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Food Poisoning

The Facts

Food poisoning is a very common illness. For most people it is usually mild, but food poisoning can be severe and even deadly for some individuals.

Most cases of food poisoning occur when people eat food or drink water containing bacteria, bacterial toxins (substances produced by bacteria), parasites, or viruses. Food poisoning can also occur when non-infectious poisons (such as poisonous mushrooms) or heavy metals (such as lead or mercury) find their way into people's stomachs.

It is estimated that about 4 million Canadians experience food poisoning each year. People at greatest risk for food poisoning are seniors, pregnant women, young children and babies, and people with chronic medical conditions (e.g., diabetes, AIDS, liver disease).


Food poisoning occurs when contaminated food or water is ingested. Contamination can occur anywhere along the process of obtaining and eating food – it can occur during growing, harvesting, processing, storing, or preparation stages. In most cases, bacteria, viruses, or parasites are transferred to food from other sources, making these organisms the most common causes of food poisoning. However, in some less common types of food poisoning, the poison or toxin is naturally part of the food (e.g., poisonous mushrooms or fish). Other less common causes include shellfish and insecticides.

Bacteria and bacterial toxins: Many bacteria can cause food poisoning, either directly or by the toxins they produce. Some of the most common include Salmonella, E. coli, Shigella, Staphylococcus, Campylobacter, and Clostridium perfringens. Many bacterial causes of food poisoning can be found in undercooked meats, poultry, eggs, dairy, processed meats, fish, custards, cream pies, and contaminated water.

Viruses: Norovirus and other viruses can cause food poisoning, most commonly through contaminated raw or uncooked produce and shellfish from contaminated water.

Parasites: Parasites such a giardia lamblia can also cause food poisoning through contaminated produce and water.

Mushrooms and toadstools: Dozens of species can cause muscarine poisoning. These poisons attack the central nervous system, causing partial or complete paralysis in severe cases.

Fish: Some fish, like the puffer fish, are naturally poisonous. A poison similar to that naturally found in the puffer fish can also occur in many edible Caribbean and Pacific species. It's called ciguatera poison, and it's produced by a tiny sea parasite called a dinoflagellate. This poison attacks the nervous system.

Another kind of fish poison, called scombroid poison, is a concentrated histamine. Fish containing toxic levels of histamine often taste unusually bitter or spicy.

Shellfish: Clams, mussels, oysters, and scallops can cause poisoning when they ingest certain poisonous dinoflagellates that produce the toxin saxitoxin. This is more likely to occur in North America between June and October. Shellfish eaten during those months are potentially dangerous.

Insecticides: There are many types of poisons found in insecticides but the most dangerous types are the organophosphates, which are basically nerve gas for insects. Such insecticides are deliberately formulated to be less harmful to humans than insects, but these chemicals can be very dangerous to people if the insecticides are not used properly.

There are many other causes of food poisoning. These include wild nuts, leaves, flowers and berries, underripe tubers, botulism, cadmium from containers, lead or arsenic from fertilizers, and acids and lead from pottery.

Symptoms and Complications

Almost all forms of food poisoning produce nausea, vomiting, abdominal cramps, and diarrhea. The bacterial causes of food poisoning tend to cause these symptoms as well as fever and headache. Symptoms can start within hours to days after eating the contaminated food and last from a day to a week.

Many non-infectious (not caused by bacteria and their toxins, viruses, etc.) food poisoning affects the central nervous system and cause symptoms typical of nerve poisons. Eating shellfish contaminated with saxitoxin, for example, will produce weakness or paralysis around the mouth in a few minutes, which slowly spreads to the rest of the body. Signs of ciguatera poisoning include face pain, headache, itching, and odd sensations of alternating hot and cold. Scombroid (histamine) fish poisoning causes the symptoms of excess histamine. Flushing and skin rash of the face, stomach pain and diarrhea appear within 1 hour.

Mushroom poisoning also attacks the nervous system. Shrunken eye pupils, tears, salivation or frothing at the mouth, sweating, vertigo, confusion, coma, and sometimes seizures appear within 2 hours of eating a poisonous mushroom. Insecticides based on organophosphates cause very similar symptoms. They're likely to be milder, since it is extremely rare for really large doses of insecticide to be eaten accidentally.

The most common complication of food poisoning is dehydration, when your body loses too much water and electrolytes (e.g., sodium, potassium). Food poisoning caused by the bacteria Listeria can cause problems for unborn babies, and E. coli infection can cause problems with the kidneys. Other complications can include arthritis and bleeding problems. Non-infectious food poisoning can occasionally lead to permanent nervous system problems and even death.

Making the Diagnosis

For food poisoning caused by bacteria, viruses, and parasites, a diagnosis can usually be made based on symptoms, history of exposure, and a physical examination. Your doctor may order blood tests to check for dehydration or ask for a stool sample to check for bacteria or parasites.

For other types of food poisoning, getting the right diagnosis early can be vital. Some poisons have specific antitoxins that will cure the poisoning completely.

When people are poisoned in groups, it is usually easier to pinpoint the cause. Often, there's only one food that all the sick people ate, and this can be studied to determine the culprit.

Treatment and Prevention

The treatment of food poisoning depends on the cause and on its severity.

For most people, food poisoning resolves quickly without treatment. For people with mild diarrhea lasting less than 24 hours, treatment should consist of drinking clear fluids such as oral replacement solutions. These solutions contain the right balance of water, salts, and sugar needed to prevent or treat mild dehydration. Use commercially available solutions when possible, but if necessary, a solution can be made by mixing 1/2 teaspoonful of salt, 6 teaspoonfuls of sugar with 1 litre of water.

It may be best to stay away from solid food during diarrhea and vomiting. Once you are able to take fluids, gradually start eating plain foods as tolerated. Avoid alcohol and caffeine while you are sick. People with severe symptoms or severe dehydration may need to be admitted to the hospital so they can receive rehydration solutions intravenously (into a vein).

Most bacterial food poisonings do not need antibiotics, but some types of infections may need antibiotic treatment.

For food poisonings that cause nervous system effects, there may be other medications or antidotes that can be used. For example, in mushroom (muscarine) and insecticide poisoning, a medication called atropine* can be used to counterattack toxic effects.

If poisoning is very severe, a patient may require a ventilator (artificial breathing machine), kidney dialysis, and or admission to a hospital intensive care unit.

You can't always prevent food poisoning, but there are some things that you can do to minimize your risk. The following are some tips:

  • Wash your hands thoroughly with soap and warm water for at least 20 seconds before and after handling food, after using the bathroom, changing diapers, or touching animals.
  • If you have a skin infection like impetigo (Staphylococcus bacteria), don't prepare food for others while spots or sores are visible.
  • Try to keep different foods and food types separate during preparation and storage.
  • Use a separate cutting board and knife for raw foods and cooked foods.
  • When reheating food, cook it thoroughly enough that the core reaches at least 75°C (170°F). This won't remove all poisons or kill all bacteria, but it helps against some common kinds.
  • Be aware that some foods are more prone to causing food poisoning than others, which means you have to handle them more carefully. Green vegetables and carrots, for example, are less likely to be toxic than fish, meat, poultry, eggs, and dairy.
  • Pay special attention to thoroughly cook meat and poultry, ensuring that recommended internal temperatures are reached.
  • If you're keeping leftovers, refrigerate them as soon as possible. Do not let them sit out for longer than one hour or cool to room temperature.
  • Do not thaw foods at room temperature - put in the refrigerator for thawing.
  • Throw out foods that could be contaminated. 2 days is usually the maximum that prepared foods should be stored in the refrigerator. Otherwise, it should be frozen.
  • Keep cold foods cold and hot foods hot.
  • Don't let kids lick the spoon if raw eggs are an ingredient.

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