Listeria monocytogenes, commonly referred to as Listeria, is a pathogen that causes listeriosis, a serious human illness. It is unlike most other foodborne pathogens because it can grow at proper refrigeration temperatures. In addition, Listeria is widely distributed in nature, and the organism has been recovered from farm fields, vegetables, animals and other environments such as food processing facilities, retail stores and home kitchens and ready-to-eat foods.
WHAT ARE THE SYMPTOMS?
L. monocytogenes causes listeriosis, a serious infection with high hospitalization rates for those who become ill. People at highest risk for a severe case include the elderly, the fetuses of pregnant women, and the immunosuppressed. It is unique among foodborne pathogens since its incubation time (time from ingestion of cells to illness) is at least seven days. Listeriosis is a rare disease with a high mortality rate, causing about 43 percent of the food poisoning deaths in the United States.1 L. monocytogenes can also cause mild, flu-like symptoms in healthy individuals when consumed at very high levels.2
A person with listeriosis has fever, muscle aches and occasional gastrointestinal symptoms such as nausea or diarrhea. If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur. Infected pregnant women may experience only a mild, flu-like illness; however, infections during pregnancy can lead to miscarriage or stillbirth, premature delivery or infection of the newborn.
Foods can become contaminated with L. monocytogenes along the continuum from farm to fork, in the produce growing environment, during processing, or during handling and preparation in retail establishments and consumers’ kitchens (ILSI, 2005).
The primary route of transmission is through the ingestion of contaminated food. The International Life Sciences Institute in 2005 described high-risk foods for causing listeriosis as those with the following properties:
Because Listeria is abundant in nature and can be found almost anywhere, there can be a constant reintroduction of the organism into the food plant, retail setting, foodservice establishment and home. It is difficult to totally eliminate this contaminant from the food-handling environment, but the goal is to control it as effectively as possible, especially where it can contaminate ready-to-eat, refrigerated foods.
Although L. monocytogenes is the only member of the Listeria family that causes human illness, the presence of any member of the Listeria family in a food processing environment may indicate that conditions are favorable for L. monocytogenes.
Effective control of L. monocytogenes requires prevention of contamination (to the extent possible) and prevention of growth through time/temperature or formulation control. Knowledge of potential harborage sites is important, as contamination is more likely to occur when the organism has become established in a niche. Food processing plant surveys have found Listeria in the following locations (listed approximately in the order of prevalence):
Control of Listeria relies on detecting and managing harborage sites with thorough and frequent cleaning. This includes daily cleaning of floors and drains, and adequate attention to less frequently cleaned areas such as HVAC systems, walls, coolers and freezers. Also, damaged equipment, cracks, crevices and hollow areas must be part of sanitation and inspection schedules. It is essential to avoid creation of aerosols during cleaning, especially of floors and drains, to avoid spread of contaminants.
The organism is killed by normal food pasteurization and cooking processes, and is typically sensitive to most sanitizers at recommended rates. Contamination may occur after the cooking process in the processing environment, at retail locations and in the home. For example, post-pasteurization contamination of food products can occur when the organism is dispersed via an aerosol. Prevention of growth is essential to avoid the potential for illness, because L. monocytogenes can grow at refrigerated temperatures, defeating one of the traditional food safety measures.
L. monocytogenes can survive on cold surfaces and can also multiply slowly at 34° F. It has also been shown to grow to a water activity as low as 0.92 and over a pH range of 4.4-9.4.3 Because the organism can grow under refrigeration, effective labeling to ensure product rotation in retail settings is an important control measure for ready-to-eat products.
Since this organism continues to elicit concern among consumers, regulators, processors and retailers, studies need to be carefully designed to ensure validity.4
The FDA/Food Safety and Inspection Service risk assessment reinforces epidemiological conclusions that foodborne listeriosis is a moderately rare, although severe, disease. A study by the Food Products Association showed it is likely that low levels of L. monocytogenes are consumed routinely with limited effect.5 It is believed that 5 percent of the general population may be asymptomatic carriers of Listeria, but the percentage may be higher in particular groups, such as slaughterhouse workers.
Extensive risk assessments and analyses have been conducted by the Food Safety and Inspection Service (FSIS)6, U.S. Food and Drug Administration/FSIS7, World Health Organization/Food and Agriculture Organization,8 and International Life Sciences Institute9 to identify factors that contribute to risk of illness. This research is important because the prevalence in the food supply does not match the rate of illness in the population, and because the outcome of illness in susceptible individuals is very severe. These assessments have generally concluded that the ability of a food to support growth of Listeria enhances risk.
Because of this, US FDA issued draft Compliance Policy Guidelines in February, 2008 based on the ability of a product to support growth of L. monocytogenes.10 The USDA retains a zero tolerance policy for RTE foods, while other countries allow up to 100 cfu/g in certain foods.
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REFERENCES AND FURTHER INFORMATION
1CDC(2006). Surveillance for Foodborne-Disease Outbreaks --- United States, 1998—2002. MMWR 55(SS10);1-34 http://www.cdc.gov/MMWR/preview/mmwrhtml/ss5510a1.htm
2Ryser, E.T. & Marth, E.H. 1991. Listeria, Listeriosis & Food Safety. New York, Marcel Dekker.
3International Commission on Microbiological Specifications for Foods. Microorganisms in Foods 5, Microbiological Specifications of Food Pathogens. Blackie Academic and Professional, New York. 1996.
4Swanson, K.M.J. 2005. L. monocytogenes Challenge Study “How To” Guidelines. Food Safety Magazine. June/July 2005.
5Gombas DE, Chen Y, Clavero RS, Scott VN. 2003. Survey of Listeria monocytogenes in ready-to-eat foods. J Food Prot. 66(4):559-69
9ILSI. 2005. Achieving Continuous Improvement in Reductions in Foodborne Listeriosis – A Risk Based Approach. Journal of Food Protection 68(9):1932-1994.