Two listeria outbreaks caused by smoked fish consumption—using whole-genome sequencing for outbreak investigations

Abstract

Listeria monocytogenes may contaminate and persist in food production facilities and cause repeated, seemingly sporadic, illnesses over extended periods of time. We report on the investigation of two such concurrent outbreaks. We compared patient isolates and available isolates from foods and food production facilities by use of whole-genome sequencing and subsequent multilocus sequence type and single nucleotide polymorphism analysis. Outbreak cases shared outbreak strains, defined as Listeria monocytogenes isolates belonging to the same sequence type with fewer than five single nucleotide polymorphism differences. We performed routine food consumption interviews of L. monocytogenes patients and compared outbreak cases with sporadic cases. Two outbreaks were defined, each consisting of ten outbreak cases in the period 2013-15. Seven outbreak cases and a fetus in gestational week 38 died. 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. Outbreak cases more often than sporadic cases stated that they consumed the relevant fish products, odds ratio 10.7. Routine collection and typing of food isolates was key to solving the outbreaks. Furthermore, these outbreaks illustrate the value of whole-genome sequencing for outbreak definition and investigation. Whole-genome sequencing combined with epidemiological investigations provided the discriminatory power to recognize low-intensity, extended time-period outbreaks and link them to food products from two different contaminated production facilities with sufficient strength for food authorities to intervene on. Cold smoked and gravad fish constitute risk products and may be responsible for more listeriosis cases than previously recognized.

Benzalkonium tolerance genes and outcome in Listeria monocytogenes meningitis

“…A global sampling study including 300 disease and environmental listerial isolates from five continents showed clonal complex 6 to be present mainly in Europe and North America [38]. A recent analysis of two L. monocytogenes outbreaks in Denmark showed ST6 to be responsible for one of the outbreaks, confirming this sequence type to be involved in disease in other European countries [39]. Additional surveillance studies are needed to determine whether clonal expansion or acquisition of emrC harbouring listerial isolates causing severe invasive disease must be regarded as a global phenomenon.…”

Closing gaps for performing a risk assessment on Listeria monocytogenes in ready‐to‐eat (RTE) foods: activity 3, the comparison of isolates from different compartments along the food chain, and from humans using whole genome sequencing (WGS) analysis

“…The improved resolution obtained by WGS enabled linking of isolates and more robust case definitions enabling isolates to be ruled in or out of outbreaks. Furthermore, WGS makes it possible to recognise extended time-period outbreaks and link clinical cases to food products and food production facilities (Gillesberg Lassen et al, 2016). Such studies demonstrate the advantages of using WGS analysis for national surveillance and outbreak detection and investigation.…”

Potential growth of Listeria monocytogenes in Italian mozzarella cheese as affected by microbiological and chemical-physical environment

“…Unfortunately, listeriosis outbreak investigations are frequently associated with processed and ready-to-eat foods, such as meats, dairy products, cold smoked or gravad fish products, fruits, and vegetables (Jackson et al, 2011;McCollum et al, 2013;McIntyre et al, 2015;Gillesberg Lassen et al, 2016;Jensen et al, 2016). Listeriosis is a serious and sometimes lethal infection that may occur after eating food contaminated with Listeria monocytogenes.…”