The Prevalence of Listeria in Food and the Environment
Listeria bacteria are common in nature.
Listeria monocytogenes is omnipresent in nature; it is found widely in such places as water, soil, infected animals, human and animal feces, raw and treated sewage, leafy vegetables, effluent from poultry and meat processing facilities, decaying corn and soybeans, improperly fermented silage, and raw (unpasteurized) milk.
Foodborne listeriosis is relatively rare but is a serious disease with high fatality rates (20%–30%) compared with other food-borne microbial pathogens. Severe L. monocytogenes infections are responsible for high hospitalization rates (91%) among the most common food-borne pathogens, may cause sporadic cases or large outbreaks, and can persist in food-processing environments and multiply at refrigeration temperatures, making L. monocytogenes a significant public health concern.
Foods commonly identified as sources of Listeria infection include improperly pasteurized fluid milk, cheeses (particularly soft-ripened varieties, such as traditional Mexican cheeses, Camembert, and ricotta), ice cream, raw vegetables, fermented raw-meat sausages, raw and cooked poultry, and cooked, ready-to-eat (RTE) sliced meats—often referred to as “deli meats.” One study found that, over a five-year period of testing, in multiple processing facilities, Listeria monocytogenes was isolated from 14% of 1,080 samples of smoked finfish and smoked shellfish.
Ready-to-eat foods have been found to be a notable and consistent source of Listeria. For example, a research-study done by the Listeria Study Group found that L. monocytogenes grew from at least one food specimen in the refrigerators of 64% of persons with a confirmed Listeria infection (79 of 123 patients), and in 11% of more than 2,000 food specimens collected in the study. Moreover, 33% of refrigerators (26 of 79) contained foods that grew the same strain with which the individual had been infected, a frequency much higher than would be expected by chance. A widely cited U.S. Department of Agriculture (USDA) study that reviewed the available literature also summarized these findings:
In samples of uncooked meat and poultry from seven countries, up to 70 percent had detectable levels of Listeria. Schuchat et al. found that 32 percent of the 165 culture-confirmed listeriosis cases could be attributed to eating food purchased from store delicatessen counters or soft cheeses. In Pinner et al.’s microbiologic survey of refrigerated foods specimens obtained from households with listeriosis patients, 36 percent of the beef samples and 31 percent of the poultry samples were contaminated with Listeria.
The prevalence of Listeria in ready-to-eat meats has not proven difficult to explain. As one expert in another much-cited article has noted:
The centralized production of prepared ready-to-eat food products. . . increases the risk of higher levels of contamination, since it requires that foods be stored for long periods at refrigerated temperatures that favor the growth of Listeria. During the preparation, transportation, and storage of prepared foods, the organism can multiply to reach a threshold needed to cause infection.
The danger posed by the risk of Listeria in ready-to-eat meats prompted the USDA to declare the bacterium an adulterant in these kinds of meat products and, as a result, to adopt a zero-tolerance policy for the presence of this deadly pathogen. The Code of Federal Regulations includes requirements for the post-lethality control of Listeria in meat and poultry products. This regulation is referred to as “The Listeria Rule” and was enacted in 2003. The rule outlines prevention and control measures that must be taken in processing facilities to reduce the risk of contamination of ready-to-eat products.