It also allowed a link to be made between fish from two different contaminated production facilities with sufficient strength for authorities to intervene.
The outbreaks in Denmark were severe, leading to eight deaths including a stillborn baby but were also ‘low-intensity’ - running over a number of years.
Two outbreaks were defined, each consisting of ten cases from 2013–15 which would have previously appeared as isolated cases.
Current guidelines do not advise pregnant women and the elderly against eating cold smoked fish products but following these outbreaks and the 2014 one, a panel looking at measures to reduce listeriosis in Denmark has recommended this be changed.
Positive isolates
Listeria monocytogenes isolates from cold smoked or gravad fish products or their two respective production environments were repeatedly found to belong to the outbreak strains, found the study.
Researchers compared patient isolates and isolates from foods and production facilities by WGS and subsequent multilocus sequence type and single nucleotide polymorphism analysis.
Using WGS, they identified two genetic clusters of L. monocytogenes in 2013–15 with different serogroups and sequence types but traced to the same type of source.
In Denmark, WGS was introduced for surveillance of the pathogen in 2013 as the first organism undergoing routine real-time use.
It was chosen because of the severity of the infections and relatively modest incidence compared with other foodborne infections.
The team identified two separate L. monocytogenes clusters of ST391 (serotype IIa) and ST6 (serotype IVb).
Outbreak one
The ST391 outbreak cluster was identified in April 2014 and initially contained four patients with the earliest laboratory sample date being 22 June 2013.
In June 2015, two L. monocytogenes isolates from environmental sampling at an unnamed company were found by WGS to be of the outbreak strain.
The DVFA stopped production at the firm and issued a sales ban on cold smoked and gravad fish products. These items were sampled as was the production environment, including 100 swab samples from equipment.
The outbreak strain was isolated from an environmental sample taken from non-product touching areas and L. monocytogenes ST121 was found.
Following cleaning and disinfection of the production area, the marketing and production bans were lifted but within weeks a new case was identified. This patient reported eating hot smoked salmon produced by the company.
Product samples were taken from hot smoked salmon and mackerel; all were negative for L. monocytogenes. The company was allowed to continue to operate; however, production of cold smoked fish products was discontinued.
As of 1 September 2015, ten cases had been identified and four adults died.
Outbreak two
On 10 September 2014, an SNP-cluster of five ST6 L. monocytogenes patient isolates, where the earliest dated back to 15 May 2013 was identified.
Less than two weeks later, WGS analysis found isolates from cold smoked halibut and trout produced by an unnamed company to be of the outbreak strain.
Upon identification of a new case in April 2015, the DVFA re-inspected and while environmental samples were negative, samples taken from gravad salmon and from frozen halibut were positive.
As of 1 September 2015, the DVFA continues to follow-up on initiatives at the company and the extensive sampling scheme. Ten outbreak cases were identified and three died.
WGS provided the possibility of linking cases that occurred over a period of years, establishing that they were continuous-source outbreaks and establish links with two different production facilities.
“In fact, the investigations provided evidence of sufficient strength for the Danish food authorities to intervene accordingly, possibly preventing future cases,” said the researchers.
Source: Clinical Microbiology and Infection
“Two listeria outbreaks caused by smoked fish consumption—using whole-genome sequencing for outbreak investigations”
Authors: S. Gillesberg Lassen, S. Ethelberg, J.T. Björkman, T. Jensen, G. Sørensen, A. Kvistholm Jensen, L. Müller, E.M. Nielsen, K. Mølbak