Food poisoning is an illness usually caused by micro-organisms, most commonly bacteria although viruses have been identified as an agent too. Illness may also occasionally arise from consumption of food contaminated by chemicals and metals, or from eating certain plants and fish which contain naturally occurring toxins. Most reported cases, however, are attributable to bacteria.

Several different types of bacteria cause food poisoning. Some types grow in food and usually need to be present in high numbers in order to cause illness. Other types are able to cause illness when only relatively few organisms are present (e.g. less than 100).

Details of food-poisoning bacteria, which are most significant in the UK, are outlined below.

Salmonella

short, thin, rod-shaped bacterium

Background

Traditionally considered the most common cause of reported cases of food poisoning in the UK, it is now second to Campylobacter. There is a range of Salmonella species which cause food poisoning; the most frequently implicated in food poisoning cases in the UK are S. enteritidis and S. typhimurium. Salmonella food poisoning may cause serious illness and can cause fatalities especially in susceptible persons, i.e. older people, babies and those who are already ill.

Sources

Naturally found in guts of animals, including farm animals and especially poultry, bacteria is transferred to meat during the slaughtering process. It is also found in or on eggs, in unpasteurised milk, rats, mice, and domestic pets, including terrapins. People may be a source of these pathogens, particularly when suffering from symptoms of Salmonella food poisoning and may continue to excrete them for a long period after recovery.

A very few people may carry the organisms without ever having experienced the illness. Those carrying the bacteria are known as carriers and may not be allowed to work within the food industry if there is a risk of transmitting pathogens to food. It has been estimated that as many as 50,000 people may be excreting salmonellae at any one time in the UK.

Effects

Salmonella bacteria cause illness by multiplying within the human body and causing an infection.

Symptoms: fever, vomiting, abdominal pains and diarrhoea, (septicaemia or peritonitis may occasionally develop).
Onset: 6 -72 hours, but usually 12-36 hours.
Duration: 1 - 8 days, but can be longer.

Controls

Food industry:

  • Control animal foodstuffs
  • Limit contamination possibilities during slaughtering.
  • Screen (e.g. clear stool samples) carriers and persons suspected of carrying the illness especially in the food industry.

Food industry and domestic:

  • Avoid use of raw eggs in foods such as mayonnaise, uncooked desserts. Catering outlets can obtain pasteurised eggs.
  • Keep raw meats and poultry away from other foods to prevent cross-contamination.
  • Keep foods at correct temperatures.
  • Keep animals away from foods.
  • Practise good personal hygiene.

Staphylococcus Aureus

round bacteria which form clusters

Background

Although it has become less common since its peak in the 1950s in the UK it is still the main type of food-poisoning associated with human contamination of food.

The majority of outbreaks are caused by direct contamination of cooked foods by hands which have picked up the bacteria from nose, throat and skin lesions. Staphylococci produce toxins in food which are resistant to heat and are therefore unlikely to be destroyed during the cooking process.

Sources

Staphylococcus is commonly found on humans. It causes skin and wound infections but may be carried naturally on the skin of healthy people and is carried in the nose and throat of almost half the population. The pathogen is sometimes found in unpasteurised milk.

Effects

Staphylococci produce toxins whilst growing in food. When the food is eaten the toxins act on the intestine to cause vomiting.

Symptoms: vomiting, stomach pains, diarrhoea.
Onset: 1-7 hours.
Duration: 6 -24 hours

Controls

Food industry:

  • Train and supervise personnel in good personal hygiene practices.

Food industry and domestic:

  • Practise good personal hygiene.
  • Pay particular attention to hand washing before food preparation involving direct handling of foods.
  • Cover cuts.
  • Avoid sneezing, coughing, etc.
  • Refrigerate foods rich in nutrients, particularly those which have been handled

Bacillus Cerus

rod-shaped bacteria

Background

Bacillus cereus is a pathogen which can produce spores which survive normal cooking. Two different types of food poisoning may be caused by Bacillus cereus but the most common in this country is the 'emetic type' with symptoms described below.

It produces a toxin in the food which appears to occur as the bacteria forms spores. The toxin is not easily destroyed by heat of normal cooking. Food-poisoning cases are associated most commonly with rice and pasta dishes which have not been kept at the correct temperatures. Between 1971 and 1978 there were 110 reported incidents of B. cereus food-poisoning in the UK of which all but two were associated with rice.

Sources

It is found in cereal products, dust and soil, but is most commonly associated with rice.

Effects

B. cereus bacteria may produce a toxin in the food which causes illness when ingested.

Symptoms: vomiting, stomach cramps and some diarrhoea.
Onset: 1 - 5 hours.
Duration: usually no longer than 24 - 36 hours.

Controls

Food industry and domestic:

  • Cook foods thoroughly.
  • Keep hot until eaten or cool rapidly and then store in the fridge.

Campylobacter

comma -shaped bacteria

Background

Campylobacter bacteria are now the most common cause of diarrhoea in the UK with most cases being caused by the type C. jejuni. Although these organisms were discovered many years ago, it was not until the late 1970s that better laboratory detection methods highlighted their significance as a cause of diarrhoea.

Campylobacter species do not grow in food and illness can be caused if food is contaminated by small numbers of the pathogen. Illness is not necessarily caused by eating contaminated food; it has also been related to drinking water and to contact with animals.

Sources

These bacteria are found in animals, birds, untreated water and foods such as raw poultry, raw meat and unpasteurised milk. Birds, especially magpies, have been found to contaminate milk by pecking through caps on bottles left on the doorstep.

Effects

Symptoms: very severe abdominal pain, diarrhoea, headaches and nausea. People are rarely sick. May be confused with appendicitis because of the severe pain and fever.
Onset: 1 -10 days (usually 2-5 days).
Duration: 1-7 days.

A second dose of illness may sometimes occur about 3 weeks after the first symptoms developed.

Controls

Food industry and domestic:

  • Cook poultry thoroughly.
  • Avoid cross-contamination from raw poultry to other foods.
  • Keep animals and birds away from food, including bottled milk.
  • Wash hands after dealing with pets.

Escherichia Coli 0157 (E. coli)

rod-shaped bacteria, small numbers cause illness

Background

There are several types of E. coli. Not all are harmful, but certain strains are pathogenic, usually causing symptoms of diarrhoea. One type, E. coli O157, causes serious illness and even death, particularly in young children and older people. E. coli O157 was the causative agent of the well-publicised food-poisoning outbreak in Scotland in 1996, in which over 500 cases of illness and 20 deaths occurred.

Sources

E. coli O157 is found in the gut of farm animals and illness is associated with eating undercooked meat and unpasteurised dairy products and by contact with farm animals. Since low numbers of this bacteria can cause illness, cross-contamination from raw to cooked or ready-to-eat foods is important. It is apparent that illness can be caused by eating cooked meats which have been contaminated by raw meats.

Effects

Symptoms: watery and sometimes bloody diarrhoea, severe abdominal cramps, and occasionally kidney damage (in more serious cases).
Onset: usually 3 - 4 days, but ranges from 1-14 days.
Duration: usually 2 weeks but longer if complications, such as kidney damage develop.

Controls

Food industry:

  • Limit contamination in the slaughterhouse.
  • Train food handlers and ensure awareness of cross-contamination potential, especially in butcherís outlets and other premises where raw and cooked meats handled.

Food industry and domestic

  • Ensure meats are thoroughly cooked.
  • Prevent cross-contamination from raw meats to other foods.

Clostridium Perfringens

rod-shaped bacteria

Background

Clostridium perfringens has been identified as a cause of upset stomachs since the 1890s and was the second most common cause of reported food poisoning in the UK in the 1980s. Outbreaks are often associated with large-scale catering where foods are prepared in advance. This type of bacteria is able to form spores which are not destroyed by normal cooking.

Sources

It is found in animal and human faeces, soil, dust, insects and raw meat.

Effects

Illness is caused by a toxin which is produced in the body after eating food containing large numbers of active Cl. perfringens bacteria.

Symptoms: abdominal pain and diarrhoea. Vomiting is rare.
Onset: 8-22 hours, usually 12-18 hours
Duration: 12-48 hours

Controls

Food industry and domestic:

  • Keep raw foods, including vegetables (which may be contaminated with pathogens from soil) away from other foods
  • Cook foods thoroughly and keep hot until eaten or cool rapidly and then store in fridge
  • Reheat food quickly and thoroughly.
  • Avoid reheating more than once

Clostridium Botulinum

rod-shaped bacteria

Background

Clostridium botulinum produces a toxin in food which causes a severe illness called botulism, with a high mortality rate. It occurs rarely in the UK: in the last 75 years there have been only 11 outbreaks of food-borne botulism. Cases have often been associated with poorly processed canned foods. The most recent outbreak in the UK was caused by canned hazelnut puree affecting 27 people and causing one death. This type of bacteria produces spores which are not killed in normal cooking and are only destroyed at very high temperatures, i.e. above 121°C for 3 minutes.

Sources

The pathogen is found in soil, fish, meat and vegetables.

Effects

Symptoms: initial short period of diarrhoea and vomiting followed by double vision, difficulties in swallowing and breathing may lead to paralysis.
Onset: 2 hours to 8 days, usually 12 - 36 hours.
Duration: may persist for 6 - 8 months.

Controls

Food industry:

  • Ensure that foods are correctly and thoroughly processed
  • Food industry and domestic:
  • Avoid use of damaged, 'blown' cans

Listeria

rod-shaped bacteria

Background

Listeria monocytogenes has been found to cause illness and, although it has been associated with food, this is not the only way in which the bacteria are transmitted.

Numbers of reported cases are relatively low in the UK. A peak occurred in 1988 with 291 cases and 63 deaths, but in 1996 there were only 112 reported cases of listeriosis. Pregnant women, newborn babies, older people and immuno-suppressed persons are most at risk from the illness.

Sources

The pathogen is found in many places in the environment, including cattle, sheep, silage, effluents and sewage. Foods which have been found to contain Listeria include unpasteurised milk products, such as soft cheeses, and meat-based pates. It is able to grow at low temperatures and may even grow very slowly at refrigeration temperatures.

Effects

Symptoms: include fever, diarrhoea, septicaemia, meningitis and abortion.
Onset: Variable, from 3 -70 days.
Duration: Variable.

Controls

Food industry and domestic:

  • Store foods at correct temperatures.
  • Use within recommended shelf-life.
  • High-risk groups are advised to avoid eating soft cheeses, p‚tÈs and ensure that cook-chill meals and ready-to-eat poultry are eaten only if heated thoroughly until piping hot.

Food-Poisoning Investigation

Many people who experience symptoms of diarrhoea and vomiting do not visit their doctor and it is estimated that there is considerable under-reporting of food-poisoning cases.

When a person is identified, however, as suffering from food poisoning, then the appropriate public health body has to be notified and an investigation may be carried out.

Initial investigation of each case is normally carried out by an Environmental Health Officer, employed by the local authority. Details are collected relating to symptoms, onset times, foods consumed prior to illness, etc. Where there is evidence of an outbreak (i.e. two or more cases which appear to be linked), then investigation procedures may involve a variety of other agencies including the health authority, Public Health Laboratory Service, etc.

Investigation of an outbreak must be conducted swiftly and thoroughly. The main objectives are to:

  • contain the spread of an outbreak i.e. prevent further cases from the original source and any possible spread of illness by those who have symptoms (e.g. affected persons who are employed within the food industry are likely to be excluded from work until free from infection);
  • prevent a recurrence;
  • establish whether legal action is required where breaches of food safety legislation have occurred.

The procedures involve collecting detailed information to establish where the outbreak occurred (place where the food was prepared/served), the food(s) involved, the organism responsible, where it may have originated, how the problem occurred, etc.

Faecal and food samples (where available) will be collected for laboratory investigation to try to isolate the organism responsible for the outbreak.

In many food-poisoning investigations, however, food samples will not be available because of the time delay between the consumption of the suspected food and the onset of symptoms, notification and subsequent investigation. Investigations cannot always establish clear and conclusive evidence concerning the exact cause of the problem.