In April 2007, the U.S. Centers for Disease Control (CDC) released preliminary data from its FoodNet surveillance system for 2006 that show significant declines in the rate of foodborne infections associated with Campylobacter, Listeria, Shigella and Yersinia and increases in illnesses attributed to Escherichia coli and Vibrio infections (Figure 1). Certainly, the less-than-stellar news on the E. coli front isn’t surprising, given the nationwide outbreaks linked to fresh produce, specifically spinach, tomatoes and lettuce, in 2006; but perhaps disappointing given the heartening declines that occurred between 2003 and 2004 when the beef industry made great strides in implementing then-new U.S. Department of Agriculture (USDA) food safety measures in processing facilities. Also of concern is the fact that, according to CDC, the number of Vibrio infections rose to its highest level since FoodNet surveillance began in 1996.
Why, then, is this article focused on Listeria monocytogenes (LM) control when the CDC’s latest foodborne illness incidence data indicates that the estimated decline in the rate of foodborne infections attributed to this pathogen is 34%, a percentage reduction that seems fairly substantial? For a few important reasons:
1. Because, as the agency notes in its findings, when compared with the 1996-1998 baseline period figures, significant declines occurred in the estimated incidence of Listeria—but most of these declines occurred before 2006. In its summary, the agency states, “In 2006, the incidence of Listeria infections remained higher than at its lowest point in 2002.” Thus, although the estimated incidence rate of listeriosis fell by 34% and accounts for just 138 cases reported in 2006 (as compared with Salmonella, which accounted for 6,655 cases [39%], 5,712 Campylobacter cases [33%] and 590 cases of Shiga toxin–producing E. coli O157, for example), the fatality rate associated with L. monocytogenes infection is nearly 25%, well above any other foodborne pathogen.
2. Because, even though of the 17,252 laboratory-confirmed foodborne infections in 2006 identified in the FoodNet surveillance, Listeria infections account for just 138 cases of foodborne illness, CDC notes that this number only represents reported cases. The agency estimates that for the total number of reported or verified case of listeriosis, the truer, extrapolated number of food poisoning cases caused by L. monocytogenes may be as high as 2,500 illnesses and 500 deaths annually.
3. Because, the 2006 FoodNet statistics show the number of Listeria cases is currently 0.31 per 100,000 people, which while approaching the nation’s Healthy People 2010 objective to reduce the number of listeriosis cases to 0.25 cases per 100,000 people, is evidence that there is more work to do to achieve this goal (Table 1).
Although the increased incidence rates reported for other foodborne pathogens in the 2006 FoodNet data should not be discounted, the fact is that L. monocytogenes contamination remains a significant challenge to the food industry for the reasons above and more. The challenges industry faces in controlling this damp-loving bug in processing plants and food handling establishments are numerous, including:
• L. monocytogenes is ubiquitous in the natural environment, found in soil, water, and vegetation, and is carried by healthy animals. We can swab or sample virtually any damp place in the environment and find it.
• L. monocytogenes is easily transferred from floors and walls or other environmental surfaces onto equipment where it finds harborage points and can then be transferred directly to foods.
• L. monocytogenes can survive and grow in refrigerated minimally pasteurized and packaged ready-to-eat (RTE) products, at temperatures as low as 28F, as well as in low-oxygen conditions.
• Similarly, although L. monocytogenes can often be present in raw foods, it also can be present in cooked foods due to post-processing contamination.
• L. monocytogenes can survive for long periods of time in the environment, on foods and on processing plant surfaces, as long as some form of moisture is available.
In fact, most of the growth and survival control techniques used to protect foods from pathogen contamination in the processing environment do not necessarily work for Listeria as well as we typically expect. Refrigeration, vacuum packaging, acidification—none of it guarantees that we’ve eradicated the problem or prevented Listeria from reappearing even years after we think we’ve overcome it. Ultimately, its ubiquity in the environment, its ability to grow in a normal range of characteristics that we normally expect to keep foods safe, and its resistance to succumb to a variety of control measures in certain at-risk foods, make L. monocytogenes a very difficult pathogen to control in the food processing environment.
This is why U.S. regulatory agencies, industry and researchers continue to work to improve methods that reduce the risk of Listeria contamination in the food supply chain. Both the U.S. Department of Agriculture’s Food Safety and Inspection Service (USDA FSIS) and the U.S. Food and Drug Administration (FDA) have developed important information and guidelines in this area. As the only federally mandated rule that requires processing facilities to comply with Listeria control measures within a specific framework, 9 CFR 430.4, Control of Listeria monocytogenes in post-lethality exposed ready-to-eat products, has been implemented in specified ready-to-eat meat and poultry product processing plants since October 2003. As we approach the fourth year of implementation, many insights have been gained by and resources developed for RTE meat and poultry product manufacturers to aid in the battle to reduce Listeria risk factors in the plant.
In addition, the knowledge gained offers a framework for RTE product manufacturers outside the oversight of FSIS to enhance their Listeria control measures and systems—particularly those operations that make higher risk products (Table 1). L. monocytogenes contamination has effected product recalls across RTE food categories. In the past two years, FDA Class I recalls for products testing positive for or linked to Listeria included potato and egg salads, frozen strawberries used as a primary ingredient for smoothies, raw milk, cole slaw, packaged fresh-cut fruit, prepackaged turkey sandwiches, smoked salmon and trout, radish, alfalfa and bean sprouts, and various chicken, turkey, vegetable and imitation seafood-flavoredspreads and dips. USDA FSIS L. monocytogenes recalls since January 2007 have involved hot dogs, sausage, deli-style ready-to-eat pork, roast beef, chicken and turkey products, hog head cheese, spicy Thai-style pasta salad with chicken breast, ham salad and dried beef, and ready-to-cook chicken breast strips.
The FSIS L. monocytogenes regulation and guidance materials are useful for any company manufacturing RTE foods that can support the growth of Listeria. These include foods with a water activity greater than 0.92, or a pH greater than 4.39. As indicated earlier, Listeria is a hardy switch-hitter that grows aerobically and anaerobically at low temperatures thereby frustrating the more commonly accepted methods of keeping food safe. When it comes to Listeria, anyone who is in the business of manufacturing high- or medium-moisture foods has a shot at becoming famous the wrong way. Here, we’ll discuss some of these resources and industry learnings within the framework of the FSIS rule to see where we are now in the progression toward reducing Listeria contamination of foods and hopefully, move us closer to achieving more significant declines in future FoodNet surveillance reports.
Background: Getting to Where We Are
In June 2003, FSIS made final its interim rule for the control of L. monocytogenes on certain RTE meat and poultry products (9 CFR 430.). As of Oct. 6, 2003, the rule required that certain RTE meat and poultry processors implement one of three risk-based alternatives for categorizing their products, develop written programs to control L. monocytogenes, and verify the effectiveness of those programs through testing. The rule also requires RTE product processors to share plant information and testing data with the agency, and emphasizes the importance of incorporating a multiple hurdle strategy comprised of a combination of microbial interventions in the RTE processing environment.
Once a manufacturer has established that the plant is producing RTE meat or poultry products that fall under the FSIS rule, the establishment must select one of the following three intervention alternatives to control for L. monocytogenes:
• Alternative 1. Employ both a post-lethality treatment and a growth inhibitor (antimicrobial agent or process) for L. monocytogenes on RTE products (Figure 2). Establishments opting for this alternative are subject to FSIS verification activity that focuses on the post-lethality treatment’s effectiveness. Sanitation is important but is built into the degree of lethality necessary for safety.
• Alternative 2. Employ either a post-lethality treatment or a growth inhibitor for the pathogen on RTE products (Figure 3). Establishments opting for this alternative are subject to more frequent FSIS verification activity than those in Alternative 1.
• Alternative 3. Employ sanitation measures only (Figure 4). Establishments opting for this alternative will be targeted with the most frequent level of FSIS verification activity. Within this alternative, FSIS places increased scrutiny on operations that produce hotdogs and deli meats because they are high-risk products for listeriosis as identified in the 2001FSIS/FDA risk ranking.
Since the implementation of the rule began, many larger RTE food processing operations have invested in one or both of the first two alternative programs, which require that processors validate the intervention treatments and provide supporting scientific data as evidence of the efficacy of those interventions to FSIS. The FSIS LM Compliance Guidelines include a draft form where companies are required to report their production volumes, as well as to identify their product category, process alternative, the log reduction or minimum log increase of the relevant interventions, and the frequency of establishment testing. The agency uses this information to develop its verification sampling program for each RTE processing operation.
One of the keys to meeting L. monocytogenes risk reduction aims and to identifying the appropriate scientific resources required is to truly understand the treatment requirements involved. Essentially, Alternative 1 requires that both treatment types are met, and Alternative 2 requires that either of the two treatments is met. The following definitions of each type of treatment further elaborates differences in these requirements:
Alternative 1: Post-lethality Treatment (PLT):
• Must cause a reduction in LM over time.
• Must be a CCP in the HACCP plan.
• Safe Harbor = 0.85 aW or pH 4.39 requiring verification only in the HACCP plan. Higher numbers must have specific validation.
• All other PLT methods must have specific validation.
• Safe Harbor = Plant has documentation showing that the intended effect occurs prior to distribution of the product into commerce.
• No requirement for product contact surface (PCS) testing. It is recommended but not required.