What is food poisoning?

In the UK, campylobacter bacteria are the most common cause of food poisoning. The bacteria are 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 symptoms) for food poisoning caused by campylobacter is usually between two and five days but it has been seen as late as ten to eleven days. The symptoms usually last for less than a week, however, if you have a weakened 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, and mold ripened soft cheeses (such as 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. During pregnancy, listeriosis can cause pregnancy and birth complications, and can result in miscarriage.

Salmonella

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

E. coli (Escherichia coli)

Escherichia coli, often known as E. coli, is 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, stomach cramps and occasionally fever. About half of people with the infection will have bloody diarrhoea. 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 of eating contaminated food 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 in man 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. Gastroenteritis is inflammation of the stomach and intestines, typically resulting from bacterial toxins or viral infection. It can result in 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 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. If ingested in large volumes, it can 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 aquatic sediments and can survive in these environments as a resistant spore. Typical symptoms for foodborne 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 typically lasts 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 virus 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
• Ameobiasis – a type of dysentery caused by a single-cell parasite (amoeba) called entamoeba histolytica (this is very rare in the UK)
• Anasakis (a fish parasite) – can be found in raw fish that hasn’t been processed properly.

The symptoms of food poisoning caused by a parasite usually develop within ten days of eating contaminated food, although sometimes it may be weeks before you feel unwell. Symptoms will vary greatly depending on the type of parasitic infection. If left untreated, the 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/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 the 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), as the harmful bacteria can be spread from the chopping board to the salad. It can also occur if raw meat is stored above ready-to-eat foods and juices 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 can 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 yourselves 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 can carry a small amount of soil particles on them which can contain 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, it is advisable to 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 can 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. The longer cooked rice is left at room temperature, the more likely it is that the bacteria or toxins could make the rice unsafe to eat. It is therefore important to ensure rice is cooked thoroughly in the first instance and to try and 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 (ie: ready-to-eat). If not, they will need to be washed and cooked.

Raw beans, especially red kidney beans can be particularly risky, as they naturally contain phytohaemagglutinin, which is a lectin that can produce toxic effects. This is particularly a problem associated with slow cooking raw red kidney beans as it doesn’t destroy the toxin; in some instances it can increase its 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 as advised by on-packet 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 Direct.

Read more...

Everything you need to know about food poisoning
How to avoid food poisoning
Food safety myths

Do you have any questions on food poisoning that you'd like answered? Let us know in the comments below...

This article was published on 14th December 2016.

All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.