Fish and Shellfish Poisoning – Types, Causes and Symptoms

What is fish poisoning?

Fish poisoning is any illness caused by toxins consumed in seafood, either fish or shellfish. Some of these toxins may be found in just about any food, be it meat, fruit or vegetables. But seafood may also contain specific toxins that cause illnesses when consumed by humans. This is known as marine toxins. Many of these toxins are heat stable and cannot be destroyed during the cooking process. Therefore it can cause an illness whether the contaminated fish is eaten raw or well cooked. These toxins do not arise from the fish itself but rather from the micro-organisms that accumulate within it.

The reason why fish poisoning is often considered separately from other types of food poisoning is that some cases are potentially life threatening. Death can result as quick as 24 hours after eating the contaminated seafood. Even if death does not occur, there are types of fish poisoning that can lead to permanent loss of short term memory and even brain damage. Therefore fish poisoning should not be taken lightly. Contrary to popular belief, fish poisoning does not only occur with poisonous species. Even widely available and commonly eaten fish can cause poisoning if the toxins within it are sufficiently high to affect humans.

Types of fish poisoning

There are host of food infections that can cause illness in humans. Some of these may cause food poisoning by the way of the toxins produced by the causative organisms. However, fish and shellfish poisoning refers to:

  • Ciguatera poisoning from fish which is caused by dinoflagellates.
  • Scombroid poisoning from fish which is caused by bacteria.
  • Amnesic, neurotoxic and paralytic shellfish poisoning from shellfish like clams, mussels and oysters which is caused by diatoms (amnesic type) or dinoflagellates (neurotoxic and paralytic types).

These are not the only type of fish poisoning but stand out due to the prevalence among poisoning with contaminated seafood. Most have the potential to be life-threatening as well.

Causes of seafood toxins

Ciguatera poisoning

Ciguatera poisoning occurs from eating fish that have accumulated toxins from dinoflagellates, a marine organism. Dinoflagellates are protists and have both plant and animal characteristics. Large tropical fish that tend to feed off smaller fish (predatory fish) are more likely to cause ciguatera poisoning. The reason being is that these large fish not only acquire dinoflagellate toxins directly, but also indirectly from eating smaller fish with ciguatoxin (the toxin produced by dinoflagellates).

Dinoflagellates are found in larger populations within tropical waters and is particularly concentrated in red tide. The region where these fish are caught is therefore an important factor as to whether it can cause ciguatera poisoning or not. For Americans, it is the following fish that is caught in areas such as Florida, Guam and neighboring islands, Hawaii, Puerto Rico and the Virgin Islands that may be a problem:

  • Barracuda
  • Grouper
  • Mullet
  • Sea bass
  • Snapper


Picture of a barracuda

Scombroid poisoning

Scombroid or scombrotoxic poisoning occurs with eating certain fish that have high levels of a substance known as histamine as a result of being contaminated with bacteria. When these bacteria spoil the fish, it breaks down the fish protein and leads to the production of histamine and substances which block histamine removal. In other words a person may be consuming large amounts of histamine, a chemical mediator of inflammation that is often associated with allergic reactions.

A number of different bacteria can cause this histamine accumulation in the fish. It occurs after the fish dies and depends on how soon the fish is refrigerated or frozen after being caught. A longer time between death and refrigeration/freezing increases the chances of bacteria breaking down the fish flesh. This type of poisoning is more likely to occur with certain fish species, particularly:

  • Albacore
  • Bonito
  • Mackerel
  • Tuna
  • Mahi mahi


Picture of mackerel

Shellfish poisoning

Shellfish poisoning occurs when eating shellfish contaminated with the toxins from different aquatic organisms. There are three main types, named according to the serious symptoms that each causes. The causative aquatic organism that is responsible for the toxins in contaminated shellfish differs among each type.

  • Paralytic shellfish poisoning is caused by toxins (saxitoxin, neosaxiton and gonyautoxins) from dinoflagellates (different dinoflagellates than ciguatera poisoning). Muscle paralysis is one of the severe symptoms. The toxin may be found in shellfish like clams, cockles, crabs, lobsters, mussels, oysters and scallops from cold coastal waters in the Pacific and New England.
  • Neurotoxic shellfish poisoning is caused by toxins (brevetoxins) from dinoflagellates (different dinoflagellates from paralytic shellfish poisoning and ciguatera poisoning). Tingling in the mouth and limbs, and incoordination are some of the symptoms. The toxin may be found in shellfish like clams, mussels and oysters from the Gulf of Mexico and the Atlantic ocean around the southern US states.
  • Amnesic shellfish poisoning is caused by toxins (domoic acid) from a species of diatoms known as Nitzchia pungens. Like dinoflagellates, diatoms are neither plants nor animals. Some of the symptoms of severe cases is permanent short-term memory loss and brain damage. The toxin may be found in shellfish like mussels.


Fish poisoning symptoms

The following symptoms may be seen in most types of fish and shellfish poisoning:

  • Nausea
  • Vomiting
  • Diarrhea
  • Abdominal cramps
  • Excessive sweating
  • Headache

In addition the following symptoms may be specific to certain types of fish poisoning.

Ciguatera Symptoms

Symptoms start with 30 minutes to several hours after eating the contaminated fish, and resolve within 1 to 4 weeks.

  • Prickling (“pins and needles”) or burning sensations.
  • Muscle weakness
  • Itching of the skin
  • Dizziness
  • Unusual tastes and alterations of temperature sensations in the mouth.
  • Hallucinations or nightmares

Death is rare.

Scrombotoxic Symptoms

Symptoms start with a few minutes to 2 hours after eating the fish, and resolves within several hours.

  • Skin rash and flushing.
  • Burning sensation in the mouth.
  • Swelling of the mouth.
  • Metallic taste

Paralytic Shellfish Symptoms

Symptoms start around 2 hours after eating the contaminated shellfish, but can be as soon as 15 minutes or as delayed as 10 hours.

  • Numbness or tingling in the face and limbs.
  • Poor coordination
  • Muscle paralysis (severe cases)
  • Respiratory failure (severe cases)

In severe cases death can occur anywhere from a few hours to a day after the onset of symptoms.

Neurotoxic Shellfish Symptoms

Symptoms start around 1 to 3 hours after consuming the contaminated seafood and resolve around to 2 to 3 days thereafter.

  • Numbness or tingling in the face and limbs.
  • Poor coordination
  • Hot and cold temperature reversal.
  • Slurred speech

Death is rare.

Amnesic Shellfish Symptoms

Symptoms start around 24 hours after eating the contaminated shellfish.

  • Dizziness and disorientation
  • Short-term memory loss (permanent)
  • Seizures (severe cases)
  • Muscle weakness or paralysis (severe cases)
  • Brain damage

Death may occur in severe cases.


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