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.
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.
The OzFoodNet MJOI guidelines provide guidance around communications required during a MJOI.
Suspect food under investigation
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.
6. Case definition
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:
- laboratory definitive evidence or
- clinical and epidemiological evidence.
Laboratory definitive evidence
Isolation or detection of Listeria monocytogenes from a site that is normally sterile, including fetal gastrointestinal contents.
- A fetus/neonate where the gestational outcome is one of the following:
- premature birth (<37 weeks gestation)
- diagnosis (within the first month of life) with at least one of the following:
- Granulomatosis infantiseptica
- meningitis or meningoencephalitis
- congenital pneumonia
- lesions on skin, mucosal membranes or conjunctivae
- respiratory distress and fever at birth and
- in the absence of another plausible diagnosis.
- or a mother has experienced at least one of the following conditions during pregnancy:
- fever of unknown origin
- influenza like illness
- meningitis or meningoencephalitis
- localised infections such as arthritis, endocarditis and abscesses
- preterm labour/abruption and
- in the absence of another plausible diagnosis.
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).
- The clinical and epidemiological evidence criteria for a confirmed case means that if the mother is a confirmed case by laboratory definitive evidence, then the fetus/neonate is also a confirmed case if they have the defined (fetus/neonate) clinical evidence, and vice versa.
- Laboratory definitive evidence in a fetus <20 weeks gestation means the mother only is a confirmed case.
7. Laboratory testing
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.
Potential source detection
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.
Cluster detection, investigation and source attribution
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.
8. Case management
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]:
- Confirm results of relevant pathology tests, or recommend the tests be done
- [Seek the doctor's permission to contact the case or relevant care-giver, and find out if the case or relevant care-giver has been told what the diagnosis is before beginning the interview]
- [Interview the case/next of kin/care-giver and treating doctor to] complete the National OzFoodNet Listeria Case Questionnaire (Appendix 2). Follow up of listeriosis cases can be particularly sensitive; cases may be deceased, and it is sometimes necessary to interview next of kin, or females who may have experienced a miscarriage or stillbirth.
- If appropriate, secure any available residual suspected foods and refer to the NSW Food Authority to arrange collection and testing
- If cases were institutionalised (e.g. in hospital or aged care) for their entire incubation period, or food at the institution is the suspected source of infection:
- Obtain records (itemised list or menu) of foods served to the case during the exposure period
- Refer environmental investigation of the institution to the NSW Food Authority
- If the case was immunocompromised, check whether they had been placed on a ‘low listeria diet’ on admission (see also Section 11. Special Situations).
- [Update NCIMS records and attached the completed questionnaire]
- Ensure clinical isolates are sent to [ICPMR] for typing
- [Complete a CDONCALL Report (note: if records are up to date, this can be printed/download from NCIMS), and send to firstname.lastname@example.org – if afterhours and urgent response is required, additionally notify CD Oncall]
- [Review epidemiological links between cases within the local jurisdiction], assess the possibility of a common source outbreak
- Maintain surveillance for further cases.
[CDB (OzFoodNet Epidemiologists) should take the following actions]:
- Capture case interview information on NELSS
- [Follow-up typing and whole genome sequencing data from ICPMR and MDU, and capture this information on NCIMS and NELSS]
- [Review epidemiological and molecular links between cases, and investigate linked cases in collaboration with PHUs, NSW Food Authority and OzFoodNet representatives in other jurisdictions where appropriate (see also Section 11. Special Situations)].
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.
Isolation and restriction
Exclusion from childcare, preschool, school or work is not necessary.
Active case finding
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.
9. Environmental and Food evaluation
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].
10. Contact management
Identification of contacts
Person to person transmission does not usually occur, so identifying contacts is not usually relevant for listeriosis.
11. Special situations
[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:
- look for common source of infection
- test any available suspected foods
- characterisation of further related non-clinical L. monocytogenes isolates should be performed to confirm the outbreak and demonstrate whether case isolates and food isolates are indistinguishable
- 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 through referral to food authorities
- in some outbreak settings, active case finding and investigation of non-invasive listeriosis (such as acute febrile gastroenteritis) may be warranted – for non-invasive febrile gastroenteritis, the median incubation period is around 24 hours (range 6 hours – 10 days).(9)
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.
Facilities such as hospitals, long term care facilities and aged care facilities
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.
Vulnerable infant populations
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.
12. References and additional sources of information
- Lamont RF, Sobel J, Mazaki-Tovi S, Kusanovic JP, Vaisbuch E, Kim SK, et al. Listeriosis in human pregnancy: a systematic review. J Perinat Med 2011;39(3):227-36.
- World Health Organization. Risk assessment of Listeria monocytogenes in ready-to-eat foods. Technical report. Geneva: World Health Organization; 2004.
- Ho JL, Shands KN, Friedland G, Eckind P, Fraser DW. An outbreak of type 4b Listeria monocytogenes infection involving patients from eight Boston hospitals. Arch Intern Med 1986;146(3):520-4.
- Muñoz P, Rojas L, Bunsow E, Saez E, Sánchez-Cambronero L, Alcalá L, et al. Listeriosis: An emerging public health problem especially among the elderly. J Infect 2012;64(1):19-33.
- Martins IS, Faria FC, Miguel MA, Dias MP, Cardoso FL, Magalhaes AC, et al. A cluster of Listeria monocytogenes infections in hospitalized adults. Am J Infect Control 2010;38(9):e31-6.
- Oevermann A, Zurbriggen A, Vandevelde M. Rhombencephalitis Caused by Listeria monocytogenes in Humans and Ruminants: A Zoonosis on the Rise? Interdiscip Perspect Infect Dis 2010:632513.
- McLauchlin J. Human listeriosis in Britain, 1967-85, a summary of 722 cases. 2. Listeriosis in non-pregnant individuals, a changing pattern of infection and seasonal incidence. Epidemiol Infect 1990;104(2):191-201.
- Heymann DL, editor. Control of Communicable Diseases Manual. 20th ed. Washington: American Public Health Association; 2015.
- Goulet V, King LA, Vaillant V, de Valk H. What is the incubation period for listeriosis? BMC Infect Dis 2013;13(1):11.
- Goulet V, Marchetti P. Listeriosis in 225 non-pregnant patients in 1992: clinical aspects and outcome in relation to predisposing conditions. Scand J Infect Dis 1996;28(4):367-74.
- Swaminathan B, Gerner-Smidt P. The epidemiology of human listeriosis. Microbes Infect 2007;9(10):1236-43.
- Chavada R, Keighley C, Quadri S, Asghari R, Hofmeyr A, Foo H. Uncommon manifestations of Listeria monocytogenes infection. BMC Infect Dis 2014;14:641.
- Ryser E, Marth E, editors. Listeria, Listeriosis, and Food Safety. 3rd ed: CRC Press; 2007.
- Dalton C, Merritt T, Unicomb L, Kirk M, Stafford R, Lalor K. A national case-control study of risk factors for listeriosis in Australia. Epidemiol Infect 2011;139(03):437-45.
- Drevets DA, Bronze MS. Listeria monocytogenes: epidemiology, human disease, and mechanisms of brain invasion. FEMS Immunol Med Microbiol 2008;53(2):151-65.
- NNDSS [Online]. Australia's National Notifiable Diseases Surveillance System (NNDSS) Summary Tables. Accessed on 09/04/2015 Available from: http://www9.health.gov.au/cda/source/cda-index.cfm.
- Pouillot R, Hoelzer K, Jackson KA, Henao OL, Silk BJ. Relative risk of listeriosis in Foodborne Diseases Active Surveillance Network (FoodNet) sites according to age, pregnancy, and ethnicity. Clin Infect Dis 2012;54(suppl. 5):S405-S10.
- OzFoodNet. Listeriosis unpublished data. 2015.
- Codex. Guidelines on the application of general principles of food hygiene to the control of Listeria monocytogenes in foods. In: Commission CA, editor. CAC/GL2007. p. 61-2007.
- Food Standards Australia New Zealand [Online]. Recall Guidelines for Ready-To-Eat foods. Accessed on 09/04 2015. Available from: www.foodstandards.gov.au/publications/pages/listeriarecallguidel5618.aspx.
- Martin T, Dean E, Hardy B, Johnson T, Jolly F, Matthews F, et al. A new era for food safety regulation in Australia. Food Control 2003;14(6):429-38.
- Food Standards Australia New Zealand [Online]. Listeria and food - advice for people at risk. Accessed on 09/04 2015. Available from: www.foodstandards.gov.au/publications/Pages/listeriabrochuretext.aspx.
- OzFoodNet. Monitoring the incidence and causes of diseases potentially transmitted by food in Australia: Annual report of the OzFoodNet network, 2010. Commun Dis Intell Q Rep 2012;36(3):E213.
- Standards Australia. Food microbiology - Method 24.1: Microbiology of food and animal feeding stuffs - Horizontal method for the detection and enumeration of Listeria monocytogenes—Detection method (ISO 11290-1:1996, MOD). 2009.
- Standards Australia. Food microbiology - Method 24.2: Microbiology of food and animal feeding stuffs - Horizontal method for the detection and enumeration of Listeria monocytogenes—Enumeration method (ISO 11290-2:1998, MOD). Microbiology of food and animal feeding stuffs. 1998.
- Anon. Therapeutic guidelines: antibiotic. Version 15 ed. Groups AE, editor: Melbourne: Therapeutic Guidelines Limited; 2014; 2014.
- Xayarath B, Freitag NE. Optimizing the balance between host and environmental survival skills: lessons learned from Listeria monocytogenes. Future Microbiol 2012;7(7):839-52.
Appendix 1: Listeriosis fact sheet
Appendix 2: PHU Checklist
Appendix 3: Listeriosis Disease Investigation form