What is food poisoning?
Food poisoning occurs when food or water contaminated with harmful microbes (germs), toxins or chemicals is eaten or drunk. When we think of food poisoning, we usually think of the typical gastroenteritis – an infection of the gut (intestines) – that usually causes diarrhoea with or without vomiting. However, sometimes other symptoms or problems can arise from eating contaminated food. Food poisoning can be caused by:
Campylobacter is the most common bacteria 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 viruses, such as norovirus or rotavirus, can contaminate food and cause food poisoning.
These are another type of microbe (germ). Parasites are organisms (living things) 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, the most common parasite that can cause food poisoning is Toxoplasma. This is a parasite that lives in the gut 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.
Toxins (poisons) 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, there is an ongoing debate about how much oily fish is safe to eat in pregnancy because of possible effects of these chemicals on a developing baby.
This is a general leaflet about food poisoning. There are separate leaflets that give more details about some of the different microbes that cause food poisoning.
How does food become contaminated?
Contamination of food can occur because of problems in food production, storage or cooking. For example:
- Not storing food correctly or at the correct temperature. For example, not refrigerating food. This is particularly a problem for meat and dairy products.
- Inadequate cooking of food (undercooking or not cooking to the correct temperature). Bacteria are often found in raw meat and poultry. Adequate cooking usually kills the bacteria.
- Contamination by someone preparing the food who has not followed food hygiene rules and has not washed their hands properly.
- Contamination from other foods (cross-contamination). For example, not washing a board used to prepare raw meat before you cut a slice of bread using the same board. Storing raw meat in the fridge above food that is ‘ready-to-eat’ and so allowing raw meat ‘juices’ to drip onto the food below.
- Bacteria can also be present in unpasteurised milk and cheese. The pasteurisation process kills the bacteria.
How does water become contaminated?
Water can become contaminated with bacteria or other microbes usually because human or animal 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 2008, there were around 69,000 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?
- The main symptom is diarrhoea, often with vomiting as well. Diarrhoea is defined as ‘loose or watery stools (faeces), usually at least three times in 24 hours’. Blood or mucus can appear in the stools with some infections.
- Crampy pains in your abdomen (tummy) are common. Pains may ease for a while each time you pass some diarrhoea.
- A high temperature (fever), headache and aching limbs sometimes occur.
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 dehydration
Diarrhoea and vomiting may cause dehydration (a lack of fluid in the body). 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.
- Symptoms of dehydration in adults include: tiredness, dizziness or light-headedness, headache, muscular cramps, sunken eyes, passing little urine, a dry mouth and tongue, weakness, and becoming irritable.
- Symptoms of severe dehydration in adults include: weakness, confusion, rapid heart rate, coma, and producing very little urine. This is a medical emergency.
Dehydration in adults is more likely to occur in:
- Elderly or frail people.
- Pregnant women.
- People with severe diarrhoea and vomiting. In particular, if you are not able to replace the fluid lost with enough drinks.
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 dehydration. They may check your temperature, pulse and blood pressure. They may also examine your abdomen to look for any tenderness.
Your doctor may ask you to collect a stool 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:
- If you suspect that you are becoming dehydrated.
- If you are vomiting a lot and unable to keep fluids down.
- If you have blood in your diarrhoea or vomit.
- If you have severe abdominal pain.
- If you have severe symptoms, or if you feel that your condition is getting worse.
- If you have a persisting high fever.
- If your symptoms are not settling; for example, vomiting for more than 1-2 days, or diarrhoea that does not start to settle after 3-4 days.
- Infections caught abroad.
- If you are elderly or have an underlying health problem such as diabetes, epilepsy, inflammatory bowel disease, kidney disease.
- If you have a weakened immune system because of, for example, chemotherapy treatment, long-term steroid treatment, HIV infection.
- If you are pregnant.
- If you suspect that you may have contracted food poisoning from eating restaurant or takeaway food (see below).
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. You can find your local food standards enforcer using this link: www.food.gov.uk/enforcement/enforceessential/yourarea/
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 dehydration, or to treat dehydration if it has developed. (Note: if you suspect that you are dehydrated, you should contact a doctor.)
- As a rough guide, drink at least 200 mls after each bout of diarrhoea (after each watery stool).
- This extra fluid is in addition to what you would normally drink. For example, an adult will normally drink about two litres a day, but more in hot countries. The above ‘200 mls after each bout of diarrhoea’ is in addition to this usual amount that you would drink.
- If you vomit, wait 5-10 minutes and then start drinking again, but more slowly. For example, a sip every 2-3 minutes, but making sure that your total intake is as described above.
- You will need to drink even more if you are dehydrated. A doctor will advise on how much to drink if you are dehydrated.
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.
Rehydration drinks 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 gut 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.
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.
Anti-diarrhoea drugs 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 anti-diarrhoea drugs 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 your gut’s activity. You should not take loperamide for longer than five days.
Note: do not give anti-diarrhoea drugs to children under 12 years. Also, do not use anti-diarrhoea drugs if you pass blood or mucus with the diarrhoea or if you have a high 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.
Paracetamol or ibuprofen are useful to ease a high temperature or headache.
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 ‘lab’ to look for infecting microbes (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:
- Wash your hands thoroughly after going to the toilet. Ideally, use liquid soap in warm running water, but any soap is better than none. Dry properly after washing.
- Don’t share towels and flannels.
- Don’t prepare or serve food for others.
- Regularly clean the toilets that you use. Wipe the flush handle, toilet seat, bathroom taps, surfaces and door handles with hot water and detergent at least once a day. Keep a cloth just for cleaning the toilet (or use a disposable one each time).
- Stay off work, college, etc, until until at least 48 hours after the last episode of diarrhoea or vomiting.
- Food handlers: if you work with food and develop diarrhoea or vomiting, you must immediately leave the food-handling area. For most, no other measures are needed, other than staying away from work until at least 48 hours after the last episode of diarrhoea or vomiting. Some special situations may arise and sometimes longer time off is needed. If in doubt, seek advice from your employer or GP.
- If the cause of food poisoning is known to be (or suspected to be) a germ called Cryptosporidium, you should not swim in swimming pools for two weeks after the last episode of diarrhoea.
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 a chronic (ongoing) 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:
- Dehydration and salt (electrolyte) imbalance in your body. This is the most common complication. It occurs if the water and salts that are lost in your stools, or when you vomit, are not replaced by your drinking adequate fluids. If you can manage to drink plenty of fluids then dehydration is unlikely to occur, or is only likely to be mild, and will soon recover as you drink. Severe dehydration can lead to a drop in your blood pressure. This can cause reduced blood flow to your vital organs. If dehydration is not treated, kidney failure may also develop. Some people who become severely dehydrated need a ‘drip’ of fluid directly into a vein. This requires admission to hospital.
- Reactive complications. Rarely, other parts of your body can ‘react’ to an infection that occurs in your gut. This can cause symptoms such as arthritis (joint inflammation), skin inflammation and eye inflammation (either conjunctivitis or uveitis).
- Spread of infection to other parts of your body such as your bones, joints, or the meninges that surround your brain and spinal cord. This is rare. If it does occur, it is more likely if diarrhoea is caused by Salmonella infection.
- Persistent diarrhoea syndromes may rarely develop.
- Irritable bowel syndrome is sometimes triggered by a bout of food poisoning.
- Lactose intolerance can sometimes occur for a period of time after food poisoning. This is known as ‘secondary’ or ‘acquired’ lactose intolerance. Your gut lining can be damaged by an episode of gut infection. This leads to lack of an enzyme (chemical) called lactase that is needed to help your body digest a sugar called lactose that is in milk. Lactose intolerance leads to bloating, abdominal pain, wind and watery stools after drinking milk. The condition gets better when the infection is over and the gut lining heals. It is more common in children.
- Haemolytic uraemic syndrome is another potential complication. It is rare and is usually associated with food poisoning caused by a certain type of E.coli infection. It is a serious condition where there is anaemia, a low platelet count in the blood, and kidney failure. It is more common in children. If recognised and treated, most people recover well.
- Reduced effectiveness of some drugs. During an episode of food poisoning, certain drugs that you may be taking for other conditions or reasons may not be as effective. This is because the diarrhoea and/or vomiting means that reduced amounts of the drugs are absorbed (taken up) into your body. Examples of such drugs are drugs for epilepsy, diabetes and contraception. Speak to your doctor or practice nurse if you are unsure of what to do if you are taking other drugs and have food poisoning.
Can food poisoning be prevented?
The Foods Standards Agency in the UK have identified the ‘4 Cs’ to help prevent food poisoning:
- Keep work surfaces and utensils clean.
- Wash and dry your hands regularly but especially after going to the toilet, before preparing food, after handling raw food and before touching ‘ready-to-eat’ food.
- Don’t prepare food for others if you have diarrhoea or vomiting.
- Cover any sores or cuts on hands with a waterproof plaster before you touch food.
- Change dishcloths and tea towels regularly.
- Make sure that you cook food thoroughly, especially meat. This will kill bacteria. Food should be cooked right through and be piping hot in the middle.
- If you are reheating food, it needs to be cooked right through and be piping hot in the middle.
- Don’t reheat food more than once.
- Food that needs to be chilled or refrigerated should be. If food is left out of the fridge, bacteria may multiply to levels that can cause food poisoning.
- Your fridge needs to be kept between 0°C and 5°C. Also, don’t leave the door open unnecessarily.
- Cool leftover food quickly and then refrigerate. Taking it out of the cooking pot and putting it into a shallow container can speed the cooling process up.
This is when bacteria pass from foods (commonly raw foods) to other foods. It can occur if foods touch directly, if one food drips onto another, if your hands or utensils or equipment such as knives or chopping boards touch one food and then another.
- Wash your hands after touching raw foods.
- Separate raw and cooked or ‘ready-to-eat’ foods.
- Keep raw meat in a sealable container at the bottom of the fridge.
- Don’t use the same surface or chopping board for preparing raw and ready-to-eat foods.
- Make sure that knives and utensils are cleaned after preparing raw foods.
Special precautions may also be needed to prevent food poisoning if you are travelling abroad – see separate leaflet called ‘Traveller’s Diarrhoea’ for further details. For example, avoid water and other drinks that may not be safe, and avoid food washed in unsafe water.
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