The risk of Listeria monocytogenes in ready-to-eat (RTE) fish products requires further attention, according to a long-awaited EFSA and ECDC study.

The European Centre for Disease Control and Prevention (ECDC), European Food Safety Authority (EFSA) and European unio Reference Laboratory (EURL) for Listeria monocytogenes, at the French Agency for Food, Environmental and Occupational Health and Safety (ANSES), began the European Listeria Typing Exercise (ELiTE) in 2010 to describe listeriosis in humans and Listeria monocytogenes in food.

Researchers found a high degree of dissemination of certain Listeria bacteria in the food chain and in people across the European unio. There was a strong link with RTE fish products in several identified clusters.

A cluster means isolates are genetically similar so likely originate from a common source. If human and food isolates are in the same cluster it is a strong indication the food may have caused the infections. However, microbiological findings alone are not enough to link food with infections, with epidemiological evidence also needed, according to the report’s authors.

Focus on fish products
Prevention and control of Listeria contamination in fish production facilities could reduce food contamination and potentially human illnesses. A review of business compliance with microbiological criteria should also be considered, particularly for fish products, according to the study.

Work covered data about public health and food from 13 and 23 EU member states, respectively, and involved three categories of RTE food: packaged hot or cold smoked or “gravad” (cured) fish, soft or semi-soft cheeses, and packed heat-treated meat products. In total, 580 human isolates and 413 food isolates were included in the research with the majority from fish samples. From the human data, at least 75 people were known to have died.

The study used molecular typing, which is a way of identifying specific strains of microorganisms, by looking at their genetic material. The method was pulsed field gel electrophoresis (PFGE), which was a well-established, standardized approach during the time period covered by the study.

PFGE is now being phased out and replaced by whole genome sequencing (WGS). ECDC and EFSA WGS databases are expected to be operational by June 2022. The project mapped clustering PFGE types with respective clonal complexes (CCs) characterized by WGS.

There were 78 separate clusters by PFGE profiles, involving 573 Listeria monocytogenes isolates. Of these, 21 included human and food Listeria monocytogenes isolates, 47 were only human, and 10 only food.

Outbreak links
In the 21 human-food clusters, almost 90 percent of food isolates were from fish products, with nearly 10 percent from delicatessen meat and 1 percent from cheese products. There were nine multi-country clusters with more than 10 cases and three involved 13, 14, and 15 countries.

The amount of Listeria in fish was generally low, but in 48 samples exceeded the microbiological limit of 100 colony forming units per gram (cfu/g). only six meat and one cheese product had counts above 100 cfu/g.

Of 78 clusters by PFGE profiles, 57 were small, up to five Listeria monocytogenes isolates per cluster. The largest was Listeria monocytogenes clone CC8. It involved 30 human and 56 food Listeria monocytogenes isolates from 15 countries. This indicates it may be common in several countries and has potentially been circulating in RTE fish production plants, according to the study.

Experts said based on the ability of Listeria to persist in the food chain for years, this clone is likely to cause large cross-border outbreaks. It was linked to 12 infections in three countries from 2015 to 2018 and 22 infections involving five countries from 2014 to 2019.

Another clone, CC121, was linked to four clusters with very few human isolates, suggesting lower virulence of the strains and possibly requiring a higher infectious dose. There was one nine-country cluster of 30 Listeria monocytogenes food isolates and no matches with human infections.