More than half of the severe listeriosis cases in the European Union belong to clusters, many of which are not being picked up fast enough by the current surveillance system, suggests a new article published in Eurosurveillance.
Invasive infection by Listeria monocytogenes (Lm) leads to relatively rare but serious food-borne disease mainly affecting elderly people, immunocompromised individuals and pregnant women. Clinical manifestations include sepsis and infection of the central nervous system, which can lead to lifelong sequelae or death [1,2]. Pregnancy-associated listeriosis can result in preterm birth, miscarriage or stillbirth [2,3]. In the European Union and European Economic Area (EU/EEA), 2,224 human cases of invasive listeriosis were reported in 2015, with an overall case fatality rate of 18.8% . Reported numbers of cases of listeriosis suggest that the incidence of disease slightly increased over the period of 2010–15. The incubation period of listeriosis is usually 3 to 21 days, but can be as long as 67 days, depending on the clinical form of the disease . Patients frequently have underlying conditions and/or are elderly, which limits the collection of exposure data in some cases. On epidemiological grounds, most cases are considered sporadic and detected outbreaks usually involve small numbers of patients, which limits statistical power in analytical epidemiological studies. As a result, most reported cases of listeriosis are difficult to link to a specific food product or food business operator.
Lm is able to form biofilms, grow at refrigeration temperature, high salt and nitrite concentrations, and can be resistant to disinfectants [6,7]. These properties contribute to its ability to persist and multiply in the food-processing environment and make it difficult to control. In the United States (US), a nationwide subtyping of Lm using pulsed-field gel electrophoresis (PFGE) was introduced in 1998. During the following 6 years, there was a more than fivefold increase in the number of outbreaks where a common food vehicle could be identified . In the subsequent 10 years, the introduction of detailed food history gathering for all listeriosis patients resulted in a further increase in the number of solved outbreaks per year, as well as a reduction of the number of cases per outbreak .
PFGE is, however, time-consuming and difficult to standardise. In recent years, it has been demonstrated that whole genome sequence (WGS)-based subtyping can provide substantial additional discrimination and, consequently, can be of benefit to outbreak investigations [9–11]. Within the EU/EEA, listeriosis is one of the priority diseases for which supranational WGS-enhanced surveillance will be initiated in 2018 . The work presented here supports the preparation for this surveillance system through a large-scale, retrospective, multi-centre study on Lm isolates from human cases from EU/EEA countries by covering the comparison and validation of analytical pipelines, the assessment of the epidemiological concordance of the results and the potential impact on public health . The analytical pipelines are based on the gene-by-gene approach recommended by the PulseNet International global consortium [14–17].