Public health priority: High in view of potential severity and potential to mitigate ongoing exposure.
PHU response time: Respond to confirmed cases within 1 working day of notification. Enter confirmed case on NCIMS on day of notification.
Case management: Notify the Communicable Diseases Branch. Provide data to National Enhanced Listeriosis Surveillance Scheme (NELSS) within 1 week of initial notification and update case details within 1 working day of new data receipt.
Contact management: Counsel and disseminate information to those exposed to a suspected common food source.
Human listeriosis is caused by a single species of Listeria, Listeria monocytogenes, which can be further characterised.
L. monocytogenes survives and multiplies in many non-human niches. Humans are an ‘accidental’ host, usually becoming infected following the consumption of contaminated food, or in the case of a fetus or newborn, vertically from their pregnant mother.
The sources of food contamination are many and varied as L. monocytogenes is widespread in the environment. For example, L. monocytogenes may be isolated from soil, surface water, decomposing organic matter, spoiled silage, sewage, commercial food-manufacturing environments, raw foods including vegetables, meats and dairy products and is carried in the gastrointestinal tract of many animals.
Although asymptomatic vaginal and faecal carriage has been reported (1) it is not considered a common source of infection in humans.
Nearly all cases of human listeriosis result from the consumption of L. monocytogenes contaminated food (2) and vertical transmission during gestation, or uncommonly, birth or shortly after birth.(1) Listeriosis can be acquired via contaminated food in the hospital or nursing home setting as food is often produced on a large scale for provision to vulnerable populations. The elderly, those with immunocompromising conditions or those whose treatment includes acid-suppressing medications are particularly at risk.(3-5)
Unusual modes of transmission include: nosocomial transmission in the newborn period (1); zoonotic transmission (6), including via direct contact with the placenta and other birth fluids of infected farm animals, particularly stillborn animals; and laboratory transmission.(7)
The outcome of exposure is highly dependent on the immune status of the host and the dose of organisms ingested.(1)
The incubation period can be up to 70 days after exposure (8) and can vary depending on the form of listeriosis. For invasive listeriosis, the median incubation period is 9 days (range 1–14 days) for central nervous system (CNS) involvement (e.g. meningitis) and 2 days (range 1–7 days) for bacteraemia.(9)
The median incubation period for pregnancy-associated listeriosis is 27.5 days (range 17–67 days).(9)
As horizontal person-to-person transmission generally does not occur this is not relevant to routine public health practice.
Listeriosis can manifest as an invasive or non-invasive infection.
Those at highest risk are generally the immunocompromised (by disease or treatment), elderly, pregnant, newborn and occasionally those who have certain chronic medical conditions (e.g. heart disease, diabetes, liver disease, renal disease, cancer, alcohol dependency) or are medicated (e.g. gastric acid inhibitors).(12-14)
Although invasive listeriosis is a relatively rare disease, the severity of the disease and the usual involvement of commercially manufactured foods, especially during outbreaks, mean that the social and economic impact of listeriosis is among the highest of the foodborne diseases. (2) The clinical impact appears to be much less for non-invasive listeriosis (febrile gastroenteritis).
Even though exposure to L. monocytogenes in food is probably relatively common (15), invasive listeriosis is an uncommon disease. In Australia, the five year mean (2011-2015) was 78 cases per year, with a notification rate of 0.3 per 100,000 population. Approximately equal numbers of cases were notified in males and females.(16)
In the United States, among women of reproductive age (15–44 years), pregnant women had a markedly higher listeriosis risk than non-pregnant women.(17) In Australia, there were 27 pregnancy-related cases between 2010 and 2014. There were 61 listeriosis-associated deaths in Australia between 2010 and 2014, comprising 51 adult and 10 fetal/neonatal deaths.(18)
During recent years, the incidence of listeriosis in most countries has remained constant, with a number of countries reporting declines in the incidence of disease. These reductions likely reflect the efforts in those countries by industry and governments including:
Most cases of listeriosis are sporadic, but outbreaks occur. Listeriosis outbreaks recognised and reported in Australia since 2005 are summarised in Table 1.
The main strategies for reducing exposure to L. monocytogenes are reducing contamination of food products and providing dietary recommendations to high risk groups to avoid potentially contaminated food. There is no evidence of acquired immunity(8) and no vaccine to prevent listeriosis.
A wide variety of foods may be contaminated with L. monocytogenes, but outbreaks and sporadic cases of listeriosis are predominantly associated with RTE foods.(2) RTE foods include commercially manufactured foods that have a long recommended refrigerated shelf-life and fresh foods that are consumed without further bactericidal treatment, e.g. cooking. Five key factors contribute strongly to the risk of listeriosis associated with RTE foods:
Food Standards Australia New Zealand (FSANZ) has regard to the Codex Alimentarius Commission guidelines19 in developing national standards for food processing controls. Regular testing programs for high risk foods to limit the maximum levels of L. monocytogenes in foods operate in Australian jurisdictions. Packaged RTE foods found to have an unacceptable level of contamination with L. monocytogenes may be recalled from sale.(20)
Local and State Government agencies responsible for the enforcement of the food safety standards regulate hygiene practices of food handlers in retail food establishments, including delicatessens and take away food premises, according to FSANZ Standards applied through jurisdictional Food Acts, to ensure the use of appropriate food handling and storage procedures (21). In the home, temperature control, limiting the length of storage periods and adequate cooking can mitigate increased risk of L. monocytogenes contamination.(22)
People in high risk groups for listeriosis should avoid high risk foods(22), for example:
Other dietary advice for people at risk of listeriosis on the Australian and New Zealand Food Standards website.
Details of confirmed cases should be entered on [NCIMS] within 1 working day of notification.
[Attempt to interview all cases (or care givers / next of kin where required) using the National OzFoodNet Listeria Case Questionnaire (Appendix 3). Attach completed questionnaires to the NCIMS record].
Once received, the [CDB staff] enter enhanced surveillance data on the OzFoodNet NELSS database.(23)
During an investigation it may be relevant to collect food from cases. This should be done in consultation with the [NSW Food Authority (NSWFA)].
Routine NELSS data is captured by CDB staff. OzFoodNet routinely analyses and disseminates this information on a fortnightly basis to OzFoodNet sites. Cases are also reported nationally to CDNA via OzFoodNet fortnightly surveillance reports.
Investigation data from a potential or declared MJOI are handled in accordance with the OzFoodNet MJOI guidelines.
On confirmation of a diagnosis of listeriosis, pathology laboratories and/or clinicians notify jurisdictions by urgent means, e.g. electronic laboratory notification or telephone, to the [local PHU].
The jurisdiction should ensure that the treating doctor is informed of the notification prior to case follow-up.
[Inform CDB via email@example.com of the notification within 1 working day of the diagnosis. Inform CD Oncall if an urgent afterhours public health response is required].
OzFoodNet central disseminates a fortnightly summary, to stakeholders, with supplemental reports as needed. Cases are also reported nationally to CDNA via OzFoodNet fortnightly surveillance reports.
Laboratory testing results should be communicated back to the investigating PHU and [CDB] for inclusion on [NCIMS] and the NELSS database.
When a specific food is suspected, the [NSW Food Authority] should be notified. National co-ordination of food recalls and subsequent communication is the responsibility of FSANZ. The National Food Incidence Response protocol exists to outline actions required by food regulators during investigations.
The MJOI protocol includes guidance on when notification under the International Heath Regulations 2005 is required.
The current case definition  for listeriosis is:
Only confirmed cases should be notified. Where a mother and foetus (≥20 weeks gestation)/neonate are both confirmed, both cases should be notified.
A confirmed case requires either:
Isolation or detection of Listeria monocytogenes from a site that is normally sterile, including fetal gastrointestinal contents.
A maternal/fetal pair where one of either the mother or fetus/neonate is a confirmed case by laboratory definitive evidence (up to 2 weeks postpartum).
Listeria infection is confirmed when L. monocytogenes is identified, mostly by culture, from sterile sites (often cerebrospinal fluid or blood), foetus/neonate (including gastrointestinal contents) or associated products of conception (e.g. amniotic fluid, placental tissue).
Listeria infection can also be identified by polymerase chain reaction (PCR) testing of specimens. Where listeriosis has been diagnosed using PCR, the sample should also be cultured to enable definitive characterisation of an isolate.
L. monocytogenes is not routinely sought in stool from sporadic cases presenting with febrile diarrhoea (non-invasive listeriosis), nor in the stool of unaffected persons outside a specific investigation or cluster/outbreak.
Serology is no longer used for the diagnosis of L. monocytogenes.
While non-culture methods for detection of Listeria spp. or L. monocytogenes in non-human samples (e.g. food and environmental) are often performed in routine testing, an isolate of L. monocytogenes should be sought as per the Australian Standard for food microbiology current at the time.(24)
When an isolate is cultured from an epidemiologically implicated food or otherwise during a cluster/outbreak investigation, the L. monocytogenes therein should be enumerated as per the Australian Standard for food microbiology current at the time.(25)
Isolates of L. monocytogenes from relevant non-human detections e.g. implicated foods, recalled foods, samples taken for any purpose during an investigation, should be forwarded to the jurisdictional reference laboratory for further characterisation to help inform attribution.
Methods of characterisation are evolving. There is a need for both rapid and definitive methods. Methods in use at any particular time will be decided by PHLN laboratories in consultation with OzFoodNet, jurisdictions and CDNA. All human L. monocytogenes isolates are characterised as part of NELSS. Non-human L. monocytogenes isolates should also be characterised.
Current methods include molecular serotyping, binary typing, multi-locus variable number tandem repeat analysis, multi-locus sequence typing, pulsed field gel electrophoresis and phylogenetic relatedness based on whole genome sequencing. Classical serotyping is no longer widely used.
Primary testing laboratories should refer isolates to jurisdictional public health laboratories for characterisation in a timely fashion.
Clusters may become evident from notification details and/or isolate characterisation details as reported by the laboratory. Routine analysis of NELSS data helps identify listeria clusters based on organism characterisation.
The case investigation should begin within 1 working day following the notification of a confirmed case.
[Note: steps reordered to delineate PHU and CDB functions]
[The PHU should take the following actions]:
[CDB (OzFoodNet Epidemiologists) should take the following actions]:
Antibiotic treatment should be prescribed by the treating physician as per the Australian Therapeutic Guidelines – Antibiotic. (26)
The case or relevant care-giver should be provided with advice about the nature of the infection and the mode of transmission (refer to Appendix 1: Listeriosis fact sheets).
Pregnant women and known immunocompromised persons should be educated about high risk foods and safe food handling and storage.
Exclusion from childcare, preschool, school or work is not necessary.
Active case finding should be initiated if there is evidence of a cluster of cases or of common exposure to a suspect source. [CDB] should be alerted to any isolation of L. monocytogenes in food served to vulnerable populations (e.g. meals on wheels, aged care, hospitals). Refer also to Section 11. Special situations.
L. monocytogenes is widely distributed in the environment and is frequently present in raw foods of both plant and animal origin. L. monocytogenes can survive and grow over a wide range of environmental conditions such as refrigeration temperatures (including the ability to survive freezing), low pH and high salt concentration and is resistant to a number of disinfectants, especially when organic matter is also present. It can remain viable in dry environments for long periods. This resilience provides a means for L. monocytogenes to contaminate and proliferate within food supplies, despite the use of common preservation methods designed to eradicate or limit the replication of other harmful microorganisms. It can persist in food processing environments resulting in post-processing contamination.(27)
Listeriosis is defined as a zoonosis, but direct transmission between ruminants and humans rarely occurs. In most cases of direct zoonotic transmission, the infections are non-life threatening cutaneous infections through contact with infected cattle or after handling of abortive material. However, ruminants, particularly cattle, contribute to amplification and dispersal of L. monocytogenes into the farm environment. Dairy farms and dairy processing facilities are frequently contaminated with L. monocytogenes compared to other environments, and its subtype populations in the farm environment encompass commonly strains that have been associated with human illness, whether sporadic or epidemic.(6)
Where a specific food has been identified as a suspected source, the [NSW Food Authority should be engaged to investigate] the premises where food was prepared and served to: determine the likelihood of disease transmission in that setting.
All L. monocytogenes isolates from foods subject to recalls should be forwarded to jurisdictional reference laboratories for further characterisation. L. monocytogenes isolated from foods implicated by cases should also be forwarded. A subset of jurisdictional food and environmental isolates should also be characterised on a regular basis.
[Food service managers in NSW Health facilities will routinely test food samples for Listeria and are required to notify their local PHU and the NSW Food Authority if L. monocytogenes is detected in food. Food is not sterile and Listeria bacteria are commonly found in food without ever causing harm. No action is required by the PHU unless there is a related case, or the food was ready-to-eat and served to patients. If there is reason to believe that a food contaminated was served to inpatients who are at higher risk for disease (e.g. because of underlying immune suppression or pregnancy), then the PHU should contact relevant clinicians (e.g. oncologists, immunologists and obstetricians) to inform them of the incident and remind them to consider listeriosis as a diagnosis in patients with consistent symptoms. Because there is no specific preventative action for people already exposed, direct contact with the patients is not useful].
Person to person transmission does not usually occur, so identifying contacts is not usually relevant for listeriosis.
[Local PHU, CDB (OzFoodNet) and NSW Food Authority work in partnership to investigate and control outbreaks. PHUs and CDB are responsible for undertaking the epidemiological investigations of cases, and the NSW Food Authority (in collaboration with local councils) conducts environmental investigations, including traceback of implicated foods, where appropriate].
If two or more cases occur (other than maternal and fetal paired cases) that are epidemiologically linked e.g. common food source or common setting, or microbiologically linked (by typing), investigation should include the following:
When an outbreak is multi-jurisdictional as defined in the OzFoodNet MJOI guidelines, PHUs, food CDB, NSWFA and jurisdictional reference laboratories collaborate on the outbreak investigation. The investigation is conducted in accordance with the MJOI guidelines.
A heightened level of concern is required for cases residing in a facility for all or part of their incubation period. Food served at the facility should be suspected until investigations determine otherwise. A single case in a facility may be sentinel for an outbreak and should trigger a thorough investigation of the source due to the vulnerability of facility populations, and be immediately reported to the facility’s manager and medical health officer. For hospitalised patients that are immunocompromised, determine whether the hospital has a ‘low listeria’ diet, and whether the case had been placed on such a diet when admitted. If the hospital does not have a ‘low listeria’ diet, discussions should be held with the hospital’s dietician, infection control and catering teams to implement such a diet and a mechanism for triaging patients into these diets when admitted.
When an outbreak occurs in a facility, PHU, CDB, NSWFA and reference laboratories of the relevant jurisdictions collaborate on the outbreak investigation in conjunction with the facility. Any epidemiologically implicated foods should be sampled and sent for laboratory testing.
If a cluster of listeriosis associated with a susceptible infant population occurs, PHU staff should ensure that the facility’s infection control procedures are reviewed and an investigation conducted to determine the likelihood, place, source and means of disease transmission – which will most probably be other than via food.
Appendix 1: Listeriosis fact sheetAppendix 2: PHU ChecklistAppendix 3: Listeriosis Disease Investigation form