Poisoning - fish and shellfish
Fish poisoning; Dinoflagellate poisoning; Seafood contamination; Paralytic shellfish poisoning; Ciguatera poisoning
This article describes a group of different conditions caused by eating contaminated fish and seafood. The most common of these are ciguatera poisoning, scombroid poisoning, and various shellfish poisonings.
This article is for information only. DO NOT use it to treat or manage an actual poison exposure. If you or someone you are with has an exposure, call the local emergency number (such as 911), or the local poison control center can be reached directly by calling the national toll-free Poison Help hotline (1-800-222-1222) from anywhere in the United States.
Poisonous Ingredient
In ciguatera poisoning, the poisonous ingredient is ciguatoxin. This is a poison made in small amounts by certain algae and algae-like organisms called dinoflagellates. Small fish that eat the algae become contaminated. If larger fish eat a lot of the smaller, contaminated fish, the poison can build up to a dangerous level, which can make you sick if you eat the fish. Ciguatoxin is "heat-stable." That means it does not matter how well you cook your fish, if the fish is contaminated, you will become poisoned.
In scombroid poisoning, the poisonous ingredient is a combination of histamine and similar substances. After the fish dies, bacteria create large amounts of the toxin if the fish is not immediately refrigerated or frozen.
In shellfish poisoning, the poisonous ingredients are toxins made by algae-like organisms called dinoflagellates, which build up in some types of shellfish. There are many different types of shellfish poisoning. The most well-known types are paralytic shellfish poisoning, neurotoxic shellfish poisoning, and amnesic shellfish poisoning.
Where Found
Ciguatera poisoning normally occurs in larger fish from warm tropical waters. The most popular types of these fish used for food include sea bass, grouper, and red snapper. In the United States, the waters around Florida and Hawaii are most likely to have contaminated fish. Worldwide, ciguatera fish poisoning is the most common type of poisoning from marine biotoxins. It is a major public health problem in the Caribbean.
The risk is greatest in the summer months, or any time a large number of algae are blooming in the ocean, such as during "red tide." A red tide occurs when there is a rapid increase in the amount of dinoflagellates in the water. However, thanks to modern transportation, anyone around the world may eat a fish from contaminated waters.
Scombroid poisoning most often occurs from large, dark meat fish such as tuna, mackerel, mahi mahi, and albacore. Because this poison develops after a fish is caught and dies, it does not matter where the fish is caught. The main factor is how long the fish sits out before being refrigerated or frozen.
Like ciguatera poisoning, most shellfish poisonings occur in warmer waters. However, poisonings have occurred as far north as Alaska and are common in New England. Most shellfish poisonings occur during the summer months. You may have heard the saying "Never eat seafood in months that do not have the letter R." This includes May through August. Shellfish poisoning occurs in seafood with two shells, such as clams, oysters, mussels, and sometimes scallops.
Always check with your local health department or fish and wildlife agency if you have any questions about the safety of eating any food product.
Symptoms
The harmful substances that cause ciguatera, scombroid, and shellfish poisonings are heat stable, so no amount of cooking will prevent you from becoming poisoned if you eat contaminated fish. Symptoms depend on the specific type of poisoning.
Ciguatera poisoning symptoms can occur 2 to 12 hours after eating the fish. They include:
- Abdominal cramps
- Diarrhea (severe and watery)
- Nausea and vomiting
Shortly after these symptoms develop, you will start to have strange sensations, which may include:
- A feeling that your teeth are loose and about to fall out
- Confusing hot and cold temperatures (for instance, you will feel like an ice cube is burning you, while a match is freezing your skin)
- Headache (probably the most common symptom)
- Low heart rate and low blood pressure (in very severe cases)
- Metallic taste in the mouth
These symptoms may get worse if you drink alcohol with your meal.
Scombroid poisoning symptoms most often occur immediately after eating the fish. They may include:
- Breathing problems, including wheezing and chest tightness (in severe cases)
- Extremely red skin on the face and body
- Flushing
- Hives and itching
- Nausea and vomiting
- Peppery or bitter taste
Below are other well-known types of seafood poisoning, and their symptoms.
Paralytic shellfish poisoning: About 30 minutes after eating contaminated seafood, you may have numbness or tingling in your mouth. This sensation may spread down to your arms and legs. You may become very dizzy, have a headache, and, in some cases, your arms and legs may become temporarily paralyzed. Some people may also have nausea, vomiting, and diarrhea, although these symptoms are much less common.
Neurotoxic shellfish poisoning: The symptoms are very similar to those of ciguatera poisoning. After eating contaminated clams or mussels, you may experience nausea, vomiting, and diarrhea. These symptoms will be followed soon after by strange sensations that may include numbness or tingling in your mouth, headache, dizziness, and hot and cold temperature reversal.
Amnesic shellfish poisoning: This is a strange and rare form of poisoning that begins with nausea, vomiting, and diarrhea. These symptoms are followed by short-term memory loss, and other less common nervous system symptoms.
Home Care
Shellfish poisoning may be a medical emergency. A person with serious or sudden symptoms should immediately be taken to an emergency medical center right away. You may need to call the local emergency number (such as 911) or poison control for appropriate treatment information.
Before Calling Emergency
The following information is helpful for emergency assistance:
- Person's age, weight, and condition
- Type of fish eaten
- Time it was eaten
- Amount swallowed
However, DO NOT delay calling for help if this information is not immediately available.
Poison Control
Your local poison control center can be reached directly by calling the national toll-free Poison Help hotline (1-800-222-1222) from anywhere in the United States. This national hotline will let you talk to experts in poisoning. They will give you further instructions.
This is a free and confidential service. All local poison control centers in the United States use this national number. You should call if you have any questions about poisoning or poison prevention. It does NOT need to be an emergency. You can call for any reason, 24 hours a day, 7 days a week.
What to Expect at the Emergency Room
If you have ciguatera poisoning, you may receive:
- Blood and urine tests
- ECG (electrocardiogram, or heart tracing)
- Fluids by IV (through a vein)
- Medicines to stop vomiting
- Medicines to help reduce nervous system symptoms (mannitol)
If you have scombroid poisoning, you may receive:
- Airway support, including oxygen, breathing tube through the mouth (intubation), and breathing machine (ventilator)
- Blood and urine tests
- ECG (electrocardiogram, or heart tracing)
- Fluids by IV (through a vein)
- Medicines to stop vomiting
- Medicines to treat severe allergic reactions (if needed), including diphenhydramine (Benadryl)
If you have shellfish poisoning, you may receive:
- Blood and urine tests
- ECG (electrocardiogram, or heart tracing)
- Fluids by IV (through a vein)
- Medicines to stop vomiting
If shellfish poisoning causes paralysis, you may have to stay in the hospital until your symptoms improve.
Outlook (Prognosis)
Fish and shellfish poisonings occur infrequently in the United States. You can protect yourself by avoiding fish and seafood caught in and around the areas of a known red tide, and by avoiding clams, mussels, and oysters during the summer months. If you are poisoned, your long-term outcome is usually quite good.
Scombroid poisoning symptoms usually only last for a few hours after medical treatment has begun. Ciguatera poisoning and shellfish poisoning symptoms may last from days to weeks, depending on the severity of the poisoning. Serious outcomes or death are rare.
There is no way for the person who prepares the food to know that their food is contaminated. Therefore, it is very important that your health care provider tell the restaurant their food is contaminated so that they can throw it away before other people become sick. Your provider should also contact the Department of Health to make sure that the suppliers providing the contaminated fish are identified and destroyed.
References
Jong EC. Fish and shellfish poisoning: toxic syndromes. In: Sandford CA, Pottinger PS, Jong EC, eds. The Travel and Tropical Medicine Manual. 5th ed. Philadelphia, PA: Elsevier; 2017:chap 34.
Kapil V, Bradberry SM, Pirmohamed M. Prescribing, therapeutics and toxicology. In: Feather A, Randall D, Waterhouse M, eds. Kumar and Clarke's Clinical Medicine. 10th ed. Philadelphia, PA: Elsevier; 2021:chap 12.
Madhosingh H. Infectious diseases. In: Harward MP, ed. Medical Secrets. 6th ed. Philadelphia, PA: Elsevier; 2019:chap 12.
Morris JG. Human illness associated with harmful algal blooms. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 284.
Nitzberg M, Smereck J. Diarrhea. In: Walls RM, ed. Rosen's Emergency Medicine: Concepts and Clinical Practice. 10th ed. Philadelphia, PA: Elsevier; 2023:chap 27.
Version Info
Last reviewed on: 1/2/2023
Reviewed by: Jesse Borke, MD, CPE, FAAEM, FACEP, Attending Physician at Kaiser Permanente, Orange County, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
