What are the causes of food poisoning?
Food poisoning is common and most of us will recognise the scenario. You ate something that looked (or smelled) a little dodgy, or you were abroad in a country where it isn't safe to drink the water, and you had a salad. Washed in the water you know you shouldn't drink. A few hours later you get tummy ache, and you are being sick (vomiting) and running backwards and forwards to the toilet. There are pet names for it in different parts of the world: Delhi belly, Kathmandu quickstep, Montezuma's revenge, Karachi crouch. But you can pick it up pretty much anywhere, including your own home.
Campylobacter is the most common germ (bacterium) that causes food poisoning in the UK. Other germs (bacteria) that can cause food poisoning include:
- Escherichia coli (usually shortened to E. coli).
- Clostridium perfringens.
These are another type of microbe. Parasites are living things (organisms) that live within, or on, another organism. Examples include cryptosporidium, Entamoeba histolytica and giardia parasites. Food poisoning caused by parasites is more common in the developing world.
In the UK, a common cause of food poisoning is Toxoplasma gondii. This is a parasite that lives in the bowels of a number of animals, including cats. Food poisoning can occur if food or water is contaminated with the stools (faeces) of infected cats, or if raw or undercooked meat from another animal carrying the parasite is eaten. The infection is known as toxoplasmosis. Symptoms of this type of food poisoning include swollen lymph glands and sometimes a skin rash.
Toxins and chemicals
Poisons (toxins) produced by bacteria can also contaminate food, as well as the bacteria themselves. For example, the bacterium Staphylococcus aureus can contaminate ice cream and its toxins can lead to food poisoning. The bacterium Bacillus cereus can contaminate rice. If contaminated rice is reheated and eaten, the toxins produced can lead to food poisoning.
Certain types of fish (including shark, marlin, swordfish and tuna) contain high levels of the chemical mercury. Eating these types of fish is not normally a problem for most people - it does not cause gastroenteritis or food poisoning. But pregnant women are advised to avoid eating shark, marlin and swordfish and to limit tuna. This is because a high level of mercury can damage the developing nervous system of an unborn baby.
Oily fish may be contaminated by chemicals called polychlorinated biphenyls. Again, this does not usually cause a problem or food poisoning for most people. However, you should limit the amount of oily fish you eat in pregnancy because of possible effects of these chemicals on a developing baby. The Department of Health recommends no more than two portions of oily fish a week.
Note: this is a general leaflet about food poisoning. There are separate leaflets that give more details about some of the different microbes that cause food poisoning.
How does food become contaminated?
Contamination of food can occur because of problems in food production, storage or cooking. For example:
- Not storing food correctly or at the correct temperature. For example, not refrigerating food. This is particularly a problem for meat and dairy products.
- Inadequate cooking of food (undercooking or not cooking to the correct temperature). Bacteria are often found in raw meat, including poultry. Adequate cooking usually kills the bacteria.
- Contamination by someone preparing the food who has not followed food hygiene rules and has not washed their hands properly.
- Contamination from other foods (cross-contamination). For example, not washing a board used to prepare raw meat before you cut a slice of bread using the same board. Storing raw meat in the fridge above food that is "ready-to-eat" and so allowing raw meat juices to drip on to the food below.
- Bacteria can also be present in unpasteurised milk and cheese. The pasteurisation process kills the bacteria.
How does water become contaminated?
Water can become contaminated with bacteria or other microbes usually because human or animal stools (faeces) get into the water supply. This is particularly a problem in countries with poor sanitation. In such countries, food may also be washed and prepared using contaminated water. So, for example, in countries with poor sanitation, you should always avoid:
- Drinking tap water.
- Having ice cubes in drinks (as the ice may have been made from tap water).
- Brushing your teeth with tap water.
- Eating salads (as the lettuce, tomatoes, etc, may have been washed in contaminated water).
- Eating uncooked vegetables (as they may have been washed in contaminated water).
Further reading and references
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