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Fish and Shellfish Poisoning


Gerald F. O’Malley

, DO, Grand Strand Regional Medical Center;

Rika O’Malley

, MD, Grand Strand Medical Center

Reviewed/Revised May 2022 | Modified Sep 2022

Certain types of fresh or frozen fish or shellfish may contain toxins that can cause a variety of symptoms.

Vomiting and diarrhea (gastroenteritis Overview of Gastroenteritis Gastroenteritis is inflammation of the lining of the stomach and small and large intestines. It is usually caused by infection with a microorganism but can also be caused by ingestion of chemical... read more Overview of Gastroenteritis ) caused by toxins is different from gastroenteritis caused by eating fish (or any other food) that has become contaminated by illness-causing bacteria or viruses.

There are three common types of poisoning caused by eating fish:

  • Ciguatera

  • Tetrodotoxin

  • Scombroid

Ciguatera poisoning

Ciguatera poisoning may result from eating any of the more than 400 species of fish from the tropical reefs of Florida, the West Indies, or the Pacific. The toxin is produced by certain dinoflagellates, which are microscopic sea organisms that the fish eat. The toxin accumulates in their flesh. Older fish and large ones (such as grouper, snapper, and kingfish) are more toxic than smaller, younger ones. The flavor of the fish is not affected. Food processing procedures, including cooking, cannot destroy the toxin.

The initial symptoms—abdominal cramps, nausea, vomiting, and diarrhea—may begin 2 to 8 hours after the person eats the fish and last 6 to 17 hours. Later symptoms may include

  • Itchiness

  • Pins-and-needles sensations

  • Headache

  • Muscle aches

  • A reversal of sensations of hot and cold

  • Facial pain

For months afterward, unusual sensations and nervousness may persist.

Doctors may treat affected people with intravenous mannitol (a drug that reduces swelling and pressure), but it is unclear whether this provides any benefit.

Tetrodotoxin poisoning

Tetrodotoxin poisoning is most common in Japan, due to eating the puffer fish (fugu), which naturally contains this toxin in certain organs. However, more than 100 other freshwater and saltwater species also contain tetrodotoxin.

Early symptoms include numbness in the face and limbs followed by increased salivation, nausea, vomiting, diarrhea, and abdominal pain. If a large amount of the toxin is eaten, muscles can become paralyzed and death may result from paralysis of the muscles that regulate breathing. The toxin cannot be destroyed by cooking or freezing.

Scombroid poisoning

After fish such as mackerel, tuna, bonito, skipjack, and blue dolphin (mahi mahi) have been caught, the tissues of the fish break down, eventually producing high levels of histamine. When ingested, histamine causes immediate facial flushing. It can also cause nausea, vomiting, stomach pain, and hives Hives Hives are red, itchy, slightly elevated swellings. The swelling is caused by the release of chemicals (such as histamine) from mast cells in the skin, which cause fluid to leak out of small... read more Hives (urticaria) a few minutes after the fish is eaten. Symptoms, which are often mistaken for a seafood allergy, usually last less than 24 hours. The fish may taste peppery or bitter. Unlike other fish poisonings, this poisoning can be prevented by properly storing the fish after it is caught.

Because symptoms are caused by histamine, they may be relieved by antihistamine drugs such as diphenhydramine.

Shellfish poisoning

Shellfish poisoning in the US can occur from June to October, especially on the Pacific and New England coasts. Shellfish such as mussels, clams, oysters, and scallops may ingest certain poisonous dinoflagellates at certain times when the water has a red cast, called the red tide.

The dinoflagellates produce a toxin that attacks nerves (such toxins are called neurotoxins). The toxin, saxitoxin, which causes paralytic shellfish poisoning, persists even after the food has been cooked.

The first symptom, a pins-and-needles sensation around the mouth, begins 5 to 30 minutes after eating. Nausea, vomiting, and abdominal cramps develop next, followed by muscle weakness. Occasionally, the weakness progresses to paralysis of the arms and legs. Weakness of the muscles needed for breathing may even be severe enough to cause death unless the person is put on a breathing machine (ventilator). Those who survive usually recover completely.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Aridol, BRONCHITOL, Osmitrol , Resectisol
Aid to Sleep, Alka-Seltzer Plus Allergy, Aller-G-Time , Altaryl, Banophen , Benadryl, Benadryl Allergy, Benadryl Allergy Children's , Benadryl Allergy Dye Free, Benadryl Allergy Kapgel, Benadryl Allergy Quick Dissolve, Benadryl Allergy Ultratab, Benadryl Children's Allergy, Benadryl Children's Allergy Fastmelt, Benadryl Children's Perfect Measure, Benadryl Itch Stopping, Ben-Tann , Children's Allergy, Compoz Nighttime Sleep Aid, Diphedryl , DIPHEN, Diphen AF , Diphenhist, DiphenMax , Dytan, ElixSure Allergy, Genahist , Geri-Dryl, Hydramine, Itch Relief , M-Dryl, Nighttime Sleep Aid, Nytol, PediaCare Children's Allergy, PediaCare Nighttime Cough, PediaClear Children's Cough, PHARBEDRYL, Q-Dryl, Quenalin , Siladryl Allergy, Silphen , Simply Sleep , Sleep Tabs, Sleepinal, Sominex, Sominex Maximum Strength, Theraflu Multi-Symptom Strip, Triaminic Allergy Thin Strip, Triaminic Cough and Runny Nose Strip, Tusstat, Unisom, Uni-Tann, Valu-Dryl , Vanamine PD, Vicks Qlearquil Nighttime Allergy Relief, Vicks ZzzQuil Nightime Sleep-Aid
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