks.
Symptoms appear within 1–6 hours, including paresthesia, temperature reversal, bronchoconstriction, rhinorrhea, conjunctivitis, ataxia, nausea, vomiting, and diarrhea.
Reported mainly in the Gulf of Mexico, Caribbean Sea, Florida, North Carolina, and New Zealand.
Symptoms are mild and self-limiting; treatment is supportive, and no deaths have been reported.
Pufferfish Poisoning (Tetrodotoxin):
Caused by tetrodotoxin produced by bacterial species such as Alteromonas and Vibrio.
Symptoms appear within 6 hours of ingestion and include perioral paresthesia, nausea, dizziness, weakness, numbness, ascending paralysis, slurred speech, respiratory failure, gastrointestinal pain, and cardiac arrhythmias.
Other symptoms include profuse sweating, salivation, hypothermia, headache, tachycardia, and hypotension.
Commonly associated with consumption of pufferfish, fugu, porcupine fish, and ocean fish.
Distributed worldwide, especially in Japan and the Indo-Pacific region.
No antidote is available; treatment involves maintaining respiration and supportive care.
Severe cases may result in loss of reflexes and acute lung dysfunction.
Scombroid Fish Poisoning (Histamine Poisoning):
Caused by ingestion of inadequately chilled, histidine-rich fish such as tuna, mackerel, mahi-mahi, and skipjack.
The toxin involved is histamine, formed during improper storage or handling.
Reported in Japan, Canada, the U.S., England, and other regions worldwide.
Symptoms occur within 10–90 minutes of ingestion and resolve within 12 hours.
Signs include facial flushing, itching, rash, nausea, vomiting, dizziness, abdominal pain, blurred vision, faintness, respiratory distress, and shock.
The illness is usually mild and self-limiting; hydration and electrolyte replacement aid recovery.
Antihistamines are the primary treatment, and drugs like isoniazid or monoamine oxidase inhibitors may be used in some cases.
Detection methods of Seafood toxins
Bioassay techniques using animals such as mice, cats, mongooses, and brine shrimp are employed to detect seafood toxins through enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay (RIA) methods.
Gastric lavage using 2 liters of 2% sodium bicarbonate can help remove the toxin within three hours of ingestion, and activated charcoal in 70% sorbitol solution may be administered to absorb residual toxins.
Clinical diagnosis is primarily based on the patient’s history of seafood consumption and observation of characteristic symptoms.
Marine biotoxins can be detected accurately using liquid chromatography–mass spectrometry (LC-MS), a sensitive and precise analytical technique.
Standard analytical methods also include high-performance liquid chromatography (HPLC) and polymerase chain reaction (PCR) for detecting histamine and other biogenic amines in seafood samples.
Prevention and Control of Seafood toxins
Seafood poisoning primarily originates from environmental sources, especially harmful algal blooms (HABs), and effective prevention and control measures should begin at the harvesting stage.
Surveillance, sampling, and testing of seafood products using sensitive detection methods should be conducted regularly before export to ensure product safety.
Prompt refrigeration and proper storage of fish at temperatures near 0°C significantly reduce the risk of shellfish and other seafood poisonings.
Regulatory agencies and seafood industries should actively provide training and information to improve the safety of seafood handling, processing, and distribution.
Implementation of a Hazard Analysis and Critical Control Point (HACCP)-based safety program serves as a reliable and effective preventive strategy for controlling seafood toxins.
References
Jong, E. C. (2017). Fish and Shellfish Poisoning. In The Travel and Tropical Medicine Manual (pp. 451–456).
Johnson, E. A. (2017). Seafood Toxins. In Foodborne Diseases (pp. 345–366).
Ansdell, V. (2019). Seafood Poisoning. In Travel Medicine (pp. 449–456).
Liu, C., & Ralston, N. V. C. (2021). Seafood and Health: What You Need to Know. In Advances in Food and Nutrition Research (pp. 275–318).
Chand, P. (2009). Seafood Neurotoxins I: Shellfish Poisoning and the Nervous System. In Clinical Neurotoxicology (pp. 441–447).
Hodgson, E. (2012). Toxins and Venoms. In Progress in Molecular Biology and Translational Science: Toxicology and Human Environments, Volume 112 (pp. 373–415).
Watkins, S. M. (2008). Neurotoxic Shellfish Poisoning. Marine Drugs, 6(3), 430–455.
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