Public awareness of the bacterium Listeria, an opportunistic pathogen, has been on the rise in recent years as the number of reported cases of listeriosis continues to grow. Listeriosis is an infection caused by the bacterium Listeria monocytogenes. The number of reported cases in Canada rose from 85 in 2003 to 239 in 2008.
In 2008, a widespread outbreak of listeriosis originated from a Maple Leaf Foods Bartor Road facility in North York, Ontario which caused infection in 57 people and took the lives of 23 of them (Table 1). In other words, 40% of the people affected in this outbreak died of this serious illness. 119 meat products produced by Maple Leaf Foods Bartor Road plant were recalled. It is believed that contamination with L. monocytogenes likely occurred during packaging.
Table 1. Listeria monocytogenes outbreak 2008
|
Province |
Confirmed Cases |
Deaths where Listeriosis was the underlying or contributing cause* |
|
Ontario |
41 |
16 |
|
BC |
5 |
2 |
|
Alberta |
2 |
1 |
|
Saskatchewan |
2 |
1 |
|
Manitoba |
1 |
0 |
|
Quebec |
5 |
2 |
|
New Brunswick |
1 |
1 |
|
Total |
57 |
23 |
*As recorded on death certificate or as assessed by attending physician.
http://www.phac-aspc.gc.ca/alert-alerte/listeria/listeria_20100413-eng.php
Occurrence
Most healthy individuals exposed to Listeria are usually at low risk of infection because our immune systems are strong enough to clear the bacterium from our bodies. Some healthy individuals and children can become infected but, they rarely become seriously ill. In fact, according to the Public Health Agency of Canada, of the 11 to 13 million annual cases of food-related illness in Canada, listeriosis cases are in the low hundreds. However, although this infection is rare, it causes serious health effects to those at risk and is responsible for ¼ of the deaths of all reported foodborne illnesses. The population at highest risk for listeriosis are infants, the elderly, pregnant women, and people with weakened immune symptoms.
Health Effects and Treatment
Most foodborne related bacteria, such as Salmonella cause infection within 12-72 hours after eating contaminated food. However, Listeria is unusual in that it can take 3 to 70 days (with a median of 3 weeks) for a mild or invasive infection to start. Once an infection occurs in humans, 1 out of 5 people die. Another difficulty when dealing with Listeria is that early symptoms of listeriosis can easily be mistaken for the flu. Generally, listeriosis is not diagnosed until laboratory reports of L. monocytogenes have been cultured from blood, cerebrospinal fluid or amniotic fluid.
The most common symptoms of listeriosis are:
- Sepsis (blood stream infection): infection results in high fever and very ill appearance. Infection of the heart valves and other organs can occur.
- Infection of the central nervous system (typically meningitis): results in high fever, intense headaches, neck stiffness, altered consciousness and convulsions.
- Miscarriage, still birth or premature delivery of a newborn. The infected pregnant mother may have a mild flu-like illness however; it can affect newborns by penetrating the endothelial layer of the placenta. If the baby is infected at the time of birth, it can develop a blood stream infection or meningitis.
Antibiotics such as ampicillin, ciprofloxacin and azithromycin are usually given intravenously to treat listeriosis. In spite of treatment however, 20-30% of cases results in death.
Disease Transmission
The majority of listeriosis cases reported are foodborne and result from eating food contaminated with L. monocytogenes. Listeria has been found in uncooked meats such as cold cuts/deli meats and hot dogs, raw and smoked fish, raw vegetables, fresh fruit such as cantaloupes, unpasteurized (raw) milk, unpasteurized dairy products and soft cheeses like feta, Brie or Camembert. Listeria can also be found in soil, stream water, sewage, and plants. Vegetables and plants can become contaminated with Listeria from soil, water or manure-based fertilizers. Farm animals that appear healthy can carry Listeria to contaminate meats and dairy products.
Listeria monocytogenes can grow in temperatures from 4ºC (refrigerator temp) to 37ºC (body’s internal temp).
Prevention
Everyone has a role to play in ensuring the consumption of safe foods. Manufacturers of foods such as the Maple Leaf Foods Bartor Road plant must go above and beyond to ensure the necessary measures are taken to prevent another outbreak. From the CEO to the night janitor, all involved have a part to play. Ensuring proper sanitation and removal of all possible sources of Listeria growth and, monitoring and auditing of safe practice procedures must be performed daily. Manufacturers of food processing equipment are responsible for the proper design of food processing equipment that will enable thorough cleaning and disinfection as well as efficient and complete disassembly and assembly for effective cleaning.
Preventing Listeria growth requires effective sanitation of food contact surfaces. The four main types of sanitizers used in the meat industry are hot water, chlorine, iodophors and quatnerary ammonia. Quaternary ammonia and iodophors are the most effective when combined with post-processing steam and heat treatments. Alcohol alone is also effective against Listeria. Increasing the duration of contact time on food contact surfaces will improve sanitization procedures.
Precautionary measures need to be taken when preparing foods likely to contain Listeria such as ensuring raw vegetables are thoroughly washed before eaten. Those especially at risk should avoid consumption of refrigerated pâtés, smoked seafood, unpasteurized milk and milk products. Risks groups should also avoid eating luncheon meats, deli-meats and hot dogs unless they are steaming hot. Listeria can even survive and grow in vacuum-packed products. Thus, refrigerated foods in the home should be kept below 4 ºC to discourage Listeria growth.
The fluids from packages such as hot dogs contain more Listeria than hot dogs themselves. Steps should therefore be taken to avoid the fluid from coming into contact with other foods, cutting boards, utensils, dishes and food preparation surfaces. Hand washing can greatly reduce the spread of contamination.
Sampling of Listeria
Testing for Listeria is typically done using a swab, environmental sponge or the Microbial-Vac system((R)). Food contact surfaces (dairy board, stainless steel) or other non-food contact areas suspected of Listeria growth (such as brick, doorways, hall drains, floors, equipment panels, and aprons) can be tested. Specimens for sampling can also be taken from solid foods (such as cheese). Water from sources containing chlorine must be collected in a pre-sterilized bottle containing sodium thiosulfate. Samples must be collected and delivered to the laboratory as soon as possible.
Sample Handling and Storage
Optimal temperature conditions during transport and storage is below 4˚C. The holding time between collection and analysis must not exceed 48 h for water samples. All samples must be labeled according to the chain of custody.
Test Methods
Here at MBL we detect and enumerate Listeria spp. in environmental samples by the 3MTM petrifilm technique. 3MTM petrifilm plates are a thin, film, dehydrated, version of the conventional petri dish agar plate. Once a result is positive for Listeria spp., a confirmation step is performed. Benefits of this method are:
- It is a cost effective tool for the detection of environmental Listeria.
- Its quantitative results allow identification of hot spots in plant and you can track Listeria over time.
- Rapid turnaround time of results from sample collection; as little as 2-4 business days. Other methods of Listeria testing can take up much longer because of the increased number of steps and longer incubation times.
- It is safer and more reliable than other methods because it does not require an enrichment step which would increase exposure to potentially pathogenic Listeria and, it requires no culture transfers.
For more information on our Listeria testing or for a complete list of microbiological tests performed at Mold & Bacteria Consulting Laboratories, please call our Ontario, Mississauga Office at 905-290-9101 or the British Columbia, Burnaby Office at 604-435-6555.
References
“Controlling Listeria Contamination in Your Meat Processing Plant”. Government of Ontario. 27 February 2007. http://www.omafra.gov.on.ca/english/food/inspection/meatinsp/controllinglisteria.htm. Retrieved 2011-12-12.
Health Canada. Sept 2006. MFLP-11: Enumeration of Listeria Species in environmental Samples using 3M Petrifilm environmental Listeria plates.
Maple Leaf Foods assessing Listeria-killing chemical. ctv.ca (ctvglobemedia). 2008-10-12. http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/20081012/maple_leaf_foods_081012/20081012?hub=Health. Retrieved 2011-12-15.
“More About Listeria”. University of Florida Medical School. http://www.med.ufl.edu/biochem/DLPURICH/morelist.html. Retrieved 2011-12-18.
Pagotto, Franco, Karine Hébert and Jeff Farber. Feb 2011. Isolation of Listeria monocytogenes and other Listeria spp.from foods and environmental samples.
Weatherill, Shiela. July 2009. Report of the independent investigator into the 2008 listeriosis outbreak. http://www.listeriosis-listeriose.investigation-enquete.gc.ca/index_e.php?s1=rpt&page=tab. Retrieved 2011-12-12.
http://www.guardian.co.uk/world/2011/sep/29/listeria-outbreak-us-cantaloupe-melons?newsfeed=true. Retrieved 2011-12-14.
The commonly used sampling method for airborne bacteria is the impaction of air onto agar media using volumetric samplers such as Andersen, SAS, or RCS. Samples can also be collected by impingement into liquid media. However, for routine air quality monitoring agar based sampling is recommended. Trypticase soy agar (TSA) media is usually used when sampling for bacteria. It is important to select a medium suitable for growth of the target organism, for example when sampling for some specific bacteria such as Legionella. Sampling time depends on the sampler used but generally not more than 10 minutes. Shorter times are recommended in heavily contaminated areas. Samples are kept cool and shipped for overnight delivery to the laboratory for incubation and identification of the dominant species.