Fish and Shellfish 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 is for information only and not for use in the treatment or management of an actual poison exposure. If you have an exposure, you should call your local emergency number.

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 doesn’t matter how well you cook your fish, if the fish is contaminated, you will become poisoned.

In Scombroid poisoning, the poisonous ingredient is histamine and similar substances. Normal bacteria on these fish create large amounts of this toxin after the fish dies if it 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 seafood. There are many different types of shellfish poisoning. The most well known types are paralytic shellfish poisoning, neurotoxic shellfish poisoning, and amnestic shellfish poisoning.

Where Found

Ciguatera poisoning normally occurs in larger fish from warm tropical waters. The most popular types of these fish that are eaten include sea bass, grouper, and red snapper. The risk is greatest in the summer months, or any time a large amount 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, today’s transportation means that anyone around the world may be sitting down to a dinner from a fish from contaminated waters.

Scombroid poisoning usually occurs in large dark meat fish such as tuna, mackerel, mahi mahi, and albacore. Since this poison develops after a fish is caught and dies, where the fish is caught doesn’t really matter. 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. In addition, most shellfish poisonings occur during the summer months. The number of poisonings also increases when there is a “red tide.” Shellfish poisoning occurs in seafood with two shells such as clams, oysters, mussels, and sometimes scallops.Sick Tummy


The harmful substances that cause Ciguatera, Scombroid, and shellfish poisoning are heat stable, so no amount of cooking will protect you from becoming poisoned if you eat fish that is contaminated. Symptoms depend on the specific type of poisoning.

Ciguatera poisoning symptoms can occur anywhere from 2 to 12 hours after eating the fish. They include:

  • Abdominal cramps
  • Diarrhea (severe and watery)
  • Nausea
  • 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 that an ice cube is burning you, while a match is freezing your skin)
  • Headache (probably the most common)
  • Low heart rate and low blood pressure (in very severe cases)
  • Metallic taste in the mouth

Scombroid poisoning symptoms usually occur immediately after eating the fish. They may include:

  • Breathing problems (in severe cases)
  • Extremely red skin on face and body
  • Flushing
  • Hives and itching
  • Nausea
  • Vomiting

There are different types of shellfish poisoning. Below are the most well known types 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. Read More …

Neurotoxic shellfish poisoning: The symptoms are very similar to Ciguatera poisoning. After eating contaminated clams or mussels, you will most likely experience nausea, vomiting, and diarrhea. These symptoms will be followed shortly by strange sensations that may include numbness or tingling in your mouth, headache, dizziness, and hot and cold temperature reversal.

Amnestic shellfish poisoning: This is a strange and rare form of poisoning that begins with nausea, vomiting, and diarrhea, which is followed by short-term memory loss, as well as other less frequent neurologic symptoms.

Home Care

Shellfish poisoning may be a medical emergency. With sudden or significant symptoms, the person should be taken immediately to an emergency medical centre or Hospital Emergency Department. You may need to call the local emergency number or poison control for appropriate treatment information.

Before Calling Emergency

Determine the following information:

  • Patient’s age, weight, and condition
  • Type of fish eaten
  • Time it was eaten
  • Amount swallowed

What to Expect at the Emergency Room

If you have Ciguatera poisoning, you may receive:

  • Medicines to stop vomiting
  • Fluids by IV (to replace fluids lost from vomiting and diarrhea)
  • A medication to help reduce neurological symptoms

If you have Scombroid poisoning, you may receive:

  • An antihistamine medication, such as diphenhydramine (Benadryl)
  • Fluids by IV (to replace fluids lost from vomiting and diarrhea)
  • Medicines to stop vomiting
  • Medicines to treat severe allergic reactions (if needed)
  • Breathing tube (in rare cases)

If you have shellfish poisoning, you may receive:

  • Medicines to stop vomiting
  • Fluids by IV (to replace fluids lost from vomiting and diarrhea)

If shellfish poisoning causes paralysis, you may have to remain in the hospital until your symptoms improve.

Outlook (Prognosis)

Fish and shellfish poisonings occur on occasion in Australia. 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 the various shellfish poisoning symptoms may last from days to weeks depending on the severity of the poisoning. Only very rarely have serious outcomes or death occurred.

Since these poisons are heat stable, 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 doctor tell the restaurant that their food is contaminated so that they may throw it away before other people become sick. Your doctor may also contact the Department of Health to make sure that the suppliers providing the contaminated fish are identified, and all possibly contaminated fish from the same lot are destroyed.

Preventing shellfish poisoning

If shellfish is cooked thoroughly and at the correct temperature then it should not result in food poisoning. Many species of shellfish contain bacteria such as e coli and viruses which include norovirus but these are destroyed during cooking.

One exception to this is oysters which many people enjoy eating raw. If this applies to you then be aware that there will always be a risk with this and any other type of raw fish.

This unfortunately, doesn’t apply to toxins such as those found in algae and marine organisms. This also includes ciguatera poisoning as these toxins are impervious to cooking, freezing, salting and pickling.

So, what measures can you take to protect yourself against shellfish poisoning?

  • Keep any fresh fish, e.g. tuna or mackerel in the fridge which will prevent them from decomposing and producing histamines which cause food poisoning.
  • Avoid eating any shellfish (or fish in general) if you are traveling in developing countries. Unless you are certain that these are free from contamination it is a good idea to avoid any of these due to the risk of bacterial or viral food poisoning.
  • Do not assume that cooking infected fish will kill these toxins because it doesn’t. Cooking these fish at high temperatures or conversely, freezing them does not destroy their toxins.
  • If you are not sure about any fish, and this includes shellfish then do not eat them.

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