A prolonged multi-country outbreak of 22 listeriosis cases caused by Listeria monocytogenes sequence type (ST) 1247, clonal complex (CC) 8 has been identified through whole genome sequencing (WGS) in five EU countries: Denmark (9 cases), Estonia (6), Finland (2), France (1) and Sweden (4). Five patients have died due to, or with, the disease. The first case had symptom onset in July 2014 in Estonia, and the most recent case occurred in Denmark in February 2019. Eight patients, out of twelve for whom a food consumption history was available, confirmed the consumption of cold-smoked fish products.
L. monocytogenes food isolates, matching the human outbreak strain by WGS, were detected at wholesale and retail level in four countries (i.e. France, Denmark, Italy and Sweden) from 13 batches of cold smoked or gravad salmon and from six batches of cold smoked trout products. Traceability information of the contaminated batches pointed to the Estonian processing Company A as the single common manufacturer of these fish products. The raw fish was received from suppliers in Norway and Finland. Environmental investigations and food testing at the Estonian processing plant showed the presence of L. monocytogenes that matched the outbreak strain in two samples on the processing line and in four batches of the final product.
The presence of L. 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. Control measures were implemented in Estonia, Denmark, France and Italy following the RASFF (Rapid Alert System for Food and Feed) notifications, but until the source of infection has been identified and controlled, new invasive listeriosis cases associated with this event may still occur.
In general, pregnant women, the elderly and immunocompromised individuals are at increased risk of invasive listeriosis, which is associated with severe clinical course and potentially death.