Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Investigation/outbreak measures

Victorian statutory requirement

Listeriosis (Group B disease) must be notified in writing within five days of diagnosis.

Laboratories are required to notify Listeria monocytogenes isolated from food or water.

Infectious agent

L. monocytogenes is a gram-positive bacterium belonging to the genus Listeria. Of the seven recognised species it is currently the only one implicated in human cases.

Identification

Clinical features
Listeriosis predominantly affects:

• people who have immunocompromising illnesses such as leukaemia, diabetes and cancer
• the elderly
• pregnant women and their foetuses
• newborn babies
• people on immunosuppressive drugs such as prednisone or cortisone.

Healthy adults are usually not affected but may experience transient, mild to moderate flu-like symptoms.

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 foetus through the placenta, which can result in stillbirth or premature birth. Babies may be severely affected with conditions such as septicaemia 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 non-pregnant cases listeriosis usually presents as an acute meningoencephalitis or septicaemia. Focal infections such as pneumonia, endocarditis, infected prosthetic joints, localised internal abscesses and granulomatous lesions in the liver and other organs have been described. Symptoms may have a sudden onset. Fever, severe headache, nausea and vomiting can lead to prostration and shock.

The reported case fatality rate has been around 30% in both pregnancy and non-pregnancy related groups.

Method of diagnosis
Listeriosis is diagnosed by isolation of Listeria monocytogenes from blood, CSF, placenta, meconium, foetal gastrointestinal contents and other normally sterile sites.

Incubation period

The incubation period is mostly unknown. Outbreak cases have occurred 3-70 days after a single exposure to an implicated product. Median incubation is estimated to be three weeks.

Public health significance & occurrence

Listeriosis is an uncommon disease in humans. In Australia in 2003 the rate was three infections per million population for non-pregnancy Listeriosis cases and 4.6 infections per 100 000 births per year for maternal-foetal infections.

Although most human cases appear to be sporadic, three large outbreaks reported overseas have clearly established L. monocytogenes to be a food-borne pathogen. These three outbreaks in the Maritime Provinces (1981), Massachusetts (1983) and Los Angeles County (1985) involved a total of 232 cases. The overall case fatality rate was 36%. The implicated foods were coleslaw, pasteurised milk and Mexican-style soft cheese.

Reservoir

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 five per cent in the general population has been reported. The significance of these carriers in the epidemiology of listeriosis is unknown.

Mode of transmission

The main route of transmission is oral through ingestion of contaminated food. Other routes include mother to foetus via the placenta or at birth. The infectious dose is unknown.

Period of communicability

Mothers of infected newborns may shed the infectious agent in vaginal discharges and urine for seven to ten days after delivery. Infected individuals can shed the organisms in their stools for several months.

Susceptibility & resistance

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 foetuses
• newborn babies
• people on immunosuppressive drugs (such as prednisone or cortisone).

There is little evidence of acquired immunity even after prolonged severe infection.

Control measures

Preventive measures
It is important to educate people in high risk groups about the foods likely to be contaminated and about safe food handling and storage.

People in high risk groups for listeriosis should avoid the following high risk foods:

• 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 or vegetables where washing procedures are unknown (excluding canned or pasteurised juices)
• pre-cooked meat products which are eaten without further cooking or heating, such as pate, sliced deli meat including ham, strassburg and salami and cooked diced chicken (as used in sandwich shops)
• any unpasteurised milk or foods made from unpasteurised milk
• soft serve ice creams
• soft cheeses, such as 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.

Safe foods include:

• freshly prepared foods
• freshly cooked foods, to be eaten immediately
• hard cheeses, cheese spreads, processed cheese
• milk- freshly pasteurised and UHT
• yoghurt
• canned and pickled food.

Safe food handling and storage:

• wash your hands before preparing food and between handling raw and ready to eat foods
• keep all food covered during storage
• 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 from dripping onto cooked and ready to eat foods
• keep your refrigerator clean and the temperature below 5°C
• strictly observe use-by and best before dates on refrigerated foods
• do not handle cooked foods with the same utensils (tongs, knives, cutting boards) used for 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 oven
• thoroughly cook all food of animal origin
• 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 (piping hot)
• reheat left overs until piping hot
• when using a microwave oven, read the manufacture?s instructions carefully and observe the recommended standing times, to ensure the food attains an even temperature before it is eaten.

Foods are regularly tested for the presence of L. monocytogenes. Processed, packaged ready to eat foods found to be contaminated with L. monocytogenes are recalled from sale.

Control of case
Treatment is usually with penicillin or amoxyl/ampicillin either alone or in combination with trimethoprim+sulfamethoxazole. For penicillin sensitive patients trimethoprim+sulfamethoxazole may be used alone (see the current edition of Therapeutic guidelines: antibiotic, Therapeutic Guidelines Limited).

Investigation/outbreak measures

• Obtain medical history from treating doctor.
• Obtain a food history from 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 at what point they became contaminated.
• Recall contaminated food if necessary.