Listeria monocytogenes in ready-to-eat (RTE) foods: attribution, characterization and monitoring


Executive summary
A virtual meeting of the Joint FAO/WHO Expert Meeting on Microbiological Risk Assessment (JEMRA) of Listeria monocytogenes (hereinafter referred to as “L. monocytogenes”) in ready-to-eat (RTE) foods: attribution, characterization and monitoring was held from 20 October to 6 November 2020.
The purpose of the meeting was to review recent data on L. monocytogenes and determine the need to modify, update, or develop new risk assessment models and tools for this pathogen. A public call for data and experts was issued to support this work. In addition, background documents on the various aspects related to the meeting were prepared ahead of time for consultation by the experts.
Prepared documents included the following:
1) assessment of past JEMRA documentation; “Risk assessment of Listeria monocytogenes in ready to eat foods: Interpretative summary (MRA4)” (FAO and WHO, 2004a) and “Risk assessment of Listeria monocytogenes in ready to eat foods: Technical report” (MRA5) (FAO and WHO, 2004b);
2) a review of current national L. monocytogenes surveillance programmes;
3) a review of current microbiological and laboratory methods for L. monocytogenes;
4) an update on the virulence markers for L. monocytogenes. The meeting participants reviewed the prepared summary documents and other information on outbreaks and disease attribution, virulence, population risk factors, advances in laboratory methods and surveillance.
The aforementioned risk assessment documents (MRA4, MRA5) (FAO and WHO, 2004a, 2004b) covered a cross-section of RTE foods (pasteurized milk, ice cream, cold smoked fish and fermented meats) linked to invasive listeriosis. Since the publication of these documents, outbreaks of listeriosis continue to occur across the globe associated with previously reported foods, but also with many previously unreported food vehicles, including fresh and minimally processed fruits and vegetables (e.g. frozen vegetables).
The expert group concluded that it would be wise to be more inclusive in future risk assessments and that a full farm-to-fork risk assessment be considered. L. monocytogenes can infect anyone; however, it continues to disproportionally affect certain highly susceptible populations. The expert group recommended that future risk assessments should review groupings of susceptible groups, based on physiological risks and other socio-economic factors.
New information has emerged on L. monocytogenes strain variants, which differ in their virulence and environmental tolerance. Based on a panel of specific genes, the expert group proposed a virulence ranking of L. monocytogenes relevant

to invasive listeriosis.
The expert group concluded that the development and implementation of effective surveillance systems are critical in addressing the control of L. monocytogenes. The use of approved standardized laboratory methods that culture and isolate strains should be the foundation so that human, food and environmental isolates can be further characterized and inventoried.
In conclusion, the expert group identified several critical gaps in the current FAO/WHO risk assessment model and collectively agreed that updating the model would be valuable for informing risk analysis strategies, including in low- and middle-income countries (LMICs). The experts prepared short examples from literature (Annex 1) to demonstrate and highlight several key principles that should be considered in the risk assessment for L. monocytogenes.

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