A Danish campaign to tackle Listeria after a large outbreak doesn’t appear to have worked, based on the recently released findings of a study.

The awareness campaign was a risk communication strategy aimed at reducing Listeria. However, the study showed the prevalence of Listeria monocytogenes in high-risk RTE facilities was largely unchanged between 2016 and 2020. Also, cases of listeriosis went up from 39 in 2016 to 86 in 2022.

To assess the presence and persistence of Listeria monocytogenes in Danish ready-to-eat (RTE) food companies in response to a Listeria awareness campaign, the production environment of seleced fish and meat firms was sampled in 2016 and 2020.

After a listeriosis outbreak that involved 41 cases and 17 deaths in 2014 caused by a contaminated RTE spiced meat roll product, the Danish Veterinary and Food Administration (Foedevarestyrelsen) launched a Listeria awareness campaign in 2015–2016 with information and educational activities aimed at producers of high-risk products, food inspectors and the public.

The number of companies that tested positive for culture was 17 out of 39 in 2016 and 11 out of 34 in 2020, indicating a limited effect of the campaign, found the study published in the MDPI journal Hygiene.

Limited success and more infections
A total of 777 environmental samples were obtained from 53 companies, of which 32 and 20 samples were positive for Listeria monocytogenes in 2016 and 2020, respectively. Four companies tested positive in both years. 

only 20 companies participated in both years and the number of samples per firm varied. Also, testing in 2020 was focused on the area after heat treatment rather than wher raw materials were handled.

Whole genome sequencing (WGS) characterized 50 isolates from 24 companies, plus another 35 from routine surveillance from 2016 to 2020.

Ten different sequence types had been implicated in cases in Denmark during the study period and five of them were also found in the food companies. Analysis of one strain suggested it had persisted over time in the production environment and may have caused human cases, with a 6-year interval.

Results showed the same sequence types can be isolated over long periods from the same companies. Isolates belonging to the same sequence type and from the same companies were genetically similar regardless of year or whether samples were from products or the environment, indicating the persistence of some sequence types.

“Prevalence of Listeria monocytogenes remained comparable between 2016 and 2020 samplings, which, taken together with the increasing trend in listeriosis cases in Denmark, may indicate that the current risk communication strategy is not working, despite the DVFA’s intensive Listeria awareness campaign and availability of comprehensive information on the agency’s website,” said scientists.

Listeria in Germany
Meanwhile, another study has looked for Listeria at fruit and vegetable producers in Germany.

From July 2020 to June 2021 in Bavaria, 39 producers of soft fruit, vegetables, and ready-to-eat raw fruits and vegetables were checked. Inspections were at the farm, primary production, and processing stages.

Environmental and food sampling as well as testing of irrigation and processing water was performed to investigate the prevalence of Listeria species including Listeria monocytogenes.

A total of 407 samples were taken including 229 swab samples from food contact materials and the environment, 59 food samples, and 119 samples of irrigation and processing water.

In 51 samples, Listeria spp. was detected. Listeria seeligeri was the most identified species, followed by Listeria innocua, Listeria monocytogenes, and Listeria ivanovii, according to the study published in the Journal of Food Protection.

Listeria monocytogenes findings
Listeria monocytogenes were identified in seven environmental and processing water samples but not in food. These isolates were detected in six different facilities, three at the primary production level, and four at processing plants.

The common isolation area of Listeria monocytogenes was the facility surroundings, especially in different gullies and drains. One processing water sample of a lettuce washing system was positive at the primary production level.

Listeria monocytogenes were found in a drain wher ready-to-eat salad with products of animal origin (POAO) were processed, highlighting the need to comply with good manufacturing and hygienic practices, especially when both types of products are processed at the same premises.

Swab samples showed the highest prevalence of Listeria. Irrigation and processing of water and food samples had a lower prevalence. More than one type of Listeria was found in 12 samples.

The detection of different Listeria species within the same company suggests the pathogen may become an issue if hygienic environmental handling is neglected, said researchers.

“In addition to water sources and quality, this study demonstrates that irrigation regime, cultivation, hygienic handling, and maintenance protocols are highly important to reduce the potential contamination of ready-to-eat soft fruits and vegetables with Listeria,” they added.