Listeria infection is an illness usually caused by eating food that has been contaminated with the bacteria Listeria monocytogenes. Listeria infection is relatively uncommon; however, the fatality rate can be as high as 30% amongst at-risk people.
Who is at risk of Listeria infection?
Listeria infection can affect people differently. Healthy people may develop few or no symptoms. However, for some people, the infection can be serious enough to require hospitalisation and may even be life-threatening.
People who are at particular risk of serious infection include anyone whose immune system has been weakened, including the following:
- People with cancer including leukaemia
- People with HIV/AIDS
- People suffering liver or kidney disease
- The elderly
- Pregnant women and their unborn babies
- Anyone on immunosuppressant medications such as prednisone or cortisone.
- Organ transplant patients
- Newborn babies
Listeria in Pregnancy
In pregnant women, Listeria infection is usually a mild illness. A high temperature before or during labour may be the only sign. However, even a mild form of the illness can affect the unborn baby and can lead to miscarriage, stillbirth, premature birth or a very ill baby at birth. It is therefore important to minimise exposure to Listeria while you are pregnant.
Healthy people may not show any symptoms.
People who fall into any of the at-risk categories above (and in some cases previously healthy people) may show some or all of the following symptoms:
- Aches and pains
- Abdominal cramps
Left untreated, Listeria infection can develop into meningitis (brain infection) and septicaemia (blood poisoning).
Symptoms typically appear approximately 3 weeks after exposure to Listeria but can appear anywhere between 3-70 days after exposure.
Listeria can be effectively treated with antibiotics if detected early.
Where is Listeria found?
Listeria bacteria are most commonly found in soil, silage, sewage, birds and animals. They are also found in some foods including raw meat, raw vegetables and some processed foods.
It is not always possible to identify the source of a person’s Listeria infection because of the time delay between exposure and symptoms appearing.
Outbreaks of Listeria infection due to foods such as soft cheeses, milk, coleslaw, hot dogs and paté have been reported in Europe, America and Australia.
How can I avoid Listeria infection?
Prevention is better than Cure! People at risk from Listeria infection can reduce their risk of infection by:
- Saying no to high risk foods (see below)
- Always handling food safely (see safe food handling and storage)
- Avoiding contact with any animal afterbirth (placenta) and with aborted animal foetuses, as listeria infection has been known to cause illness and abortion in animals.
High risk foods
These foods should be avoided in pregnancy to minimise the chance of Listeria infection:
- Ready-to-eat seafood such as smoked fish and smoked mussels, oysters or raw seafood such as sashimi or sushi.
- Pre-prepared or stored salads, including coleslaw and fresh fruit salad
- Drinks made from fresh fruit and/or vegetables where washing procedures are unknown (excluding pasteurised or canned juices).
- Pre-cooked meat products which are eaten without further cooking or heating, such as paté, cured meat (including ham, prosciutto and salami), and cooked diced chicken (as used in sandwich shops).
- Any unpasteurised milk or foods made from unpasteurised milk.
- Soft serve ice cream.
- Soft cheeses e.g. brie, camembert, ricotta and feta (these are safe if cooked and served hot).
- Ready-to-eat foods, including leftover meats, which have been refrigerated for more than one day.
- Dips and salad dressings in which vegetables may have been dipped.
- Raw vegetable garnishes.
- Freshly prepared foods.
- Freshly cooked foods, to be eaten immediately.
- Hard cheeses, cheese spreads and processed cheese.
- Milk – freshly pasteurised and UHT.
- Canned and pickled food.
Safe food handling and storage
Safe food handling and safe storage of food are important for everyone. To anyone at risk of the serious complications of Listeria infection, such practices are especially important. Unlike most other food-contaminating bacteria, Listeria can grow in the refrigerator. However, Listeria bacteria are readily killed during cooking. You can reduce the risk of developing Listeria infection and other food-borne illnesses, such as gastroenteritis, by following some basic food hygiene and food storage rules:
- 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.
- Store raw meat, raw poultry and raw fish on the lowest shelves of your refrigerator to prevent them dripping onto cooked and ready-to-eat foods.
- Keep your refrigerator clean and the temperature below 5°C.
- Strictly observe use-by or best-before dates on refrigerated foods.
- Do not handle cooked foods with the same utensils (tongs, knives, and cutting boards) used on raw foods, unless they have been thoroughly washed with hot soapy water between uses.
- All raw vegetables, salads and fruits should be well washed before eating or juicing, and consumed fresh.
- Defrost food by placing it on the lower shelves of a refrigerator, or use a microwave. Thoroughly cook all food of animal origin, including eggs (no runny yolks during pregnancy!).
- 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 75°C (piping hot).
- When using a microwave, read the manufacturer’s instructions carefully and observe the recommended standing times, to ensure the food attains an even temperature before it is eaten.
Any further questions
For further advice, contact your local doctor, specialist, community health centre or maternal and child health nurse.