What is Listeriosis?

According to Centre for Disease Control and Prevention (CDC), listeriosis is a serious but treatable and preventable disease caused by the Gram-positive, rod-shaped bacterium, Listeria monocytogenes.

Signs and Symptoms

Symptoms of listeriosis are variable ranging from a mild flu-like feverish illness, nausea, vomiting, cramps, headache, constipation and diarrhoea to more severe inflammation of the central nervous system (meningitis) and septicaemia. The infection is particularly severe in high-risk populations such as immuno-compromised persons, pregnant women and their unborn babies, neonates, the elderly (> 65 years), individuals with diabetes, cancer, chronic liver or kidney disease. A case fatality rate of 20% has been estimated in these high risk groups. In most healthy individuals, the infection is often asymptomatic but can cause an acute febrile gastroenteritis which resolves itself within a few days. Direct contact with the pathogen can also cause eye and skin infections.

Transmission

The most common mode of transmission is ingestion of the bacteria, mainly through consumption of contaminated food such as raw or unpasteurised milk and soft cheeses, ready-to-eat meats and smoked fish products as well as fresh produce consumed raw or minimally processed. In the workplace, transmission can occur through direct contact with diseased farm animals or animal tissues during birthing or butchering processes. Laboratory hazards include accidental autoinoculation and exposures to contaminated or infected tissues and cultures. With the exception of mother-to-foetus transmission, person-to-person transmission of Listeria has not been reported.

Statistics/Epidemiology

Although statistics regarding listeriosis cases as an occupational disease is limited, it has been reported that human acquisition of the infection from animal sources can occur as an occupational hazard especially in poultry workers, abattoir workers and veterinary surgeons. In the USA, 16 cases of cutaneous listeriosis on hands of veterinarians and farmers were reported between 1957 and 2009 following occupational exposure to aborted cow or sheep foetuses. It is important to note that in a majority of the cases, direct skin inoculation occurred when the animal workers were not wearing birthing gloves. As such, the risk of Listeria infection in occupational settings cannot be ruled out.

Potential sources of exposure

L.monocytogenes is ubiquitous in nature and is found in soil, water, infected animals, effluent from poultry and meat processing facilities. This bacterium is also a persistent environmental contaminant in food processing facilities and abattoirs where it can survive on difficult to clean surfaces such as crevices of equipment and biofilm in drains. Bacteria in biofilms can show increased resistance to sanitizers, disinfectants, and antimicrobial agents.

Commonly exposed workers

• Food handlers and cleaners in food processing facilities
• Laboratory workers who work with L. monocytogenes cultures
• Healthcare workers
• Veterinarians
• Abattoir workers

Control measures to limit exposure in the workplace

Laboratories:

For laboratories working with L.monocytogenes control measures include but are not limited to:-

• Biosafety level 2 facilities and adherence to policy and procedures.
• Signs with biohazard symbols must be posted at the entrance of the laboratory where the organism is being used and on any equipment that may be used to manipulate the bacterium.
• Procedures that may generate aerosols or involves high concentrations of L.monocytogenes must be conducted in a biosafety cabinet.
• All laboratory personnel must be trained on the hazards of the agent, as well as proper handling and disposal prior to working with the agent.
• Access to the laboratory must be limited to authorised personnel.
• Appropriate and well fitted personal protective equipment (PPE) must be provided and workers should be trained on the correct use, removal and disposal of PPE.

Food processing:

In a food processing facility where raw or ready-to-eat food potentially contaminated with L.monocytogenes is handled:-

• The importance of good hand washing technique cannot be over-emphasised because L. monocytogenes can accidentally end up on food-handler’s hands during food preparation.
• Floor drains are a common source of inoculum for L. monocytogenes and should be thoroughly cleaned and disinfected on a regular basis to prevent build up of inoculum.
• Workers should use appropriate and well fitted PPE when cleaning floor drains to minimise exposure from aerosols generated during high pressure spray.

Animal workers:

For veterinarians and abattoir workers, precautions that must be followed when handling animals include:-

• Treating all farm animals as potential sources of contamination and adherence to strict hygiene procedures at all times.
• Educating workers on the sources and transmission routes of the pathogen.
• Use of correct and well fitted gloves when direct skin contact with infected tissue or animals is unavoidable e.g. during birthing.

References

• Conly, J.M. and Johnston, B.L. 2008. Listeria: a persistent food-borne pathogen. Canadian Journal of Infectious Diseases and Medical Microbiology, 19(5): 327-328.
• Centre for Disease Control and Prevention: Listeria (Listeriosis).  (Accessed 11 December 2017).
• Dealing with Listeria: A guide to manufactures. (Accessed 10 December, 2017).
• Fighting Food Manufacturing Fears: How to Control and Eliminate Listeria.  (Accessed 6 December, 2017).
• Godshall, C.E., Suh, G. and Lorber, B. 2013. Cutaneous listeriosis. Journal of Clinical Microbiology. DOI: 10.1128/JCM.01974-13.
• Kerr, K.G., Birkenhead, D., Seale, K., Major, J. and Hawkey, P.M. 1993. Prevalence of Listeria spp. on the hands of food workers. Journal of Food Protection, 56(6): 525-527.
• National Institute for Communicable Diseases: Situation report on Listeriosis outbreak, South Africa, 2017. Date of issue: 4 December 2017.
• Seyoum, E.T., Woldetsadik, D.A., Mekonen, T.K., Gezahegn, H.A. and Gebreyes, W.A. 2015. Prevalence of Listeria monocytogenes in raw bovine milk and milk products from central highlands of Ethiopia. The Journal of Infection in Developing Countries, 9(11): 1204-1209.

Compiled by: Dr Noncy Gomba and Ms Larissa Singh
Immunology & Microbiology Section