Food Poisoning from fish

18 October 2021

How fish and shellfish can cause food poisoning

In this article we’ll examine the various types of food poisoning associated with eating fish.

There are essentially three ways in which fish can cause food poisoning:

1. Fish that are naturally poisonous

2. Fish and shellfish which have become contaminated in the environment.

3. Fish and shellfish which cause food poisoning because of poor practices during production.

Ciguatera.

The source of Ciguatera is certain types of algae. Small fish the eat the algae and carnivorous fish consume the smaller species. The toxin is transferred to humans when we consume either the small, or carnivorous fish.

The algae are associated with dead coral, and therefore affected by human activities and climate change. Hence to describe as ‘natural’ may be a mute point.

Example: In June 2020 five people in The Netherlands were taken ill, and the poisoning was likely caused by eating red snapper imported from India.

The onset time is around six hours, and the symptoms last between a few days or weeks. The symptoms are usually nausea, vomiting, and muscle pain. This can be followed by neurological symptoms including headache, dizziness and irregular heartbeat.

Similarly, illness can result from eating muscles which have fed on poisonous plankton, as detailed below.

Puffer fish (also known as Fugu, Globefish and Blowfish).

It has been said the fish contains a toxin one hundred times more poisonous than cyanide, and the liver alone can contain enough poison to kill five men.

The toxin is tetrodotoxin, a neuro toxin which can affect the central nervous system. In pufferfish it is found mainly in the eggs, liver and skin.

Other fish which contain tetrodotoxin include goby, shellfish, blue-ringed octopus and shellfish. In the majority of cases, there is insufficient toxin to cause illness.

Symptoms are usually between 30 minutes and 6 hours after consuming the food. Vomiting, nausea, and diarrhoea are followed by numbness in the face, muscle weakness and slurred speech. Cardiovascular affects include hypertension. If the patient survives the first 24 hours, they usually make a full recovery as the toxin is expelled in urine.

Mistakes in processing and production contribute to food poisoning cases.

It can take up to ten years to become a sushi master; a rigid learning programme to remove the parts of the fish which could contain tetrodotoxin.

In Japan, it is a legal requirement to prepare and detoxify the fish before they are sold.

In 2018 blunt head blowfish were sold at a supermarket with the livers still intact. According to reports this type of blowfish contains only very week or no poison.

Scombrotoxin

Histidine is an amino acid found in dark-fleshed fish such as tuna, mackerel and pilchards.

If the fish is stored above 4C spoilage bacteria convert the histamine to histamine, and when consumed the reactions are similar to those of an allergic reaction.

Refrigerating of fish should prevent toxin formation.

It’s responsible for about 65% of the outbreaks of food poisoning associated with fish in England and Wales.

Shellfish.

As noted above, some shellfish contain toxins having fed on poisonous plankton. Alternatively, pathogens such as Norovirus can be absorbed from sewage.

You may also come across the terms Diarrhetic shellfish poisoning (DSP) and paralytic shellfish poisoning (PSP).

This refers to the symptoms;

Diarrhetic (DSP) = diarrhoea and vomiting.

Paralytic (PSP) = tingling and numbness of the mouth almost immediately. This can spread to the arms and legs and potentially death. When death occurs, it is usually caused by respiratory paralysis.

Norovirus

Shellfish such as oysters, clams and mussels are filter feeders, extracting nutrients from water as it passes through. As a result, they may also absorb norovirus which will be present in higher concentrations than the surrounding water.

Potential sources of contamination include faulty waste water plants, failing septic systems, and vomiting overboard near shellfish beds.

Norovirus is destroyed by normal cooking temperatures, and therefore most outbreaks are connected to oysters eaten raw.

In March 2001 several outbreaks were reported in France following the consumption of oysters, with over 164 patients and two hospitalisations.

Heavy rains in February may have led to the contamination of water, with water flushed from the ground.

Chemicals

Whether via land or sea, industrialisation and chemical processing has the potential to enter the food chain.

As an example, the NHS advise pregnant women against eating shark, swordfish or marlin because they contain higher levels of mercury.

Mercury is present in very small quantities in the sea. A number of industrial processes cause this including coal burning, cement production, and smelting.

Fish such as shark are of greatest risk because they are long lived, and at the top of the food chain. Similar to Ciguatera discussed earlier, the mercury may be absorbed by algae, consumed by smaller fish, and transferred through the food chain.

Bioaccumulation is where a substance is absorbed at a faster rate than it’s excreted. Therefore, older fish such as shark may gradually absorb the chemical such as mercury, which builds in their muscle tissue over time.

As an example; herring contains mercury levels at 0.1 parts per million, whilst shark contains 1 part per million. Deposited in the muscle tissue, the mercury is transferred to humans on consumption.

In addition, raw fish should be considered a source of pathogenic bacteria, in the same way as raw meat and poultry. Pathogens detected in raw fish include Listeria, Vibrio, Clostridium and Salmonella.

The usual separation and segregation controls to prevent cross contamination should also be applied to fish. Poor practices during storage and preparation can result in cross contamination.

Standard practices of refrigeration and stock rotation is also required to prevent bacterial multiplication. For dark fish such as tuna, the recommended temperature is 4C or colder to prevent action of spoilage bacteria as discussed above. Storing on ice may therefore be required.

Raw fish may also contain parasites.

The Food Standards Agency identify parasites such as Anisakis larvae (parasite worms) as potentially being present in some wild fish including:

- Salmon

- Herring

- Cod

- Other fish species including monkfish

Their advice states that cooking to 60C for at least one minute will kill most parasites, although trematodes may require more stringent heat treatment to kill them. Trematodes may be parasitic in a molluscan host.

Freezing for the correct time/temperature (eg -20C for at least 24 hours) also kills parasites.

Therefore, under hygiene legislation, certain products including sushi and sashimi intended to be eaten raw must be frozen.

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Availability Heuristic in Hospitality Safety

by Nick Dore Hygienie • 22 February 2026

This week I’ve read about the Availability Heuristic, with information taken from The Decision Lab.com. I’ve then tried to relate this information to safety. The availability heuristic is a mental shortcut that allows us to make choices easier and faster. We base decisions on information that comes easily to mind, rather than objective analysis of facts. For example, people may overestimate the dangers of plane crashes, shark attacks, and rare diseases if there have been recent events, vividly reported on the news. In some respects, it’s important that we do easily recall major events associated with safety. Knowledge of these incidents and help change attitudes and improve culture. The main danger, as I see it, is when memorable events aren’t easily recalled. This can lead us to underestimating the danger. For example, in food safety, we might not easily recall food poisoning outbreaks related to long, slow cooking. This might lead us to underestimate the danger, take short cuts, and not follow the correct procedures. The effect is increased when we, personally have not experienced such an incident. Recent, positive, memorable events can also result in overconfidence. For example, a recent Five rating and glowing praise from EHO can can result in overconfidence, and lessen focus on the standards that brought us the reward. This is closely related to over confidence bias, where subjective confidence in our abilities is greater than objective evidence. Often illustrated by the fact around 44% of UK marriages end in divorce, but most newly weds would estimate the likelihood of divorce for them to be around 0%. The ‘availability short cut’ is deeply ingrained and largely necessary. When starting a car journey, it’s not feasible to analyse every factor of our forthcoming journey to evaluate the risk. Being aware that car accidents do occur is sufficient to focus our attention on driving safely. As with most bias I’ve read about, it’s difficult to avoid. Even being aware of its existence doesn’t necessary mean we can overcome its dangers. In addition, as noted above, the knowledge of risks and consequences of mistakes can help drive improvements. Perhaps as a food safety and health and safety trainer I should use case studies and real-life examples to illustrate route cause failings, rather than specific subjects. For example, if I describe an horrific incident involving a deep fat fryer, learners are likely to easily recall the dangers of hot oil. They’re perhaps less likely to recall the dangers of taking shortcuts (for example not allowing enough time for the oil to cool). However, taking short cuts can equally result in accidents involving chemicals, working at height, or many aspects of food safety. I’m not in any way an expert in psychology, I’m just interested in how it relates to safety. Through my company I provide food safety and health and safety training at levels three and four. More information is available on my website Hygienie.org

Authority Bias in Safety

by Nick Dore • 17 February 2026

Authority bias is our tendency to be influenced by authority figures. A 1960’s experiment had members of the public (volunteers) ask questions to people hidden behind a screen. Those answering the questions were played by actors. Under the guidance of authority figures, the volunteers administered an electric shock for every wrong answer. The actors would cry out in pain, and so far as the volunteers were concerned, the pain was real. Under instruction, the current was increased for every wrong answer, some exceeding a level that would be fatal. I know a city centre restaurant where someone from head office arrived unannounced one weekend to monitor the sites performance. By Monday morning they had gained access to the safe and the takings. At no point did anyone challenge their authority, or check they were from head office. Many will have experienced the frustration of having their work suggestion dismissed…. Only for later, a senior manager make the same suggestion and having it adopted. If a group decision is to be made, the decision will usually reflect the opinions of the most senior manager in the room. Our tendency is to focus on the messenger rather than the message. There are positive aspects to authority bias. During a global pandemic it helps that millions of people will follow the advice of authority figures. Of course, some people will lean in the opposite direction and have a distrust of all authoritarian advice. This can result in conspiracy theories, particularly is someone with authority, an ‘expert’ encourages the distrust. Most people would advocate a balance. For example, to follow professional advice, but where possible, to fact check and seek alternative opinions. Which brings us to safety. Employees must follow safety policies and food safety management systems. Environmental health officers’ documented actions on inspection reports must be completed. However, when EHO’s recommend soaking cloths (for wiping down surfaces) in a bleach solution, it’s reasonable to consider alternative methods. If EHO’s insist food must be cooled to below 8°C in 90 minutes you might question is this is feasible. When a safety officer insists you wear a hard hat, they must be worn. Although on one course a delegate, who was ex forces told me that on manoeuvres they camouflaged vehicles with netting. During this task they were made to remove army helmets and replace with construction hard hats. Of course, I’ve no way of fact checking this. I do recall one company who, for years had been using the wrong chemical to disinfect surfaces. If an employee identifies such mistakes, it’s good they question rather than blindly follow. Unfortunately, whether these concerns are heard may depend on whether they’re an officer, senior manager, or perceived to be an expert. (PS, I’m not an expert in any of the above, I’m just interested in the subject) The original source of this material was an article in ‘thedecisionlab.com’

Attention Bias in Safety

by Nick Dore • 17 February 2026

I’m continuing to read and consider how aspects of psychology might be relevant to safety. This week, I was reading about Attention bias. This is where our attention might be biased towards certain elements in our environment, whilst ignoring others. It’s like ‘zooming in’ on certain information which renders us blind to other factors. The implications for safety audits and checks are clear. If we have a pet hate (like, oh, I don’t know, people putting things other than food on chopping boards; car keys, glasses, delivery notes and such) we might focus on this and miss other contraventions. There are wider concerns for management such as ignoring someone for promotion because we’re focussed on their weaknesses whilst ignoring strengths and potential. Or focussing on one measurement of an employee’s productivity. It’s possible to ruin work and personal relationships by focussing on a specific flaw. The tendency to focus on the negative can also be detriment to our own mental health. There are several factors that can bias our attention. External events such as the past performance of an individual, emotional stimuli such as anger, and internal states such as hunger (which can bias our attention towards donuts and chocolate). Avoiding attention bias is difficult. Our brains have a limited capacity of focus, and a mental shortcut such as this helps maintain cognitive efficiency. In some circumstances it helps to avoid stimuli. So, when giving up smoking, our habit might be linked to a cup of tea. When drinking a cup of tea, the stimulus focuses our attention on cigarettes, and it’s hard to stop thinking about having a smoke. I’m not sure if this is relevant to safety. And in some ways, attention bias is a useful trait in safety. There are evolutionary reasons for the bias. Those early humans more aware of dangers in their environment were more likely to survive and pass on their genes. Being aware of hazards is clearly a good thing in safety. I also considered this bias in terms of how we can influence others. For example, if a manager is angry or confrontational with an EHO, there may be a strong tendency for the EHO to focus on negative information. Conversely, if we’re calm, confident and welcoming, others are more likely to focus on positive information about us. The original source of this material was an article in ‘thedecisionlab.com’ I claim no expertise in psychology; I am just keen to learn more. I train food safety and health and safety to levels three and four. Through my website, Hygienie.org I offer online and live streaming courses to individuals and businesses.