Literature searches covering 1990-2015 by The Institut de Recerca i Tecnologia Agroalimentàries (IRTA) and the Universidad de Córdoba (UCO) resulted in 308 eligible records.
Information was extracted about the study, RTE product (population) and analytical methodology, risk factors (exposure and comparators) and results (outcome) about prevalence and concentration of L. monocytogenes, as well as serotypes.
Because L. monocytogenes is able to multiply at low temperatures (2 to 4 °C), RTE foods with a relatively long shelf-life (fishery, meat products and cheese) are of particular concern.
There has been an increasing trend of listeriosis between 2008 and 2015, with the proportion of cases in the over 64 age group increasing from 56.2% in 2008 to 64.1% in 2015.
Listeriosis affected 2,206 people, causing 270 deaths in 19 member states – the highest since 2008, according to the 2015 zoonoses report from EFSA and ECDC.
Product and risk factor
‘Dairy products’ was included in most records (N=139), followed by ‘meat products’ (110), ‘seafood’ (79), ‘composite food’ (62, including meals such as pasta- and rice-based salads, pre-cooked chilled foods, sandwiches, sushi, pastry and desserts), ‘produce’ (58) and ‘other types of products’ (16, including egg products and other un-specific/non-described “RTE products” in general).
The (risk) factors ‘HACCP system’, ‘Education and training of food handlers’ and ‘Cleaning and disinfection programme’ were reported in eight (2.6%), six and three of the reviewed records, respectively.
‘Food contact surface testing’ was present in 32 records (10.4%) and in 27 of them L. monocytogenes was detected. ‘Non-food contact surface testing’ was in 24 records (7.8%) and in 18 L. monocytogenes was found.
‘Food handler’s testing’ was present in seven records and in three the pathogen was detected.
Type of exposure was a balanced distribution among processes such as ‘(re)packaging’, ‘cutting/slicing’ and ‘assembling with other ingredients/additives’.
Piecing together data
Prevalence data in terms of number of positives when investigating presence/absence of the pathogen were available for 778 outcomes.
In total, in 29.5%, 21.9%, 41.2%, 63.5% and 52.9% of the studies with meat products, seafood, dairy, produce and other, respectively, L. monocytogenes was not detected.
“The impact of some of the (risk) factors considered in this review is hard to be assessed, as the studies usually do not provide the outcome (prevalence and/or level values) as a function of the risk factors,” said the researchers.
“Only few of the reviewed studies aimed to assess the impact of an intervention on the L. monocytogenes prevalence in naturally exposed RTE foods.
“Among them, an eradication programme based on thoroughly cleaning and disinfecting production machines and lines caused a drastic reduction of L. monocytogenes prevalence in the environment and in the RTE product (smoked rainbow trout).”
Serotypes 1/2a, 1/2b, 1/2c and 4b were the most reported for all food categories, except produce.
RTE food samples were mostly taken at the manufacturer (33%) or retail (35%). Mainly for milk the farm was also described to be the sampling place.
In 287 out of the 308 reviewed records, L. monocytogenes was investigated by a detection method, i.e. presence/absence in the analysed RTE food aliquot.
Among these, the procedure recognised by the Commission Regulation (EC) No 2073/2005 was applied in 30%, either in its original version (ISO11290-1:1996) or amended (ISO 11290-1:1996/Amd 1:2004).
Those of the Food and Drug Administration (FDA) and others such as Health Protection Agency (UK) Standard method, FIL-IDF143:1990 or modifications of the ISO 11290-1 were used.
Among the studies following alternative ISO validated methods, the VIDAS LMO2 was reported in most of the cases (eight records) followed by RAPID'L.MONO (three).
Some studies applied a “custom” detection procedure using Listeria Enrichment Broth (LEB), University of Vermont broth (UVM) or Half Fraser/Fraser broth as enrichment media while ALOA (Agar Listeria Ottavani & Agosti), PALCAM Listeria agar, Oxford Listeria Selective Agar and/or McBride Listeria agar were used as plating or streaking selective media.
Enumeration was used in 189 records. The official Standard ISO 11290-2:1998 or its amendment ISO 11290-2:1998/Amd 1:2004 was used in 40%.
Custom analytical procedures using ALOA, PALCAM Listeria agar, Oxford Listeria Selective Agar and/or McBride Listeria agar as plate count method were used in 26 studies. The MPN was used in seven records using Fraser broth as selective media.