Food poisoning occurs when food or water contaminated with harmful germs (microbes), toxins or chemicals is eaten or drunk. It usually causes diarrhoea, with or without being sick (vomiting). However, other symptoms or problems can be caused by eating contaminated food. In most cases, symptoms clear away over several days but sometimes it takes longer. The main risk is lack of body fluid (dehydration). The main treatment is to have lots to drink to try to avoid dehydration. Sometimes antibiotic medicines or other treatments may be needed. Any suspected case of food poisoning from eating takeaway or restaurant food should be reported to your local Environmental Health Office. You should take care to follow the '4 Cs' to help prevent food poisoning (see below).
What is food poisoning?
Food poisoning occurs when food or water contaminated with harmful germs (microbes), toxins or chemicals is eaten or drunk. When we think of food poisoning, we usually think of the typical gastroenteritis - an infection of the bowels (intestines) - that usually causes diarrhoea with or without being sick (vomiting). However, sometimes other symptoms or problems can arise from eating contaminated food. Food poisoning can be caused by:
Campylobacter is the most common germ (bacterium) that causes food poisoning in the UK. Other bacteria that can cause food poisoning include salmonella, Escherichia coli (usually shortened to E. coli), listeria, shigella and Clostridium perfringens.
Some germs (viruses), such as norovirus or rotavirus, can contaminate food and cause food poisoning.
These are another type of microbe. Parasites are living things (organisms) that live within, or on, another organism. Examples include cryptosporidium, Entamoeba histolytica and giardia parasites. Food poisoning caused by parasites is more common in the developing world.
In the UK, a common cause of food poisoning is Toxoplasma gondii. This is a parasite that lives in the bowels of a number of animals, including cats. Food poisoning can occur if food or water is contaminated with the faeces of infected cats, or if raw or undercooked meat from another animal carrying the parasite is eaten. The infection is known as toxoplasmosis. Symptoms of this type of food poisoning include swollen lymph glands and sometimes a skin rash.
Poisons (toxins) and chemicals
Toxins can be produced by bacteria that contaminate the food. For example, the bacterium Staphylococcus aureus can contaminate ice cream and its toxins can lead to food poisoning. The bacterium Bacillus cereus can contaminate rice. If contaminated rice is reheated and eaten, the toxins produced can lead to food poisoning.
Certain types of fish (including shark, marlin, swordfish and tuna) contain high levels of the chemical mercury. Eating these types of fish is not normally a problem for most people, it does not cause gastroenteritis or food poisoning. But pregnant women are advised to avoid eating shark, marlin and swordfish and to limit tuna. This is because a high level of mercury can damage the developing nervous system of an unborn baby.
Oily fish may be contaminated by chemicals called polychlorinated biphenyls. Again, this does not usually cause a problem or food poisoning for most people. However, you should limit the amount of oily fish you eat in pregnancy because of possible effects of these chemicals on a developing baby. The Department of Health recommends no more than two portions of oily fish a week.
How does food become contaminated?
Contamination of food can occur because of problems in food production, storage or cooking. For example:
How does water become contaminated?
Water can become contaminated with germs (bacteria) or other microbes usually because human or animal stools (faeces) get into the water supply. This is particularly a problem in countries with poor sanitation. In such countries, food may also be washed and prepared using contaminated water. So, for example, in countries with poor sanitation, you should always avoid drinking tap water, having ice cubes in drinks and eating salads or uncooked vegetables.
How common is food poisoning?
It is common. In 2010, there were about 57,000 notifications of food poisoning cases in the UK. However, the total number of people with food poisoning was likely to have been much higher because people with mild symptoms do not usually report them.
How long does it take for food poisoning to develop?
For most cases of food poisoning, symptoms tend to come on within one to three days of eating the contaminated food. However, for some types of food poisoning, this 'incubation period' can be as long as 90 days.
What are the usual symptoms of food poisoning?
If vomiting occurs, it often lasts only a day or so but sometimes longer. Diarrhoea often continues after the vomiting stops and commonly lasts for several days or more. Slightly loose stools may persist for a week or so further before a normal pattern returns. Sometimes the symptoms last longer.
Symptoms of lack of fluid in the body (dehydration)
Diarrhoea and vomiting may cause dehydration. Consult a doctor quickly if you suspect you are becoming dehydrated. Mild dehydration is common and is usually easily reversed by drinking lots of fluids. Severe dehydration can be fatal unless quickly treated because the organs of your body need a certain amount of fluid to function.
Dehydration in adults is more likely to occur in:
How is food poisoning diagnosed and do I need investigations?
Most people will recognise food poisoning from their typical symptoms. If symptoms are mild, you do not usually need to seek medical advice or receive specific medical treatment.
However, in some circumstances, you may need to seek medical advice when you have food poisoning (see below about when to seek medical advice). The doctor may ask you questions about recent travel abroad or any ways that you may have eaten or drunk contaminated food or water. The doctor will also usually check you for signs of calck of body fluid (dehydration). They may check your temperature, pulse and blood pressure. They may also examine your tummy (abdomen) to look for any tenderness.
Your doctor may ask you to collect a stool (faeces) sample. This can then be examined in the laboratory to look for the cause of the infection. If you are very unwell, you may need admission to hospital. If this is the case, further investigations may be needed such as blood tests, scans or a lumbar puncture. This is to look for spread of the infection to other parts of your body.
When should I seek medical advice?
Seek medical advice in any of the following situations, or if any other symptoms occur that you are concerned about:
Reporting the food poisoning episode
You should report any suspected case of food poisoning from eating takeaway or restaurant food to your local Environmental Health Office. This is so that the business can be investigated by environmental health officers. Further actions may be taken if there is a problem with their food hygiene practices. This will hopefully help to reduce the chance that other people will get food poisoning.
If your doctor suspects or confirms that you have food poisoning, they are also required by law to report this.
What is the treatment for food poisoning?
Symptoms often settle within a few days or so as your immune system usually clears the infection. Occasionally, admission to hospital is needed if symptoms are severe, or if complications develop (see below).
The following are commonly advised until symptoms ease:
Fluids - have lots to drink
The aim is to prevent lack of body fluid (dehydration), or to treat dehydration if it has developed. (Note: if you suspect that you are dehydrated, you should contact a doctor.)
For most adults, fluids drunk to keep hydrated should mainly be water. Also, ideally, include some fruit juice and soups. It is best not to have drinks that contain a lot of sugar, such as cola or pop, as they can sometimes make diarrhoea worse.
These are recommended in people who are frail, or over the age of 60, or who have underlying health problems. They are made from sachets that you can buy from pharmacies. (The sachets are also available on prescription.) You add the contents of the sachet to water. Rehydration drinks provide a good balance of water, salts and sugar. The small amount of sugar and salt helps the water to be absorbed better from the bowels (intestines) into the body. They do not stop or reduce diarrhoea. Do not use home-made salt/sugar drinks, as the quantity of salt and sugar has to be exact.
Antisecretory medicines are a newer group of treatments. They are designed to be used with rehydration treatment. They reduce the amount of water that is released into the gut during an episode of diarrhoea.
Eat as normally as possible
It used to be advised to 'starve' for a while if you had food poisoning. However, now it is advised to eat small, light meals if you can. Be guided by your appetite. You may not feel like food and most adults can do without food for a few days. Eat as soon as you are able - but don't stop drinking. If you do feel like eating, avoid fatty, spicy or heavy food at first. Plain foods such as wholemeal bread and rice are good foods to try eating first.
Antidiarrhoeal medicines are not usually necessary. However, you may wish to reduce the number of trips that you need to make to the toilet. You can buy antidiarrhoeal medicines from pharmacies. The safest and most effective is loperamide. The adult dose of this is two capsules at first. This is followed by one capsule after each time you pass some diarrhoea up to a maximum of eight capsules in 24 hours. It works by slowing down the activity of your bowels. You should not take loperamide for longer than five days.
Note: do not give antidiarrhoeal medicines to children aged under 12 years. Also, do not use antidiarrhoeal medicines if you pass blood or mucus with the diarrhoea or if you have a high temperature (fever). People with certain conditions should not take loperamide. Therefore, read the leaflet that comes with the medicine to be safe. For example, pregnant women should not take loperamide.
As explained above, if symptoms are severe, or persist more than several days, your doctor may ask for a sample of the diarrhoea. This is sent to the laboratory to look for infecting germs (microbes like bacteria, parasites, etc). Sometimes an antibiotic or other treatments are needed if certain bacteria or other infections are found to be the cause.
Stop the spread of infection to others
Some infections causing diarrhoea and vomiting are very easily passed on from person to person. If you have diarrhoea, the following are also recommended to prevent the spread of infection to others:
Are there any complications that can occur from food poisoning?
Complications are uncommon in the UK. They are more likely in the very young, pregnant women, or the elderly. They are also more likely if you have an ongoing (chronic) condition such as diabetes or if your immune system may not be working fully. For example, if you are taking long-term steroid medication or you are having chemotherapy treatment for cancer. Possible complications include the following:
Can food poisoning be prevented?
The Foods Standards Agency in the UK has identified the '4 Cs' to help prevent food poisoning:
This is when bacteria pass from foods (commonly raw foods) to other foods. It can occur if foods touch directly, if one food drips on to another, if your hands or utensils or equipment such as knives or chopping boards touch one food and then another.