What is food poisoning?

Ever wondered what might have caused your food poisoning? How long it takes to get food poisoning or how long you may experience symptoms? What about the difference between ‘use by’ and ‘best before’ dates on food packaging? We asked the Food Standards Agency to explain exactly what the different types of food poisoning are, what causes food poisoning and which foods we should be especially careful with.

What are the different types of food poisoning?

The term ‘food poisoning’ is very broad. It refers to illness caused through eating food or drink and covers a range of causes including pathogens, toxins, allergy and excessive alcohol consumption to name but a few. The term ‘foodborne illness’ is defined by the World Health Organisation (WHO) as “diseases, usually either infectious or toxic in nature, caused by agents that enter the body through the ingestion of food or water”. Foods can sometimes be contaminated with bacteria or the toxins produced by bacteria, viruses and, in rare cases, parasites.

Some of the most common types are listed below:

Campylobacter

In the UK this is the most common cause of food poisoning. Campylobacter are a type of bacteria usually found on raw or undercooked meat (particularly poultry), unpasteurised milk and untreated water. The incubation period (the time between eating contaminated food and the start of your symptoms) is usually between two and five days but it has been seen as late as ten to 11 days. The symptoms typically last for less than a week – however, if you have a weakened or compromised immune system, you could develop complications.

Listeria monocytogenes

Listeria bacteria may be found in a range of chilled ready-to-eat foods, including pre-packed sandwiches, cooked sliced meats and pâté, as well as mould ripened soft cheeses (like brie or camembert). All of these foods should be stored appropriately and eaten by their use by dates. This is particularly important for vulnerable, high-risk groups such as the elderly, those with a weakened immune system and pregnant women. Listeriosis in the elderly has a high mortality rate and during pregnancy it can cause pregnancy and birth complications, and even result in miscarriage.

Salmonella

Salmonella bacteria are often found in raw or undercooked meat, raw eggs, milk and other dairy products. The incubation period is usually between 12 and 72 hours, and symptoms include diarrhoea, stomach cramps, nausea, vomiting and fever, and typically last around four to seven days. Salmonella is responsible for the highest number of hospitalisations for food poisoning in the UK.

E coli (Escherichia coli)

Escherichia coli, often known as E coli, is a bacteria found in the digestive systems of many animals, including humans. Most strains are harmless but some can cause serious illness. Shiga toxin producing strains of E coli, or STECs, are responsible for most food-related E coli infections. One common strain is E coli O157 (which is often called VTEC). Most cases of E coli food poisoning occur after eating undercooked beef (particularly mince, burgers and meatballs) or drinking unpasteurised milk. The incubation period for food poisoning caused by E coli is typically one to eight days but symptoms predominantly arise after three to four days. Symptoms include diarrhoea (often bloody), stomach cramps and occasionally fever. Symptoms can last from a few days to several weeks. A small number of people with E coli O157 infection go on to develop a serious condition called haemolytic uraemic syndrome (HUS). This can sometimes lead to kidney failure and death, although this is rare. The risk of HUS is highest in children aged less than five years.

Shigella

Shigella bacteria can contaminate any food that has been washed in contaminated water. Symptoms, such as diarrhoea and nausea, typically develop within seven days and can last for up to a week. An infection caused by shigella bacteria is known as bacillary dysentery or shigellosis.

Clostridium perfringens

Clostridium perfringens is found naturally in the environment and in foods. It also forms part of the normal gut flora of humans and animals. Spores of C perfringens can survive during undercooking or unrefrigerated storage, producing vegetative (growing) cells. If ingested in large volumes, it can result in gastroenteritis (inflammation of the stomach and intestines) – this results from bacterial toxins or viral infection. Additional symptoms include vomiting and diarrhoea. The main sources appear to be meat that is prepared and left warm for longer than two hours before consumption and stews, casseroles and gravy that have been inappropriately stored.

Bacillus cereus

Similar to C perfringens, the spores of the bacillus bacteria commonly contaminate raw foods and food materials, many of which have been in contact with soil or are of vegetable origin. The spores of some species, especially bacillus cereus and the bacillus subtilis group, can survive cooking and multiply under favourable warm conditions – for example, the longer cooked rice is left at room temperature the more likely it is that bacteria or toxins will make the rice unsafe to eat. If ingested in large volumes, it may result in gastroenteritis.

Botulism

Botulism is caused by botulinum toxin, which is a poison produced by the bacterium clostridium botulinum. It can be found in soil and river or sea water sediment, and can survive in these environments as a resistant spore. Raw honey is another common source when it becomes contaminated from the hive, dust, water or pollen in the environment. Typical symptoms for food-borne botulism include diarrhoea and vomiting. The disease can progress to generalised paralysis that includes the arms, legs, trunk and respiratory muscles. Most cases make a full recovery, but the recovery period can be many months.

Viruses

The virus that most commonly causes diarrhoea and vomiting is norovirus. It’s easily spread from person to person, through contaminated food or water. Raw shellfish, particularly oysters, can also be a source of infection. The incubation period is typically 24-48 hours and the symptoms usually pass in a couple of days.

Hepatitis E is a liver disease caused by the Hepatitis E virus. Two genotypes can circulate in several animals including pig and deer without causing disease to them, although it can occasionally infect humans and cause illness. The virus can also be transmitted by contaminated water. The incubation period following exposure to the Hepatitis E ranges from two to ten weeks, with an average of five to six weeks. Common early symptoms include fever, nausea and vomiting, jaundice and an enlarged liver.

Parasites

In the UK, food poisoning caused by parasites is rare. Parasitic infections that can be spread in contaminated food include:

Giardiasis – an infection caused by a parasite called giardia intestinal
Cryptosporidiosis – an infection caused by a parasite called cryptosporidium
Amoebiasis – a type of dysentery caused by a single-cell parasite (amoeba) called entamoeba histolytica (this is very rare in the UK)
Anisakis (a fish parasite) – can be found in raw fish that hasn’t been processed properly

Symptoms usually develop within ten days of eating contaminated food, although sometimes it may be weeks before you feel unwell and your symptoms may vary depending on the type of parasitic infection. If left untreated, symptoms can last a long time – sometimes several weeks to several months.

What causes food poisoning?

The main cause of food poisoning is a result of eating contaminated food or water. There are a number of reasons why this may have occurred.

For example:

Not cooking food thoroughly (particularly poultry and some meats which cannot be served rare)
Not washing hands properly before handling/preparing food, and after going to the toilet or putting things in the bin
Keeping cooked food unrefrigerated for a long period of time
Eating perishable foods past their ‘use by’ date
Eating food that has been touched by someone who is ill or has been in contact with someone who has been ill
Cross-contamination (where harmful bacteria are spread between food, surfaces and equipment). Cross-contamination can occur, for example, if you prepare raw chicken on a chopping board and don’t wash the board before preparing food that won’t be cooked (such as salad). It can also occur if raw meat is stored above ready-to-eat foods in the fridge
Drinking water which has not been properly treated (this is a problem in less-developed countries)

Which foods require particular care?

Any type of food can be a risk if it is cross-contaminated, however, there are a number of high-risk food types that we all need to be aware of. Vulnerable groups (the very young, the elderly, those who are pregnant or those that have a lower immune response) need to be particularly careful about the food choices they make and what potential risks are associated with them.

All meat may contain bacteria and it’s important that they are handled carefully and cooked properly. The only exceptions are whole cuts of beef and lamb (steak and joints) which can be served rare as the bacteria is located on the outside of the meat. It’s also important not to wash raw meat before cooking as this can easily spread bacteria around the food preparation area and onto yourself via splashing. Thorough cooking will kill any bacteria present.

For more detail on how to cook meat safely, see the NHS website for guidance on cooking meat safely and storing, preparing and cooking meat.

It is important to carefully wash fruit and vegetables in a bowl of cold water. To do this, rub them under the water until the soil has been removed. This is because some vegetables may carry a small amount of soil particles on them which can harbour E coli. This soil should be removed, especially on anything consumed raw (for example carrots). Peeling the skin will also reduce the risk.

Salad items should also be washed unless specified that they have already been pre-washed (for example ready-to-eat bagged lettuce). If in any doubt, wash before consumption.

Uncooked rice can sometimes contain spores of bacillus cereus, a bacterium that can cause food poisoning. When the rice is cooked, the spores may survive. If the rice is left standing at room temperature, the spores can grow into bacteria. These bacteria will multiply and may produce toxins that cause vomiting or diarrhoea. It is important to ensure rice is cooked thoroughly in the first instance and to try to cook only as much as you’ll need. If this isn’t possible, cook the rice, cool within one hour and store in the fridge, consuming within 24 hours. When reheating, make sure it is steaming hot throughout before serving.

It is important to check the packaging on raw beansprouts as this will confirm whether they have been processed for immediate consumption (ready-to-eat). If not, they will need to be washed and cooked thoroughly.

Raw beans, especially red kidney beans, can be risky as they naturally contain phytohaemagglutinin, which is a lectin (a family of proteins) that may produce toxic effects. This can be a problem when you slow cook raw beans as you may not destroy the toxin and, in some cases, it can even increase the toxicity. Canned beans purchased in the supermarket are safe to consume as they have been pre-soaked and boiled to destroy any toxins present.

There are also a number of other natural toxins such as diarrhetic shellfish poisoning (DSP) and paralytic shellfish poisoning (PSP) to name a couple. Symptoms usually involve intense abdominal pains and diarrhoea. The incubation period tends to be short, with symptoms often manifesting themselves after several hours following the consumption of infected shellfish.

In general, all foods with a ‘use by’ date should be consumed (or frozen) within the date specified. They should also be stored correctly and in accordance with the on-pack instructions.

What’s the difference between ‘use by’ and ‘best before’ dates?

The use by date is about safety. Foods can be eaten (and most can be frozen) up until the use by date but not after. You will see use by dates on food that goes off quickly, such as meat products or ready prepared salads. Use by dates are set based on the way the food will be stored (eg. refrigerated).

The best before date is about quality and not safety. The food should be safe to eat after this date but may not be at its best quality (flavour and texture might not be as good). Best before dates appear on a wide range of frozen, dried, tinned and other foods. The best before date will only be accurate if the food is stored according to the instructions on the label.

If you suspect food poisoning, visit the NHS website or call NHS 111.

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