Listeria infection or listeriosis, is an illness which can be contracted by eating food contaminated with bacteria known as Listeria monocytogenes.
Below is a detailed guide to listeria and how you can reduce your risk of contracting the harmful bacteria.
Who is at higher risk of contracting listeriosis?
- Pregnant women and their unborn babies
- Newborn babies
- The elderly
- Anyone whose immune system has been weakened by disease or illness, for example, Cancer or Diabetes
- Anyone on medications which can suppress the immune system
- Organ transplant patients.
Where is listeria bacteria found?
Listeria bacteria can be found in soil, silage and sewage. They have also been found in foods, including raw meat, raw vegetables and some processed foods. Outbreaks of Listeria infection have been reported from eating contaminated soft cheeses, milk, coleslaw, hot dogs and paté.
As Listeria bacteria are commonly found in the environment, they are impossible to eradicate. Some exposure to the bacteria is unavoidable. Most people are, however, at low risk of Listeria infection.
What are the symptoms of listeriosis?
Symptoms of Listeriosis differ in those affected – healthy people may develop few or no symptoms and for others the infection can become serious enough to require hospitalisation and may threaten their life.
In persons at risk, symptoms may include:
These symptoms can progress to more serious symptoms, such as meningitis (a brain infection) and septicaemia (blood poisoning). Less common symptoms of Listeriosis include diarrhoea, nausea and abdominal cramps. In pregnant women, Listeriosis is usually a mild illness and a high temperature before or during labour may be the only indicator that you have Listeriosis. However, even a mild form of the illness can affect an unborn baby and can lead to miscarriage, stillbirth, premature birth or a very ill baby at birth.
How common is listeriosis?
Listeriosis is fairly uncommon but the fatality rate amongst those at higher risk of the illness is high. In Victoria, Australia, between 8 and 24 cases of Listeria infection are reported to the Department of Human Services annually.
Can listeriosis be treated?
Listeria infection can be treated with antibiotics if treatment is started early, however it is sometimes not possible to distinguish which particular food caused a person’s illness as symptoms can take 3 to 70 days to appear after eating the contaminated food.
What can I do to reduce the risk of infection?
Avoid high risk foods and always practice good food handling and hygiene. High risk foods include the following:
- Ready-to-eat seafood (eg. smoked fish, smoked mussels, oysters or raw seafood such as sushi).
- Pre-made salads, including coleslaw.
- Pre-cooked meat products which do not require further cooking or heating (eg. paté, sliced deli meat, and cooked diced chicken)
- Unpasteurised milk or foods made from unpasteurised milk
- Soft serve ice-cream
- Soft cheeses (eg. brie, camembert, ricotta – but all are safe if cooked and served hot)
It is perfectly fine to eat the following foods:
- All freshly cooked foods
- Hard cheeses, cheese spreads, processed cheese
- Milk – freshly pasteurised and UHT (longlife)
- Canned and pickled food
Good food handling and safe storage of food are important for everyone but especially important for those in the higher risk group. Reduce the risk of developing Listeriosis and other food-borne illnesses by following some basic food hygiene and food storage rules, including:
- Wash your hands before preparing food and between handling raw and ready-to-eat foods
- Keep all food covered
- Place all cooked food in the refrigerator within one hour of cooking
- Refrigerate raw meat, raw poultry and raw fish below cooked or ready-to-eat foods to prevent any drip from contaminating these foods
- Keep your refrigerator clean and the temperature below 5°C
- Strictly observe use-by or best-by dates on refrigerated foods
- Do not handle cooked foods with the same implements (tongs, knives, cutting boards) used on raw foods, unless they have been thoroughly washed with hot soapy water between uses
- Wash raw vegetables and fruit well before eating
- Don’t let food defrost on the kitchen sink – defrost food by placing it on the lower shelves of a refrigerator or use a microwave oven.
- Thoroughly cook all animal products before eating them, including eggs
- Keep hot foods hot (above 60°C) and cold foods cold (at or below 5°C).
- Reheat food until the internal temperature of the food reaches at least 70°C. You can purchase cooking thermometers from your supermarket or variety store.
- When using a microwave oven, read the manufacturers instructions carefully and observe the recommended standing times, to ensure the food attains an even temperature before it is eaten.
Unlike most other food-contaminating bacteria, Listeria can grow in the refrigerator. However, Listeria bacteria are readily killed during cooking. At-risk people can further reduce their risk by:
- Eating only freshly prepared foods.
- Re-heating left-over foods till piping hot.
- Avoiding dips and salad dressing in which vegetables may have been previously dipped.
- Avoiding ready-to-eat foods which have been refrigerated for more than a day.