On this page
- Key messages
- Notification requirement for listeriosis
- Primary school and children’s services centre exclusion for listeriosis
- Infectious agent of listeriosis
- Identification of listeriosis
- Incubation period of Listeria monocytogenes
- Public health significance and occurrence of listeriosis
- Reservoir of Listeria monocytogenes
- Mode of transmission of Listeria monocytogenes
- Period of communicability of listeriosis
- Susceptibility and resistance to listeriosis
- Control measures for listeriosis
- Outbreak measures for listeriosis
- Listeriosis is an ‘urgent’ notifiable condition that must be notified immediately to the department by medical practitioners and pathology services. Laboratories are required to notify Listeria monocytogenes isolated from food or water.
- Listeriosis mainly affects certain vulnerable groups. Healthy adults are usually not affected, but may experience transient, mild to moderate flu-like symptoms.
- L. monocytogenes is widespread in the environment and commonly isolated from sewage, silage, sludge, birds, and wild and domestic animals.
- People in high-risk groups for listeriosis should avoid certain high-risk foods.
Notification requirement for listeriosis
Listeriosis is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately by telephone upon initial diagnosis (presumptive or confirmed). Pathology services must follow up with written notification within 5 days.
Laboratories are required to notify Listeria monocytogenes isolated from food or water.
These are Victorian statutory requirements.
Primary school and children’s services centre exclusion for listeriosis
Exclusion is not required.
Infectious agent of listeriosis
L. monocytogenes is a Gram-positive rod bacterium belonging to the genus Listeria. Of the seven recognised species, it is currently the only one implicated in human cases. There are at least 13 serovars of L. monocytogenes.
Identification of listeriosis
Listeriosis predominantly affects:
- people who have immunocompromising illnesses such as leukaemia, diabetes or cancer
- the elderly
- pregnant women and their fetuses
- newborn babies
- people on immunosuppressive drugs such as prednisolone or cortisone.
Healthy adults are usually not affected but may experience transient, mild to moderate flu-like symptoms. Ingestion of contaminated food by healthy, nonpregnant individuals can produce a self-limited febrile gastroenteritis.
Infection in pregnant women may be mild, and a temperature before or during birth may be the only sign. However, the infection can be transmitted to the fetus through the placenta, which can result in stillbirth or premature birth. Babies may be severely affected with conditions such as bacteraemia or meningitis (early-onset neonatal listeriosis).
Late-onset neonatal listeriosis generally affects full-term babies who are usually healthy at birth. The onset of symptoms in these babies occurs several days to weeks after birth (a mean of 14 days), possibly as a result of infection acquired from the mother’s genital tract during delivery or postnatally through cross-infection.
In nonpregnant immunosuppressed patients, listeriosis usually presents as an acute meningoencephalitis, brain stem encephalitis (rhomboencephalitis), brain abscess or bacteraemia. Focal infections such as pneumonia, endocarditis, infected prosthetic joints, localised internal abscesses, and granulomatous lesions in the liver and other organs have also been described. Symptoms may have a sudden onset; however, in 60 per cent of cases, presentation is subacute (>24 hours). Fever, severe headache, nausea and vomiting can lead to prostration and shock.
The reported case-fatality rate has been around 30 per cent in both pregnancy- and nonpregnancy-related groups.
Listeriosis is diagnosed by isolation of L. monocytogenes from blood, cerebrospinal fluid, placenta, meconium, fetal gastrointestinal contents and other normally sterile sites.
Incubation period of Listeria monocytogenes
The incubation period is mostly unknown. Outbreak cases have occurred 3–70 (mean 31) days after a single exposure to an implicated product. Median incubation is estimated to be 3 weeks.
Public health significance and occurrence of listeriosis
Listeriosis is an uncommon disease in humans. In Australia in 2013, the rate was three infections per million population.
Reservoir of Listeria monocytogenes
L. monocytogenes is widespread in the environment and commonly isolated from sewage, silage, sludge, birds, and wild and domestic animals. It has caused infection in many animals, and resulted in abortion in sheep and cattle. The bacteria are commonly isolated from poultry. It is a common contaminant of raw food.
Asymptomatic vaginal carriage occurs in humans, and faecal carriage of up to 5 per cent in the general population has been reported. The significance of these carriers in the epidemiology of listeriosis is unknown.
Mode of transmission of Listeria monocytogenes
The main route of transmission is oral, through ingestion of contaminated food. Other routes include mother-to-fetus transmission via the placenta or at birth. The infectious dose is unknown. Horizontal human-to-human infection has not been documented.
Period of communicability of listeriosis
Mothers of infected newborns may shed the infectious agent in vaginal discharges and urine for 7–10 days after delivery. Infected individuals can shed the organisms in their stools for several months.
Susceptibility and resistance to listeriosis
Although healthy people can be infected, the disease generally affects vulnerable groups in the community, such as:
- people who have immunocompromising illnesses (such as leukaemia, diabetes, cancer)
- the elderly
- pregnant women and their fetuses
- newborn babies
- people on immunosuppressive drugs (such as prednisolone or cortisone).
There is little evidence of acquired immunity, even after prolonged severe infection.
Control measures for listeriosis
Listeria bacteria are common throughout the environment, including in soil, water and the digestive systems of animals. Unlike many bacteria, they can continue to multiply under refrigeration temperatures.
Refrigerated foods can become contaminated with the bacteria, which can then grow to dangerous levels under certain conditions.
Foods most commonly associated with Listeria include:
- foods with a long shelf-life that are kept under refrigeration; and
- foods that are eaten without any further treatment such as cooking that would kill the bacteria.
The following foods are considered high-risk for Listeria and should be avoided by at-risk people, including pregnant women:
- raw fruit, vegetables, and herbs where washing procedures are unknown, including raw vegetable and herb garnishes.
- ready-to-eat seafood that will not be further cooked – including smoked seafood (such as fish, mussels, and oysters), raw seafood (such as sashimi, sushi and oysters), and cooked/chilled seafood (such as peeled prawns).
- pre-prepared, pre-cut, or pre-packaged fruits and vegetables – including those available from greengrocers, supermarkets, buffets, salad bars and sandwich bars.
- drinks made from fresh or frozen fruit and vegetables where washing procedures are unknown, such as juices and smoothies (excluding pasteurised or canned juices)
- cold meat products eaten without further cooking or heating, as commonly found in delis, sandwich shops, or pre-packaged in supermarkets – such as pate, ham, salami, processed/fermented meat products, and cold pre-cooked chicken.
- unpasteurised milk or foods made from unpasteurised milk
- soft-serve ice-creams
- soft cheeses – such as brie, camembert, ricotta, blue-vein, and feta.
- refrigerated ready to eat dips such as hummus, pesto, guacamole, and tahini.
- raw or lightly cooked sprouts including green sprouts like alfalfa, and bean sprouts.
- sandwiches containing any of the high-risk ingredients mentioned above.
- ready-to-eat foods – including leftover meats, which have been refrigerated for more than one day and will not be further reheated/cooked.
Hints for handling and preparing food
When you handle and prepare , you should take care to:
- Wash and dry your hands before preparing food, and between handling raw food and ready-to-eat foods.
- Wash raw fruit and vegetables thoroughly under running water before eating.
- Cook all foods of animal origin, including , thoroughly.
- Don't use the same boards and knives for cooked foods that you used for raw foods unless they have been washed in warm, soapy water and dried.
- Defrost food by placing it on the lower shelves of the fridge or use a microwave.
- Always follow advice on food labels. Cooking foods before eating them can kill many bacteria, including Listeria.
Store food with care
When you store , you should:
- Keep food covered.
- Place cooked food in the fridge within one hour of cooking.
- Put raw meat, poultry and fish below cooked or ready-to-eat food in the fridge to prevent drips that could contaminate pre-prepared food.
- Do not use refrigerated foods beyond their use-by dates.
- Keep your fridge clean.
Follow these tips on food temperatures:
- Your fridge temperature should be below 5 °C.
- 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 is piping hot.
- Ensure microwaved food reaches the desired temperature throughout before eating.
Outbreak measures for listeriosis
- Obtain a medical history from the treating doctor.
- Obtain a food history from the patient.
- Test any available suspected foods.
- Assess the possibility of common-source outbreaks if there is a cluster of cases.
- Epidemiological investigation of cases should be used to detect outbreaks and to determine source.
- Molecular subtyping should be used to determine the association between isolates from cases and any foods positive for L. monocytogenes.
- Investigate the source of any foods found to be positive for L. monocytogenes to determine the point at which they became contaminated.
- Recall contaminated food, if necessary.
Reviewed 16 January 2023