Shellfish toxins

Shellfish poisoning is a risk for anyone who enjoys travelling and especially to areas of the developing world. Many species of fish such as oysters, clams and mussels contain potent toxins – known as marine toxins, which can cause food poisoning.

These marine toxins are caused by bacteria and viruses which invade shellfish, and other types of fish via consumption of contaminated algae or marine organisms in the surrounding water.

Toxins are found within the head, liver and intestines of fish.

Types of fish/shellfish

These include herbivorous and carnivorous fish as well as shellfish.

Herbivorous fish are fish which feed upon vegetable matter within oceans and seas. This includes aquatic plants, plankton and algae (e.g. seaweeds). Examples of herbivorous fish include trout and red snapper.

Carnivorous fish eat meat or flesh of other animals which includes fish, seals and even humans. Examples of these include moray eels, sea bass, piranhas and sharks.

Shellfish are found in freshwater as well as sea water and include shrimps, prawns, oysters, cockles, clams and mussels. They are also known as ‘filter feeding molluscs’ which means that they use a filter process as they feed upon algae and plankton within the surrounding water.

Types of shellfish poisoning

All of these fish contain toxins which are responsible for the following types of food poisoning:

• Ciguatera poisoning
• Scombroid poisoning
• Shellfish poisoning

Ciguatera poisoning

A good example of this is ciguatera poisoning which occurs when fish such as sea bass consume small marine organisms known as ‘dinoflagellates’ which are found in or near coral reefs.

These dinoflagellates contain toxins, e.g. ciguatoxin, which is consumed by fish and increases in strength and concentration as they move up the food chain. These toxins are highly potent by the time they reach the human food chain.

Once consumed by humans they cause ciguatera food poisoning – an unpleasant gastrointestinal illness which is followed by neurological symptoms such as depression and fatigue.

Ciguatera poisoning is discussed in more detail within our fish food poisoning section.

Scombroid poisoning

Scombroid poisoning occurs when fish has decayed or become ‘spoiled’ due to inadequate storage, i.e. has not been stored at the correct temperature within a fridge.

As the fish decays the bacteria within it produce toxins such as histamines which cause symptoms very similar to an allergic reaction. These symptoms include nausea, vomiting and diarrhoea as well as flushing, blurred vision and a severe headache.

These symptoms usually clear up by themselves although serious cases will require hospital treatment.

Scombroid poisoning is discussed further in our fish food poisoning section.

Shellfish poisoning

There is more than one type of food (or fish) poisoning caused by these molluscs which include:

• Paralytic poisoning
• Neurotoxic poisoning
• Diarrhoeic poisoning
• Amnesic poisoning

These are all caused by shellfish feeding on contaminated algae or plankton (dinoflagellates) which contain a variety of toxins such as saxitoxin or brevetoxins.

If you eat shellfish which contain these toxins then expect to develop one of the following forms of fish poisoning.

Paralytic poisoning

This is the most common type of shellfish poisoning. It is caused by the consumption of shellfish which contain a number of chemicals that are derived from saxitoxin.

Saxitoxin is a neurotoxin which when consumed, attacks the nervous system within humans as well as causing gastrointestinal illness.

Symptoms of paralytic poisoning include:

• Tingling
• Burning
• Drowsiness
• Lack of co-ordination/clumsiness
• Slurred speech
• Dry mouth
• Choking feeling in throat

This also includes the usual symptoms of food poisoning, for example nausea, vomiting and diarrhoea.

These symptoms appear 30 minutes to an hour after consumption and can cause serious damage such as muscle paralysis and respiratory failure which can be fatal. This poisoning is particularly serious in children.

Neurotoxic poisoning

Another type of poisoning which occurs following consumption of infected shellfish. In this case, the shellfish contain a type of toxin called ‘brevetoxins’which cause symptoms very similar to those of paralytic poisoning or ciguatera poisoning.

Symptoms of neurotoxic poisoning include:

• Numbness/tingling in the mouth, arms and legs
• Dry mouth
• Poor co-ordination
• Slurring of the speech
• Drowsiness
• Nausea
• Vomiting
• Diarrhoea

This also causes both neurological and gastrointestinal symptoms.

Recovery takes 2 to 3 days from neurotoxic poisoning.

Diarrhoeic poisoning

Diarrhoea is the main symptom - hence the name but it also includes other symptoms such as nausea, vomiting, chills and abdominal pains.

This type of poisoning is caused by a variety of toxins which includes okadaic acid and yessotoxin. Okadaic acid is the toxin which is directly responsible for causing persistent diarrhoea.

These symptoms develop very quickly, usually within an hour after eating contaminated shellfish. They last for a day or so and tend to resolve themselves without the need for treatment.

The one exception to this is if someone experiences severe diarrhoea which leads to dehydration. In this case they will require fluid replacement therapy via an intravenous drip.

Depletion of fluids caused by diarrhoea and/or vomiting can be dealt with at home if it is not severe. This means drinking plenty of fluids such as water and adding electrolytes to them to replace those lost through this illness.

These electrolytes are essential minerals such as sodium (salt) and potassium which come in powder form and can be added to a glass of water. They can be purchased over the counter at a local pharmacy.

Amnesic poisoning

This is a very rare form of poisoning which occurs as a result of eating infected shellfish. These shellfish will have consumed this when feeding upon a type of algae called ‘diatoms’ or brown algae which produces the toxin domoic acid.

This toxin is found in sardines and anchovies as well as shellfish.

This neurotoxin causes a range of symptoms which include:

• Vomiting
• Abdominal pain
• Diarrhoea

These symptoms of gastrointestinal illness appear within 24 hours of eating infected shellfish.

These are then followed by neurological symptoms which include:

• Headache
• Mental confusion
• Dizziness/disorientation
• Memory loss
• Visual disturbances
• Seizures

In severe cases, paralysis and even death may occur.

There is no known antidote so anyone with these symptoms requires hospital treatment as soon as possible.

Diagnosing shellfish poisoning

This usually involves a discussion about the symptoms, a physical examination and questions about the type of shellfish eaten.

If there any samples available of the infected shellfish or leftovers then these can be tested for signs of toxins via laboratory analysis.

Treatment for shellfish poisoning

This often depends upon the species of shellfish and the type of toxin (or toxins). But it is important to remember that there are no specific treatments for these toxins.

As all of these forms of poisoning cause vomiting and diarrhoea then fluids will need to be replaced which have become depleted as a result of these. This involves drinking fluids which contain electrolytes to restore essential vitamins and minerals.

Some cases will require hospital treatment especially those which involve children or people with a medical condition or a weakened immune system. This treatment will be supportive and involve fluid replacement and medication.

Mannitol is prescribed in cases of ciguatera poisoning and is given intravenously.

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 norovirusbut 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 travelling 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.