Food poisoning

Food poisoning is defined as an acute illness, which usually includes one or more gastrointestinal symptoms, caused by the recent consumption of food or drink.

Although food poisoning can occur as a result of substances like heavy metals, pollutants or natural plant toxins, most people think of it as microbiological in origin - caused by bacteria, viruses and protozoa. Micro-organisms can cause food poisoning in two ways, by directly invading the intestinal wall (e.g. salmonella), or by producing a toxin. This toxin may be produced by living micro-organisms in the gut, or may be in food already. If the food has been cooked after it was infected, the micro-organisms may have been destroyed, but their toxins can remain and still cause symptoms of food poisoning.

If food poisoning occurs within a few hours after consumption of the offending food, this tends to suggest that it is toxic in origin. Symptoms of Salmonella infection take 12-36 hours to appear, and campylobacter can take up to 5 days. Salmonella species can invade the blood, causing fever and even meningitis and multiple abscesses. Food contaminated with the toxins of Clostridium botulinus (the botulism agent) may cause no gastrointestinal symptoms at all, and botulism is often fatal because it is not diagnosed in time. Symptoms of botulism, which include fatigue, swallowing difficulties and double vision, may take 18 hours to 1 week to appear.

Gastroenteritis is often caused by E coli (a common cause of travellers' diarrhoea and infant diarrhoea) and Giardia lamblia. However, these are not normally food-borne but water-borne organisms. Campylobacter causes fever, bloody diarrhoea, abdominal pain and nausea. Listeria, sometimes found in soft cheeses, can be fatal to an unborn child if eaten by a pregnant woman.

It is usually not possible to tell from the taste or smell of food whether it could cause food poisoning. Food allergy may occasionally cause identical symptoms to those of food poisoning.