L. monocytogenes serogroup IVb, multi-locus sequence type 6 (MLST 6) has been associated with 32 cases between December 2015 and February 2018.
Suspected infections are in Finland, Austria, Denmark, Sweden and the UK.
Four EU Member States reported seven new confirmed cases, two of which were fatal, according to an ECDC update to a December assessment.
ECDC: No conclusive results yet
The European Centre for Disease Prevention and Control (ECDC) said the close relation between isolates in five countries is suggestive of a multi-country common-source outbreak.
The agency said it did not find any human isolate genetically closely related to the South African outbreak strain, where most clinical isolates from patients were also sequence type 6 (ST6).
“This means that the molecular typing information does not indicate a link between these two outbreaks,” it told FoodQualityNews.
“ECDC and EFSA, together with the affected countries, are currently investigating the outbreak to identify the vehicle(s) of infection and the point of contamination. We do not have conclusive results yet.”
Investigations in Finland and Sweden point to frozen corn as a possible vehicle of infection.
Fifteen cases were detected from different parts of Finland between 2016 and 2018.
Frozen corn from Hungary and packed in Poland
The National Institute for Health and Welfare (THL) in Finland said one MLST6-case reported eating unheated frozen corn.
It recommended people part of listeriosis risk groups should heat all frozen vegetables. Cooking at temperatures higher than 65°C kills the bacteria.
The institute added one out of six interviewed cases reporting corn consumption was ‘not very strong epidemiological evidence’.
SOK, part of the S Group network operating in the retail and service sectors, recalled Rainbow Sweet Corn - frozen maize [Rainbow Makea maissi – pakastemaissia] (450g) in January after one lot was found to contain Listeria. It covered best before dates between 6.1.2019 - 9.6.2019.
L. monocytogenes was found at 140 CFU/g in the corn from Hungary and packed in Poland. Distribution also included Estonia and Sweden but not other cases that reported infections, according to the RASFF portal.
Listeria isolated from patients and the above product were later found to be the same type.
ECDC and EFSA outbreak assessment
The outbreak was detected through whole genome sequencing (WGS) which demonstrated a close genetic match between human isolates.
Core genome multi-locus sequencing typing (cgMLST) analysis showed all human isolates cluster by ≤5 allelic differences from the Finnish representative outbreak strain, L. monocytogenes MLST 6.
ECDC and the European Food Safety Authority (EFSA) are preparing a joint outbreak assessment which should be published by the end of this month.
During 2012-2016, between 1,754 and 2,555 L. monocytogenes cases were reported annually to the European Surveillance System (TESSy) by 30 EU/EEA countries.
PCR serogroup IVb is the most commonly reported (44% of all cases with information on serogroup), with between 332 and 403 notifications annually from 13 EU/EEA countries.