Scientists have traced the source of a multi-year Listeria outbreak in Finland to ready-to-eat, plant-based food products, including fava beans.

Six cases were reported to the Finnish Infectious Diseases Registry between 2015 and 2019. Patient interviews and hospital menus were used to target traceback investigations of implicated foods. In 2021 and 2022, similar Listeria was detected from a ready-to-eat (RTE) product with fava beans.

Listeriosis cases were linked to the product based on an investigation that combined data on the hospital menu served to patients during the incubation period and a comparison of Listeria monocytogenes isolates detected at the production plant and from patients.

Contamination of the fava bean product after pasteurization and during processing or packaging was the likely cause of the outbreak, said researchers in the journal Eurosurveillance.

Patients up to 2019; product positives years later
In April to June 2019, four cases were identified. One person fell sick in 2015, and another in 2018. The ages of patients ranged from late 40s to mid-70s, and both men and women were affected. The 2019 cases were treated in two hospital wards in southern Finland, while the two cases in 2015 and 2018 were from eastern and western Finland.

Patients had underlying conditions such as kidney failure, cancer, hypertension, or diabetes or were otherwise immunocompromised. Two people died within 15 days of listeriosis diagnosis.

The 2019 patients were given snacks prepared by the hospital food service during dialysis treatment. Snacks had been served several times over several weeks before the onset of symptoms. No food served to cases was available for sampling, and Listeria monocytogenes were not detected from 10 surface samples taken from the hospital kitchen in July 2019.

In a routine inspection of hospital kitchen facilities in May 2019, the food control officer identified deficiencies in cleanliness, cleaning equipment storage, washbasin equipment, and the automatic temperature monitoring system for chilled and frozen food.

In December 2021 and August 2022, Raisio, the producer of fava bean products, issued a recall of affected batches and informed consumers of potential Listeria contamination.

Concerning the 2022 recall, Raisio said the Listeria problem affected products packed in a subcontractor’s facility, which underwent cleaning and sampling before operations were restarted.

In November 2021, Listeria monocytogenes was detected in a sample of a fava bean product the food business took. This was analyzed at a local official food laboratory, and the whole genome was sequenced at the Finnish Food Authority (Ruokavirasto).

In August 2022, two isolates from samples of another fava bean product from the same company were sequenced at the Finnish Food Authority. All three isolates were similar to those from the six cases. No Listeria monocytogenes were found in fava bean product samples in 2019 and 2020.

In December 2021, 591 surface samples were taken from the company’s processing environment, and Listeria monocytogenes were detected in 41 samples from floors and nearby surfaces and from a ripening machine.

Heating requirement added
Products were made from whole fava beans produced in Finland. They have been sold nationwide since 2016 and are widely available to consumers. An explanation for no other cases could be that the novel fava bean product is more prevalent among younger age groups, who tend to be in better health and are less susceptible to severe illness, said scientists.

Inspections by the manufacturer and local food control authority indicated that Listeria persisted in the production environment and products were contaminated after pasteurization. However, the origin of contamination was not found.

Official controls at the production plant from 2019 to 2021 noted deficiencies around labeling and sampling, cleanliness management of the site, separation of activities requiring different hygiene levels, and the working hygiene of employees. In December 2021, measures to control Listeria monocytogenes contamination in the food handling area included an updated sampling of products and the production area, intensifying cleaning and disinfection procedures, and further staff training.

In November 2022, Raisio was asked to add a heating requirement to pack labels for fava bean products since the source of Listeria in products and in the production environment was not found. Researchers said that finding Listeria in surface samples indicates that cleaning and disinfection measures were insufficient to control the pathogen.

In March 2023, the heating requirement was extended to all products manufactured and packed in the facility, except for items that undergo sufficient Listeria destruction treatment in their final packaging.

Researchers recommended national guidelines for people with weakened immune systems, including plant-based RTE products as a risk food for Listeria and appropriate labeling so consumers can follow the instructions.