Policy on Listeria monocytogenes in ready-to-eat foods (2023): Background and scientific basis
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L. monocytogenes is a bacterium that is widespread in nature. It has been isolated from feces, sewage, silage, soil, fertilizer, vegetable matter and many foods (Farber and Peterkin, 1991; Farber and Peterkin, 2000; McLauchlin et al., 2004; Soni et al., 2014). A recent meta-analysis estimated the prevalence of L. monocytogenes in deli meat to be 2.9%, soft cheese to be 2.4%, and packaged salad at 2.0% (Churchill et al., 2019), suggesting that the prevalence of L. monocytogenes in RTE foods may be less than 5%.
It is estimated that approximately 1% of humans carry L. monocytogenes in their intestines without developing symptoms (Müller, 1990; Iida et al., 1998; Grif et al., 2001). Depending on the method used for detection, the organism's prevalence has been reported to be as high as 10% in the feces of healthy humans (Hafner et al., 2021). However, L. monocytogenes is recognized as the causative agent of the infection known as listeriosis. Listeriosis can be invasive or non-invasive. Invasive listeriosis usually develops in vulnerable individuals while non-invasive listeriosis can develop in people belonging to any population. Several modes of transmission have been identified: mother-to-fetus infection in utero or infection during childbirth, infant-to-infant, animal-to-human and transmission through the consumption of food containing L. monocytogenes (McLauchlin et al., 2004).
Serious infections of L. monocytogenes (that is, invasive listeriosis) in healthy adults are relatively rare. The highest incidence of listeriosis is among people with weakened immune systems, pregnant people and adults ages 60 and over (FAO and WHO, 2004; WHO, 2018; PHAC, 2022). In Canada, invasive listeriosis is a nationally notifiable disease. Invasive listeriosis is characterized by septicemia and meningoencephalitis and may result in death. Symptoms can start as early as 3 days and as late as 3 months after exposure to L. monocytogenes (Government of Canada, 2016; WHO, 2018). In pregnant people, symptoms are typically mild. However, the passage of the organism through the placenta may cause miscarriage, stillbirth, or perinatal septicemia and meningitis in the newborn baby. L. monocytogenes is the leading cause of death associated with foodborne illness in Canada, for which the cause is known (Thomas et al., 2015a). Invasive infection with L. monocytogenes is associated with a high case-fatality rate, that is, 20 to 30% of foodborne listeriosis infections in vulnerable populations are fatal (WHO, 2018). Listeriosis can also lead to serious and long-lasting health problems in infected individuals (Roberts et al., 2009).
In all likelihood, Canadians consume foods that may contain low levels of L. monocytogenes on a regular basis. However, the incidence of listeriosis remains relatively low. In 2008, the rate of listeriosis reported in Canada reached its highest point of 7 cases per million population, which was largely attributable to 2 outbreaks involving 57 and 40 confirmed cases each (Gaulin and Ramsay, 2010; Currie et al., 2015; Thomas et al., 2015b). Moreover, from 2011 to 2020, the national reported rate of listeriosis has remained fairly stable, ranging from 5.3 cases per million population in 2016 to 3.3 cases per million population in 2017 (PHAC, 2023). These Canadian rates are comparable to those reported in the United States (for example, 2.8 cases per million population in 2008 to 2016, excluding pregnancy-associated cases) (Pohl et al., 2019) and in the European Union (for example, 4.9 cases per million population in 2021) (EFSA and ECDC, 2022).
Scientific basis for the policy
A number of foodborne listeriosis outbreaks have been documented in Canada (see Appendix B) and throughout the world. These have been attributed to a wide variety of foods such as meat spreads, deli meats, sausages, dairy products made from pasteurized and unpasteurized milk, fish, produce and prepackaged foods such as sandwiches and salads (Desai et al., 2019).
The foods implicated in L. monocytogenes outbreaks are typically those in which L. monocytogenes is present at (or can grow to) levels that could present a health risk to consumers and are not normally further prepared before consumption. The ability of L. monocytogenes to grow at temperatures of -0.4 to 45°C, pH values of 4.4 or higher and aw values of 0.92 or higher are important characteristics that play a role in food safety (ICMSF, 1996). Nevertheless, the potential of acquiring foodborne listeriosis generally increases depending on several factors (FAO and WHO, 2004; CAC, 2009a; Buchanan et al., 2017) such as:
- host susceptibility (for example, underlying health status, immune status, medications)
- strain virulence
- the amount and frequency of consumption of a food containing L. monocytogenes
- the frequency, distribution and level of L. monocytogenes in the food
- the potential for the growth of L. monocytogenes in the food during refrigerated storage
- the refrigerated storage temperature
- the duration of refrigerated storage before consumption
In regard to the latter 3 bullets, quantitative modelling done by the European Food Safety Authority (EFSA) (EFSA BIOHAZ Panel et al., 2018) predicted that the expected number of human invasive listeriosis cases per year could be reduced by 37% if the growth of L. monocytogenes is prevented after consumer purchase, thus emphasizing the importance of consumer education.
The Listeria policy takes into account the intended consumers of RTE foods (for example, vulnerable populations), the potential for the growth of L. monocytogenes to occur in the food, as well as the presence or levels of L. monocytogenes in the food. These factors are used to determine the health concern that RTE foods pose to consumers. The potential for the growth of L. monocytogenes to occur depends on factors such as pH, aw, food formulation, the background microorganisms, the use of food additives (see Appendix C), storage conditions and shelf-life.
A definitive dose-response model for L. monocytogenes in humans has not been established. However, based on current case data from around the world, the likelihood of any 1 food containing low numbers of L. monocytogenes resulting in illness is considered to be very low (FAO and WHO, 2004). Foods that contain low levels of L. monocytogenes (for example, less than 100 Colony Forming Units (CFU)/g) pose very little risk (Chen et al., 2003; FAO and WHO, 2004). Furthermore, recent quantitative modelling suggested that the consumption of RTE foods containing more than 2000 CFU/g of L. monocytogenes is responsible for more than 90% of invasive listeriosis cases (EFSA BIOHAZ Panel et al., 2018). A U.S. risk assessment, which included a risk categorization of foods, further supports the fact that RTE foods differ in their ability to support the growth of L. monocytogenes, and therefore, differ in their risk to cause foodborne listeriosis (FDA and USDA, 2003).
Consequently, the Listeria policy separates RTE foods that support the growth of L. monocytogenes from those in which its growth is limited to levels not exceeding 100 CFU/g or in which it will not occur throughout the stated shelf-life (see the section on Category 2 ready-to-eat foods). Internationally, the Codex Alimentarius Commission, the Commission of European Communities and Food Standards Australia New Zealand (FSANZ) have proposed similar approaches to L. monocytogenes in RTE foods, to protect the health of consumers while applying fair practices in food trade (CAC, 2009a; FSANZ, 2014; European Commission, 2020).
Nevertheless, an outbreak in 2015 linked to certain ice cream products consumed by hospital patients emphasized the significance of host susceptibility and underlying health/immune status for listeriosis (Pouillot et al., 2016). All known exposures related to this outbreak were likely due to the consumption of milkshakes rather than to the original ice cream product (Chen et al., 2016a; Farber et al., 2021). Despite widespread exposure, no illnesses were reported among the general population linked to the consumption of the associated ice cream. L. monocytogenes was recovered at low levels in 99% (2307/2320) of the ice cream samples tested (Chen et al., 2016b; Pouillot et al., 2016).
This outbreak demonstrated the potential for listeriosis to occur in highly vulnerable individuals if they consume products that do not support the growth of, but do contain low levels of L. monocytogenes (Pouillot et al., 2016). Furthermore, a recent study on healthcare-associated foodborne outbreaks between 2001 and 2018 occurring in Organisation for Economic Co-operation and Development (OECD) countries identified L. monocytogenes as being responsible for the majority of healthcare-associated foodborne outbreaks in the hospital setting (Boone et al., 2021). These listeriosis outbreaks were associated with high case fatality rates and mainly affected vulnerable individuals.
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