More than 20 people are part of a multi-country Listeria outbreak traced to fish and five have died since 2014.

An outbreak assessment by the European Food Safety Authority (EFSA) and the European Centre for Disease Prevention and Control (ECDC) identified 22 listeriosis cases caused by Listeria monocytogenes sequence type (ST) 1247, clonal complex 8 through whole genome sequencing (WGS).

Both agencies warned new cases cannot be ruled out until the point of contamination in an Estonian food processing plant has been identified.

It builds on an assessment from March this year when 18 cases were identified. At that time, the Estonian fish processor, a firm called M. V. Wool, denied the link.

Outbreak cases yearly since 2014
Denmark has nine cases, Estonia six, Finland two, France one and Sweden four. The first case had symptom onset in July 2014 in Estonia. The most recent occurred in Denmark in February this year.

“Outbreak cases have been reported every year since 2014, with a small clustering of cases between the end of 2015, 2016 and 2018, and the start of the following year. Estonia, Finland and Sweden reported cases in the first two years of this outbreak; since January 2017, all cases with the exception of one case in France in 2018 have been reported in Estonia and Denmark,” according to ECDC.

Eight patients, out of twelve for whom a food consumption history was available, confirmed eating cold-smoked fish products.

Information on age and gender is available for 20 patients with the median age being 76 years most are older than 50 except for one neonatal case reported in 2014. Eleven patients are male and nine are female.

Food and environmental investigations
Listeria monocytogenes food isolates, matching the human outbreak strain by WGS, were detected at wholesale and retail level in France, Denmark, Italy and Sweden from 13 batches of cold smoked or gravad salmon and from six batches of cold smoked trout products.

In 16 batches, Listeria was detected at less than 10 colony forming units per gram (cfu/g), in one batch at <40 cfu/g, and in two cold-smoked trout batches, it was enumerated at 400 cfu/g and 570 cfu/g.

Traceability information pointed to the Estonian firm M.V Wool as the common manufacturer of fish products. Raw fish was received from suppliers in Norway and Finland.

As of March 20, 2019, the processing company can only release cold-smoked and salted fish products after the verification of ‘absence of Listeria monocytogenes in 25 gram’, following a request from the Estonian authority.

Environmental investigations and food testing at the Estonian processing plant showed Listeria monocytogenes that matched the outbreak strain in two samples on the processing line and in four batches of final product.

“The presence of Listeria monocytogenes matching the outbreak strain over several years in the fish products suggests the persistence of the microorganism at the Estonian company’s premises. Further investigation is needed to identify points of (cross-) contamination in the food processing plant,” according to ECDC.