Chapter reviewed and updated in December 2017. A description of changes can be found at Updates to the Communicable Disease Control Manual.


New Zealand Epidemiology

Although most cases of listeriosis are sporadic, outbreaks have occurred in New Zealand. The highest rates of disease are in immunocompromised individuals and neonates.

More detailed epidemiological information is available on the Institute of Environmental Science and Research (ESR) surveillance website.

Further information on foodborne illness is available on the Ministry for Primary Industries website.

Case definition

Clinical description

Listeriosis most commonly presents with diarrhoea, often associated with fever, myalgia and vomiting. Bacteraemia most often occurs in pregnant women (usually in the third trimester), the elderly and immunosuppressed. In pregnant women, the fetus may become infected, sometimes leading to miscarriage, stillbirth, premature delivery, newborn septicaemia or meningitis. The elderly and immunosuppressed may present with septicaemia, meningitis or pyogenic foci of infection.

Laboratory test for diagnosis

Laboratory definitive evidence for a confirmed case requires identification of Listeria monocytogenes from a normally sterile site, including the foetal gastrointestinal tract by one of the following:

  • isolation (culture) of L. monocytogenes
  • detection of L. monocytogenes nucleic acid.

Case classification

  • Under investigation: A case that has been notified, but information is not yet available to classify it as confirmed.
  • Probable: Not applicable
  • Confirmed: A clinically compatible illness accompanied by laboratory definitive evidence.
  • Not a case: A case that has been investigated and subsequently found not to meet the case definition.

Cases can be further classified, if appropriate, as follows.

Pregnancy associated case:

  • Cases are classified as pregnancy-associated if illness occurs in a pregnant woman, fetus, or infant aged ≤28 days old; for these cases it is the pregnant woman or mother who is notified as the case but information regarding the fetus or infant should be included on the case form.
  • All other cases are considered not to be associated with pregnancy.

Spread of infection


L. monocytogenes can be detected in soil, water, silage and food. Reservoirs include humans, domestic and wild animals and fowl. Listeria can multiply in refrigerated foods, unlike most pathogens, and can grow in biofilms.

Incubation period

Variable. Outbreak cases have occurred 3–70 days following exposure to a contaminated food product. Median incubation period is estimated to be 3 weeks.

Mode of transmission

L. monocytogenes can survive and grow at normal refrigeration temperatures and ingestion of contaminated foods such as unpasteurised milk or cheese, contaminated pasteurised soft cheeses, contaminated vegetables or meat products such as pâté, or shellfish have been major sources of infection. In perinatal infections, the fetus is infected in utero or during delivery.

Period of communicability

Mothers of infected infants may shed the bacteria in vaginal discharges and urine for 7–10 days after delivery. Infected individuals may shed the organism in their stool for several months, even after resolution of symptoms.


Notification procedure

Attending medical practitioners or laboratories must immediately notify the local medical officer of health of suspected cases. Notification should not await confirmation.

See Appendix 5: Escalation pathways for more information

Management of case


Obtain a food history (use the specific food-source questionnaire available through ESR); details of pregnancy for cases of perinatal infection; and for other cases, medical co-morbidity and ingestion of potentially contaminated foodstuffs.

Ensure samples from symptomatic people and any foodstuffs implicated have been cultured for L. monocytogenes. Discretion should be applied before testing of food items linked to sporadic cases. Testing may be of value if food items were consumed shortly after purchase and were stored in their original unopened packaging before consumption; however, testing of leftover items that have been stored in previous-opened packaging is unlikely to be useful in the investigation of a sporadic case, and may not be a good use of resources. Testing may be more liberally undertaken in an outbreak situation.

Molecular subtyping may be used to determine the association between isolates from cases and any foodstuffs that test positive for L. monocytogenes. Investigate the source of contamination of any foods found to test positive for L. monocytogenes. Recall contaminated foodstuffs if necessary.




Advise the case and their caregivers of the nature of the infection and its mode of transmission. Asymptomatic mothers of neonatal cases can shed the organism for up to 10 days after delivery.

Management of contacts


All people who have been exposed to the same food material suspected to be the source of infection.


Investigate contacts who are symptomatic.






Advise all contacts to seek early medical attention if symptoms develop.

Other control measures

Identification of source

Where food/food businesses are thought to be involved inform the Ministry for Primary Industries.



Health education

Advise pregnant women, the elderly and immunosuppressed people to avoid the following foods:

  • smoked fish or shellfish, pre-cooked fish and uncooked fish or seafood products (including sushi and sashimi) that are chilled or frozen (unless reheated thoroughly and eaten hot)
  • pre-cooked meat products such as pâté and sliced deli meat (chicken, ham, salami)
  • pre-prepared or stored salads (including fruit salad) and coleslaws
  • raw (unpasteurised) milk and foodstuffs that contain unpasteurised milk
  • soft-serve ice creams
  • surface-ripened soft cheese (for example, brie, camembert, ricotta, blue vein, feta). Hard cheeses, processed cheeses, cream cheese, cottage cheese and yoghurt are safe.

(Refer also to Food safety: People with low immunity.)

Educate the public about safe food preparation (see Appendix 3: Patient information).

Advise pregnant women, the elderly and immunosuppressed people to avoid contact with potentially infective farm material, such as aborted animal fetuses.


National reporting

Ensure complete case information is entered into EpiSurv.

If a cluster of cases occurs, contact 0800GETMOH - CD option, and outbreak liaison staff at ESR, and complete the Outbreak Report Form.

Where food/food businesses are thought to be involved inform the Ministry for Primary Industries.