Food poisoning is an illness usually caused by micro-organisms, normally bacteria but sometimes viruses. People can also get food poisoning from eating food contaminated by chemicals and metals, or from eating certain plants and fish which contain naturally occurring toxins.

Several different types of bacteria can cause food poisoning. Some types grow in food and usually need to be present in high numbers to cause illness. Other types are able to cause illness with less than 100 organisms.

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

Salmonella

short, thin, rod-shaped bacterium

Salmonella food poisoning is very common and may cause serious illness. It can cause death especially in older people, babies and those who are already ill.

Sources

These bacteria are found in guts of animals, including farm animals and especially poultry, and are transferred to meat during the slaughtering process. They are also found in eggs, in unpasteurised milk, rats, mice and domestic pets. People may be a source of these pathogens long after being ill with Salmonella. A very few people may carry the organisms without ever having experienced the illness. They are called carriers and may not be allowed to work within the food industry.

Effects

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

Symptoms: fever, vomiting, abdominal pains and diarrhoea.
Onset: 6-72 hours, but usually 12-36 hours.
Duration: 1-8 days, but can be longer.

Controls in both the Food Industry and the Home

  • Limit contamination during slaughtering.
  • Screen carriers in the food industry.
  • Avoid use of raw 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

This is the main type of food poisoning associated with human contamination of food. Most outbreaks are caused by touching cooked foods with hands which have picked up the bacteria from the nose, throat and skin. Staphylococci produce toxins in food which are resistant to heat and are therefore unlikely to be destroyed during cooking.

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 cause vomiting.

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

Controls in both the Food Industry and the Home

  • Practise good personal hygiene.
  • Pay particular attention to handwashing before food preparation.
  • Cover cuts.
  • Avoid sneezing, coughing, etc.
  • Refrigerate foods, particularly those which have been handled.

Bacillus Cereus

rod-shaped bacteria

This pathogen can produce spores which survive normal cooking. As the bacteria form spores they produce a toxin which is not easily destroyed by cooking.

Sources

It is found in cereal products, dust and soil, but most commonly in rice and pasta which has not been kept at the correct temperature.

Effects

The bacteria may produce a toxin in the food which causes illness when eaten.

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

Controls in both the Food Industry and the Home

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

Campylobacter

spirally curved, rod-shaped bacteria

These bacteria are the most common cause of diarrhoea in the UK. They do not grow in food and illness can be caused if food is contaminated by only small numbers of the pathogen. Illness 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 of bottles left on the doorstep.

Effects

Symptoms: very severe abdominal pain, diarrhoea, headaches and nausea. People are rarely sick.
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 in both the Food Industry and the Home

  • 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

Not all E. coli are harmful. Certain strains cause diarrhoea. One type, E. coli O157, causes serious illness and even death, particularly in young children and older people.

Sources

E. coli O157 is found in the gut of farm animals. Illness may come from eating undercooked meat and unpasteurised dairy products and from contact with farm animals. 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 in both the Food Industry and the Home

  • Limit contamination in the slaughterhouse.
  • Train food handlers, especially butchers.
  • Ensure meats are thoroughly cooked.
  • Prevent cross-contamination from raw meats to other foods.

Clostridium Perfringens

rod-shaped bacteria

These bacteria cause upset stomachs. They are often associated with catering where foods are prepared in advance for a lot of people. 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 bacteria.

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

Controls in both the Food Industry and the Home

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

Clostridium Botulinum

rod-shaped bacteria

These bacteria produce a toxin in food which causes a severe illness called botulism which can be fatal. It occurs rarely in the UK. Cases have often been associated with poorly processed canned foods. 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: 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 in both the Food Industry and the Home

  • Ensure that foods are correctly and thoroughly processed.
  • Avoid use of damaged cans.

Listeria

rod-shaped bacteria

Listeria monocytogenes in food has been found to cause illness, but this is not the only way in which the bacteria are transmitted. 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 such as cattle, sheep and sewage. It is able to grow at low temperatures and may even grow very slowly at refrigeration temperatures. Soft cheese and meat pâtès can contain Listeria.

Effects

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

Controls in both the Food Industry and the Home

  • Store foods at correct temperatures and use within recommended shelf-life.
  • High-risk groups are advised to avoid eating soft cheeses, pâtès and to 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 have diarrhoea and vomiting do not visit their doctor, so many cases of food poisoning may not be reported.

When a person is known to have food poisoning, then the appropriate public health body has to be notified and an investigation may be carried out.

At first an Environmental Health Officer, employed by the local authority, looks into the case. Details of symptoms, onset and foods eaten are collected. Where there is evidence of an outbreak (i.e. two or more cases appear to be linked), then investigation may involve the health authority, Public Health Laboratory Service, etc.

Investigation of an outbreak must be conducted swiftly and thoroughly 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);
    prevent a recurrence;
  • decide if someone has broken food safety law.

Information is collected about where the food was prepared or served, the food(s) involved, the organism responsible, where it may have come from, how the problem occurred, etc. Faecal and food samples (where available) will be collected for laboratory investigation.