Food poisoning

Food borne illness or food poisoning is caused by consuming food contaminated with pathogenic bacteria, toxins, viruses, prions or parasites. Such contamination usually arises from improper handling, preparation or storage of food. Food borne illness can also be caused by adding pesticides or medicines to food, or consuming or by accidentally consuming naturally poisonous substances like poisonous mushrooms or reef fish. Contact between food and pests, especially flies, rodents and cockroaches, is a further cause of contamination of food.

Some common diseases are occasionally food borne mainly through the water vector, even though they are usually transmitted by other routes. These include infections caused by Shigella, Hepatitis A, and the parasites Giardia lamblia and Cryptosporidium parvum.

World Health Organization definition

Food borne illnesses are defined by the World Health Organization as diseases, usually either infectious or toxic in nature, caused by agents that enter the body through the ingestion of food. Every person is at risk of food borne illness

Symptoms and mortality

Symptoms typically begin several hours after ingestion and depending on the agent involved, can include one or more of the following: nausea, abdominal pain, vomiting, diarrhea, fever, headache or tiredness. In most cases the body is able to permanently recover after a short period of acute discomfort and illness. However, food borne illness can result in permanent health problems or even death, especially in babies, pregnant women (and their fetuses), elderly people, sick people and others with weak immune systems. Similarly, people with liver disease are especially susceptible to infections from Vibrio vulnificus, which can be found in oysters.

Incubation period

The delay between consumption of a contaminated food and appearance of the first symptoms of illness is called the incubation period. This ranges from hours to days (and rarely years), depending on the agent, and on how much was consumed.

During the incubation period, microbes pass through the stomach into the intestine, attach to the cells lining the intestinal walls, and begin to multiply there. Some types of microbes stay in the intestine, some produce a toxin that is absorbed into the bloodstream, and some can directly invade the deeper body tissues. The symptoms produced depend on the type of microbe.

Infectious dose

The infectious dose is the amount of agent that must be consumed to give rise to symptoms of food borne illness. The infective dose varies according to the agent and consumer's age and health. In the case of Salmonella, as few as 15-20 cells may suffice.

Pathogenic agents


Bacterial infection is the most common cause of food poisoning. In the United Kingdom during 2000 as follows: Campylobacter jejuni 77.3%; Salmonella 20.9% and Escherichia coli O157:H7 1.4%, all others less than 0.1%.

Symptoms for bacterial infection are delayed because the bacteria need time to grow, so symptoms are usually not seen 12-36 hours after eating contaminated food.

Common bacterial food borne pathogens are:

  • Aeromonas hydrophila, Aeromonas caviae, Aeromonas sobria
  • Bacillus cereus
  • Brucella spp.
  • Campylobacter jejuni which causes Guillain-Barre syndrome
  • Corynebacterium ulcerans
  • Coxiella burnetii or Q fever
  • Crohn's disease
  • Escherichia coli O157:H7 enterohemorrhagic (EHEC) which causes hemolytic-uremic syndrome
  • Escherichia coli - enteroinvasive (EIEC)
  • Escherichia coli - enteropathogenic (EPEC)
  • Escherichia coli - enterotoxigenic (ETEC)
  • Listeria monocytogenes
  • Plesiomonas shigelloides
  • Salmonella spp.
  • Shigella spp.
  • Streptococcus
  • Vibrio cholerae, including O1 and non-O1
  • Vibrio parahaemolyticus
  • Vibrio vulnificus
  • Yersinia enterocolitica and Yersinia pseudotuberculosis


In addition to disease caused by direct bacerial infection, some foodborne illnesses are caused by exotoxins which are excreted by the cell as the bacterium grows. Exotoxins can produce illness even when the microbes that produced them have been killed. Symptoms typically appear after 1-6 hours depending on the amount of toxin ingested.

  • Clostridium botulinum
  • Clostridium perfringens
  • Staphylococcus aureus

For example Staphylococcus aureus produces a toxin that causes intense vomiting. The rare but potentially deadly disease botulism occurs when the anaerobic bacterium Clostridium botulinum grows in improperly canned low-acid foods and produces a powerful paralytic toxin.

Preventing bacterial food poisoning

Bacteria need warmth, moisture, food and time to grow. The presence, or absence, of oxygen, salt, sugar and acidity are also important factors for growth. In the right conditions, one bacteria can multiply using binary fission to become four million in eight hours. Since bacteria cannot be smelt or seen, the only way to ensure that food is safe is to follow good food hygiene, for example, not allowing raw or partially cooked food to touch dishes, utensils, hands or work surfaces previously used to handle properly cooked food or ready to eat food.

High salt, high sugar or high acid levels keep bacteria from growing, which is why salted meats, jam, and pickled vegetables are traditional preserved foods.

The most frequent causes of bacterial foodborne illness is cross-contamination or inadequate temperature control. Therefore control of these two matters is especially important.

Thoroughly cooking food until it is piping hot, i.e. above 70?C (158?F) will quickly kill virtually all bacteria, parasites or viruses, except for Clostridium botulinum and Clostridium perfringens, which produces a heat-resistant spore that survives temperatures up to 100?C (212?F). Once cooked, hot foods should be kept hot - above 63?C (145?F) stops multiplication.

Cold foods should be kept cold, below 5?C (41?F). However, Listeria monocytogenes and Yersinia enterocolitica can both grow at refrigerator temperatures.

Natural toxins

In contrast several foods can naturally contain toxins that are not produced by bacteria and occur naturally in foods, these include:

  • Aflatoxin
  • Alkaloid, see Hemlock
  • Ciguatera poisoning
  • Grayanotoxin (Honey intoxication)
  • Mushroom toxin
  • Phytohaemagglutinin (Red kidney bean poisoning)
  • Pyrrolizidine alkaloid
  • Shellfish toxin, including Paralytic shellfish poisoning, Diarrhetic shellfish poisoning, Neurotoxic shellfish poisoning, Amnesic shellfish poisoning and Ciguatera fish poisoning)
  • Scombrotoxin
  • Tetrodotoxin (Fugu fish poisoning)


Most food borne parasites are typically zoonoses.


  • Taenia saginata
  • Taenia solium


  • Trichinella spiralis


  • Giardia lamblia
  • Sarcocystis hominis
  • Sarcocystis suihominis
  • Toxoplasma gondii


  • Fasciola hepatica


  • Acanthamoeba and other free-living amoebae
  • Anisakis sp.
  • Ascaris lumbricoides
  • Cryptosporidium parvum
  • Cyclospora cayetanensis
  • Diphyllobothrium spp.
  • Entamoeba histolytica
  • Eustrongylides sp.
  • Nanophyetus spp.
  • Trichuris trichiura


  • Hepatitis A
  • Hepatitis E
  • Norwalk virus
  • Rotavirus

Other pathogenic agents

  • Prions, resulting in Creutzfeldt-Jakob Disease


The vast majority of reported cases of food borne illness occur as individual or sporadic cases. In most cases these originate, and occur, in the home. An outbreak occurs when two or more people suffer food borne illness after consuming food from a contaminated batch.

Often, a combination of events contributes to an outbreak, for example, food might be left at room temperature for many hours, allowing bacteria to multiply which is compounded by inadequate cooking which results in a failure to kill the dangerously elevated bacterial levels.

Outbreaks are usually identified when those affected know each other. However, some are identified by public health staff from unexpected increases laboratory results for certain strains of bacteria.

previous.gif (72x17 -- 347 bytes) next.gif (72x17 -- 277 bytes)

Are you a Doctor, Pharmacist, PA or a Nurse?

Join the Doctors Lounge online medical community

  • Editorial activities: Publish, peer review, edit online articles.

  • Ask a Doctor Teams: Respond to patient questions and discuss challenging presentations with other members.

Doctors Lounge Membership Application

Tools & Services: Follow DoctorsLounge on Twitter Follow us on Twitter | RSS News | Newsletter | Contact us