Eating high-risk foods
From 2011 to 2021, 722 listeriosis cases were reported, ranging from 42 to 93 per year. The median age of patients was 75 but ranged from 8 to 101 years old, and the incidence rate was 11-fold in those older than 75 compared to other age groups.

Most interviewed patients in the study were immunocompromised because of an underlying illness or medication use. Interview data were obtained for 304 cases from 2016 to 2021. For patients with underlying diseases, the main ones were heart disease, diabetes, other cancers than leukemia, lung disease, and chronic kidney disease.

In Finland, whole genome sequencing (WGS) was introduced in 2015 for Listeria confirmation and typing, and interviews of listeriosis cases were launched in 2016 at the Finnish Institute for Health and Welfare (THL). Since starting interviews with patients, eight outbreaks with 133 patients could be solved with both epidemiological and microbiological evidence. They were linked to fish products, fresh or frozen vegetables, and meat and lasted for between three and seven years.

Two weeks before the start of symptoms, 71 of 297 had been in inpatient treatment, 84 of 294 had eaten in a restaurant, 38 of 285 had traveled in Finland, and 14 of 283 had been abroad. The most consumed risk foods recorded were ready-to-eat meat cold cuts and cured and cold- or hot-smoked fish. Many patients had high-risk foods or reported improper food storage.

In Finland, listeriosis in pregnant women is rare. They are advised to eat fish products only properly heated and to avoid sushi, roe, and foods containing raw fish. Unheated meat cold cuts are deemed safe if consumed well before the use-by date.

“This suggests that recommendations about high-risk foods and proper food storage should also be highlighted to other risk groups than pregnant women, as well as to relatives and people taking care of the elderly,” said researchers.

Of the cases, 96 of 286 reported a habit of checking their fridge’s temperature once a week, 25 said once a month, and 73 said less frequently. Also, 179 reported throwing out outdated products once a week, 29 said once a month, and 25 said less frequently. Almost 100 cases did not know of risky foods for listeriosis before infection.

Matching Listeria from patients to a food source
From 2011 to 2021, the Finnish Food Authority received 4,939 Listeria monocytogenes isolates from foods and production plants from 23 local official food control labs. Isolated strains represented 3,353 samples—more than 100 food samples contained above 100 Listeria colony-forming units (CFU)/g. 

The overall occurrence of Listeria in food products in Finland is unknown, as no national sampling surveys have been conducted since 2016.

Since 2018, the Finnish Food Authority has been typing food isolates with a similar method to that used for patient isolates, enabling comparison of Listeria strains. Researchers found matching food isolates that gave microbiological evidence to support epidemiological findings in outbreak investigations.

Researchers combined data from Finnish national listeriosis surveillance, patient interview responses, and laboratory data of patient samples. They compared them to Listeria findings from food and food production plants collected during outbreak investigations from 2011 to 2021.

Using WGS, from 2015 to 2021, scientists identified 23 clusters with five or more patient isolates, many of which persisted for years. In 12 clusters, food or environmental isolates were also detected.