Updated 19 May 2015

Food poisoning

This refers to the consumption of food contaminated by bacteria, viruses or parasites.


Food poisoning is characterised by symptoms which usually include abdominal cramps, nausea, vomiting and diarrhoea. The similarity of the symptoms often complicates diagnosis. A thorough investigation of a patient's medical history and of foodstuffs recently consumed, will often assist the medical practitioner in making the correct diagnosis. It is important to do this, as different infections may require different treatments.

The severity of the symptoms experienced is usually closely related to the victim's age, medical history and general state of health. It can happen that two people who ingest the same infected foodstuff may be affected in very different ways. In severe cases of food poisoning, dehydration and kidney damage could occur. The 2011 outbreak of Escherichia coli in Europe, is a good example, of the virulence of some of the pathogens that cause food poisoning. More than 2000 individuals have been taken ill and 22 or more fatalities have been reported (News24, 2011). The patients who succumbed to the E. coli infection developed haemolytic-uremic syndrome (HUS), which can cause permanent damage to the kidneys and nervous system, and prove fatal (Kennedy, 2011). Such severely affected patients may require hospitalisation and dialysis.

What causes food poisoning?

The most common causes of food poisoning are bacteria, followed by viruses, parasites, chemicals and toxins.


Microbes, such as viruses and bacteria that cause illness are generally known as ‘‘germs’’. These microbes can infect food or drink at any time from cultivation, to storage, to preparation of the different foodstuffs. There are more than 20 organisms that can cause food poisoning. Of these, Campylobacter, Salmonella, E. coli, Staphylococcus aureus, Shigella, Clostridium perfringens, Clostridium botulinum, Bacillus cereus and Listeria monocytogenes are encountered most frequently.

Several bacteria, such as certain strains of E. coli, Salmonella, Shigella and Campylobacter get into the lining of the intestine, causing secretion of fluid, and sometimes bleeding. The loss of fluid, if severe, and especially in the very young and very old, can be fatal.

Inadequate cooking, and unhygienic handling and storage of foods give bacteria an ideal opportunity to get into the food and to multiply. Bacteria are responsible for the vast majority of food poisoning cases. Bacteria multiply fast –– all they need is warmth, moisture, food and time. Once inside the stomach, they have all four and continue to multiply at an alarming rate –– given the right conditions one bacterium can multiply to 4 million in just eight hours.

Bacteria that cause food poisoning

Campylobacter.This is the most common cause of food poisoning. These bacteria are found in meat and undercooked poultry, untreated water and unpasteurised milk. Poultry and cattle faeces as well as pets with diarrhoea can be a source of these bacteria.

Thorough cooking of food and pasteurisation of milk will destroy campylobacter bacteria. It is also important to avoid having direct contact with animal waste and to wash hands thoroughly after coming into contact with animals or animal waste. It is essential to wash one’s hands and all utensils before preparing food and when handling raw meat.

Abdominal cramping and pain, diarrhoea, which is sometimes bloody, nausea and vomiting, as well as fever, are all symptoms of this kind of bacterial infection.

Usually no special treatment is needed. In general, people recover without treatment in two to five days. Laboratory tests on the stool of an infected person will determine the presence of campylobacter bacteria. In severe cases, a health care professional can prescribe an antibiotic. It is very important to prevent dehydration in the case of patients who suffer from diarrhoea by providing the patient with plenty of liquids, such as oral rehydration mixture or black tea. Very few people die from a campylobacter infection.

Salmonella. Salmonella has been found in many different types of food, including poultry and eggs, raw meat, and sometimes unwashed vegetables or fruit, unpasteurised milk and other dairy products. It can also be spread by means of food that has been prepared on surfaces, such as cutting boards, that were previously used to process raw meat. These bacteria are found in the gut and faeces of humans and animals and can also contaminate water supplies. Salmonella survive refrigeration and are the second most common cause of food poisoning.

Proper cleaning of foodstuffs and cooking utensils and proper cooking of food will destroy salmonella. It is also important that people who have handled any foodstuffs that fall into the risk group mentioned above should wash their hands properly before continuing with food preparation. If the local water supply is contaminated, it is advisable to boil all water before drinking or using it for food preparation.

The symptoms of salmonella poisoning are headaches, a fever, diarrhoea and abdominal cramps.

Cases of salmonella poisoning mostly clear up by themselves and do not require special treatment. It is nevertheless wise to consult a doctor if the diarrhoea becomes severe, as dehydration may result. The doctor may prescribe medication and oral rehydration fluids.

Typhoid is caused by one of the salmonella bacteria and is a more serious infection than the other salmonella infections and requires antibiotic treatment. (See the article on typhoid fever)

E. coli. The full name of this bacterium is Escherichia coli (E. coli). Both harmless and disease-causing strains of this organism are found in the intestinal tracts of animals and humans. It thrives in raw or undercooked meat (especially beef products), unpasteurised milk dairy products and has also been found in raw vegetables and apple juice. During the 2011 E. coli outbreak a variety of raw vegetables, including lettuce, tomatoes and bean sprouts, have been suspected as the food vehicles responsible for transmitting this organism (News24, 2011).  It is also very often called 'the traveller's bug' because it frequently affects people who visit foreign countries, as it also found in untreated water. Swimming in water that is sewage-infected could also result in an E. coli infection. The 2011 outbreak of E. coli is an example of the spread of this bacteria throughout Europe through contaminated food.

There is a particular strain of E. coli that is called Enterohaemorrhagic E. coli or EHEC, which can lead to bloody diarrhoea and kidney failure in immune-compromised persons, children, and the elderly. The 2011 outbreak of EHEC mainly occurred in older women, but did not affect children (Kennedy, 2011).

Proper cooking will destroy E. coli bacteria in beef and other meat products. Fruit and vegetables must be properly washed before eating and travellers must only drink bottled water when visiting a country where the cleanliness of the tap water is questionable. It is best not to swim in rivers in these countries and people should stay away from underchlorinated swimming pools.

The symptoms of E. coli are similar to the symptoms of other bacterial infections, and include exhaustion, occasional vomiting, watery and sometimes bloody diarrhoea, abdominal cramps and severe nausea.

In most cases, treatment is not needed. The infection usually clears up within five to ten days by itself. Health care professionals advise against taking diarrhoea medications in mild cases, so that the bacteria can be expelled from the body. However, if an E. coli infection should become serious with severe vomiting and bloody diarrhoea, it is essential to seek medical attention as soon as possible, as patients may be infected with a virulent strain of EHEC and develop HUS.

Staphylococcus aureus. These bacteria are present on the human skin, in cuts on the skin, in people's noses and in boils. They are also found in unpasteurised milk. S. aureus contaminates food via droplets from the nose, or through touching by unwashed hands. People who have boils or impetigo should not prepare food for others until the infection has cleared.

The danger of these bacteria, is that they are capable of surviving both refrigeration and the cooking process. Although they can survive the refrigeration process, S. aureus bacteria do not multiply at such low temperatures. They are usually destroyed by pasteurisation and cooking, but not always. These bacteria are often found in cooked meats, in poultry, both cooked and undercooked, and in many different types of food which are prepared, but not cooked immediately. They are especially fond of creamy, dairy foods like custards and home-made mayonnaise used for making potato salads. Foods left at room temperatures provide an ideal breeding ground for this type of bacteria. The organism produces toxin while the food is cooling down, and the ingestion of this toxin is what causes the symptoms. When the toxin is already present, symptoms usually start very quickly.

A person who has been infected with Staphylococcus aureus, usually starts vomiting between two to six hours after ingestion. Vomiting is severe, as are abdominal pains. Sometimes the patient also suffers from headaches and fever. Dehydration and loss of electrolytes (sodium and potassium) could also be severe. Symptoms usually do not last long and can be over in as little as twelve hours.

Treatment usually consists of giving the patient adequate fluids, and administering fluids and electrolytes intravenously, if needed. Fatalities usually only occur in the very young, the very old or in people whose health is compromised by some other condition. Always contact your doctor or go to your nearest hospital emergency department if you develop severe vomiting, abdominal cramps, headache and fever.

Clostridium perfringens. This type of bacteria can be found in a wide range of animal faeces. It is found in soil, in raw meat and poultry, animal wastes and sewage.

The spores of this organism are very tough and survive for long periods in the environment. The spores can contaminate food if food handling is unhygienic, and are not always killed during the cooking process. It is particularly associated with foods such as gravies, stews, pies, and cooked meat dishes, such as large joints of meat and poultry.

When food is either cooked slowly, or only partially reheated, the spores can germinate and reproduce rapidly. Pies, which are often kept at moderate heat for hours on end before being sold, are a common source of this type of bacteria. Once the bacteria are ingested, they produce a toxin in the gut, which causes the symptoms - predominantly diarrhoea and cramps. Treatment consists of adequate rehydration and a light diet. Seek medical assistance if the diarrhoea becomes severe.

Listeria monocytogenes:This can cause an infection called listeriosis when food contaminated with the microorganism is eaten. L. monocytogenes bacteria are common and capable of contaminating food from the farm to the plate. They are also not new pathogenic bacteria, but the incidence of listeriosis has increased in recent years due to the methods we use to store, handle and consume foods. In general, L. monocytogenes does not affect healthy individuals severely, but tends to be more dangerous in immune-compromised persons, pregnant women, their unborn children and their infants, young children, cancer, AIDS and leukaemia patients, diabetics, renal or liver transplant recipients, and the elderly.

Research indicates that the foods most commonly infected with L. monocytogenes, are ready-to-eat foods and foods stored in refrigerators for long periods. L. monocytogenes bacteria are able to grow and multiply at low temperatures.   

Listeriosis symptoms include fever, headache, tiredness, aches and pains. In some cases abdominal cramps, nausea and diarrhoea may occur. Severe cases may develop meningitis or septicaemia. In pregnant women, the mother may only experience mild symptoms, but listeriosis can lead to miscarriage, premature birth or in rare case, stillbirth of her baby.

Anyone who has eaten ready-to-eat food and develops any of the above mentioned symptoms, should consult a doctor and mention the possibility of a listeria infection, particularly if the patient is a pregnant woman.

Precautions to avoid contamination of foods withL. monocytogenes include: hygienic handling and preparation of food; avoiding foods with a higher risk of listeria contamination, such as food from salad bars, sandwich bars, delicatessen and buffets; eating only freshly prepared and cooked foods; reheating cooked foods to steaming hot; avoiding all foods that have passed their ‘sell by’ or ‘best before’ dates. Eat fresh foods as soon as possible after purchase and do not store cooked foods in the refrigerator for longer than one or two days. Ordering hot meals when eating out can prevent infection (Food Standards Australia New Zealand, 2011).

Botulism. The bacterium Clostridium botulinum produces a nerve toxin which causes a relatively rare, but serious, paralytic illness known as botulism. Foodborne botulism can be fatal and its danger lies in the fact that many people can be infected by eating the same contaminated food. Infant botulism is caused when the spores of the bacteria grow in the intestines and release toxin.

Botulism is most often found in home-canned foods, which have a low acid content. It is also found in fermented fish, and potatoes baked in tin foil, which are not immediately eaten or refrigerated. The spores germinate in certain foods under the right conditions, and then form the toxin. High temperatures destroy the botulism toxin, so it is advisable to boil home-canned food for ten minutes before eating it.

The bacterial toxin brings about muscle paralysis, which can cause slurred speech, drooping eyelids, double vision and general muscle weakness. Botulism is a difficult condition to treat, as the muscle paralysis makes it difficult to diagnose. It is a very serious condition, which can be fatal, because it causes respiratory failure in serious cases and can result in someone being put on a respirator. If diagnosed early, foodborne botulism can be treated with an antitoxin, but recovery still takes several weeks. If you have eaten canned foods and develop any of the above mentioned symptoms, consult your doctor immediately and mention that you may have eaten food that was contaminated with C.botulinum.

Bacillus cereus. These bacteria are found in dust and soil, as well as in an astonishingly wide range of foodstuffs. These range from pasta and rice dishes to meat and vegetable dishes, soups, dairy products, sauces and pastries.

These bacteria flourish in all these foods when they are not refrigerated after cooking. The spores of Bacillus cereus are not easily destroyed by heat and will often survive cooking and reheating of food. They germinate when the food is allowed to cool slowly, and can produce two types of toxin. One type of toxin is similar to the toxin of S. aureus, is produced in the food, and causes severe vomiting. The other toxin is produced by the organisms when they reach the gut, and cause a more predominantly diarrhoeal illness.

Symptoms of this type of food poisoning can take anything from one to five days to develop. Severe diarrhoea, which may be bloody, is a symptom of this type of poisoning. In extremely severe cases, infection may lead to kidney failure in young children and even death.

Shigella. The Shigella bacteria is spread by people who are infected with shigella and prepare food with hands that were not properly washed after a visit to the toilet. It can also be spread when people eat food contaminated by flies that have bred in infected faeces. Swimming in or drinking infected water can also lead to contamination, as can eating vegetables grown in fields where there was sewage present.

It is often found where people live in crowded and unhygienic conditions and sometimes spreads quickly through whole institutions where hygiene may be inadequate. These bacteria spread extremely easily and can infect large numbers of people in a short space of time.

To prevent shigellosis, it is essential to wash your hands after going to the toilet or changing nappies. Hands should always be washed before preparing any food or drinks.

The symptoms of shigellosis include nausea, vomiting, watery and sometimes bloody diarrhoea and abdominal pain.

This infection usually clears up by itself, but sometimes antibiotics are prescribed. Anti-diarrhoea medication is also not advised for this type of food poisoning.


Chemicals can also be responsible for food poisoning. The ingestion of certain plants like poisonous mushrooms, potatoes that have gone green, the oleander bush, or foodstuffs that have been excessively sprayed with insecticides can be the source of food poisoning in humans. The ingestion of an animal that contains poison, such as shellfish exposed to red tide, can also lead to food poisoning.

The symptoms of chemical poisoning vary according to the poison that has been ingested. Symptoms can include dizziness, excessive tear production, vomiting, diarrhoea, salivation, sweating, stomach cramps, occasional convulsions, constriction of the pupils, muscle weakness and breathing difficulty. These symptoms can manifest from a few minutes after ingestion of the poison, to a couple of hours, or even days. Chemical poisoning can be fatal and should be treated as a medical emergency.

Medical staff may attempt to remove the toxin either by gastric lavage (stomach pump), or by administering ipecac syrup to induce vomiting, or laxatives to empty the intestine. Intravenous fluids may be necessary if a patient is dehydrated, pain medication may be necessary, and in the case of respiratory failure, a ventilator.


Two parasites that can commonly be spread by contaminated water are called Giardia lamblia and Cryptosporidium and are spread when drinking water is infested with these parasites. Healthy people will mostly recover, but they can be fatal to people whose immunity is compromised in some way.

Signs and symptoms of food poisoning

These vary tremendously depending on which kind of bacteria, virus, parasite or chemical someone has ingested.

General symptoms of food poisoning usually include stomach cramps, vomiting, diarrhoea and fever. For specific symptoms of the main types of bacterial poisoning, see the separate listings above. If food poisoning is suspected, it is always better to err on the side of caution, and seek medical treatment, especially if the source of contamination is unknown.

Diagnosis of food poisoning

Identifying the specific source of contamination is not always easy, as several days may have passed between ingestion of the contaminated food and a patient's arrival at a hospital or treatment centre.

Doctors will question a patient closely as to what they have eaten in the preceding days to try and determine what type of infection they are dealing with. Symptoms of different kinds of food poisoning are not dissimilar from each other, complicating diagnosis. They mostly include stomach cramps, diarrhoea, vomiting and often fever.

Blood tests and stool analysis can sometimes be done to get conclusive results.

Treatment of food poisoning

While most types of food poisoning will clear up without treatment within a few days, it is important to seek medical help if:

·    The source of the contamination is unknown

·    The person has eaten mushrooms or seafood

·    The person develops bloody diarrhoea

·    The person develops breathing problems

·    The vomiting and diarrhoea still persist after 48 hours

·    It is a pregnant woman, an infant or elderly person

·    The person's immunity is compromised by some other infectious disease

·    The person develops a fever.

In cases of ingestion of certain chemicals, medical staff will attempt to remove the toxin from the intestinal tract, either by means of gastric lavage (stomach pump), medication to induce vomiting or laxatives to induce diarrhoea.

Rehydration of a patient is important and is usually done by administering fluids and electrolytes intravenously. Antibiotics are sometimes given if there are signs of infection in the intestinal tract. In certain cases anti-diarrhoeal medication is prescribed, but this is generally not advised in bacterial infections.

In serious cases of chemical poisoning, which may result in respiratory failure, patients may need to be ventilated. If the toxin has been identified, there are antitoxins available in some cases. Food poisoning is a condition which needs to be taken seriously as it could be fatal.

Prevention of food poisoning

There is much that can be done to prevent food poisoning:

·    Never drink water that may be contaminated. Boil drinking water first if you are unsure about whether it is safe to drink. Also do not swallow swimming pool water or sea water contaminated with sewage.

·    Washhands regularly. This is the most important preventative measure. Hands should be washed after you have been to the toilet, before and after food preparation and after you have touched raw meat, fish or poultry. Teach all the members of your family to wash their hands with soap after visiting the toilet.

·    Try and avoid coming into contact with any animal faeces. If you should, wash your hands carefully with anti-bacterial soap.

·    Cooking utensils and cutting boards should be kept clean by being washed with hot soapy water on a regular basis. Do not reuse utensils that have touched raw meat, fish or chicken. Put kitchen cloths and sponges in the microwave for 60 seconds as this will kill all bacteria present in them.

·    All fruit and vegetables should be washed properly before eating. If you suspect that your water supply may be contaminated, first boil and cool the water you use for washing fruit and vegetables to prevent cross-contamination.

·    All food, especially meat, should be cooked thoroughly. Undercooked poultry can be a significant source of bacteria.

·    Food should either be kept hot or cold. After two hours at room temperature, bacteria start multiplying in foodstuffs. Refrigerate leftovers immediately and use within a day, of possible. Throw out any old food in the fridge. To avoid contamination with Listeria monocytogenes bacteria, avoid chilled foods that have been stored for long periods at salad bars, sandwich bars, or buffets.

·    Once food has thawed, it should not be refrozen.

·    Never eat anything from bulging or dented cans.

·    Don't eat anything that has passed its expiry date. Don't eat, or even taste anything, that looks suspect. Get into a habit of always checking these dates when buying food.

·    Avoid home-bottled foods, particularly those which don’t contain any acid.

·    Don't eat wild mushrooms, unless you really know which ones are dangerous and which are safe.

·    Avoid seafood that does not come from a reputable source or has been kept chilled for long periods.

Written by Dr Andrew Whitelaw, University of Cape Town and Groote Schuur Hospital

Updated by Dr I van Heerden, DietDoc, Monday, June 13, 2011


Food Standards Australia New Zealand (2011). Listeria and Food. Canberra. (

Kennedy M & agencies (2011). E. coli outbreak: WHO says bacterium is a new strain. Guardian 2 June 2011. (

News24 (2011). Germany now says it’s bean sprouts. News24, 5 June 2011. (


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