Implementing whole genome sequencing at the European level could save up to five months in detecting multi-country Listeria outbreaks, according to a recent study.

More than half of severe listeriosis cases in the European Union are part of clusters, many of which are not detected fast enough by the current surveillance system. The study, led by the European Centre for Disease Prevention and Control (ECDC), analyzed 2,726 human Listeria monocytogenes isolates from 27 countries from 2010 to 2015.

About a third of infections identified as part of a cluster affected more than one country often lasted for several years. However, only two listeriosis outbreaks were reported in the EU in 2016 and five in 2015, which suggests that many have gone undetected, according to the researchers.

Use of whole genome sequencing (WGS) to characterize listeriosis cases at the EU level could speed up detection of clusters by up to five months, compared to epidemiological investigations at the country level, which sometimes involve pulsed-field gel electrophoresis (PFGE). There is also no globally accepted unique strain nomenclature to classify isolates.

The research report, “Retrospective validation of whole genome sequencing-enhanced surveillance of listeriosis in Europe, 2010 to 2015,” was authored by Ivo Van Walle, Jonas Torgny Bjorkman, Martin Cormican, Timothy Dallman, Joël Mossong, Alexandra Moura, Ariane Pietzka, Werner Ruppitsch, Johanna Takkinen and the European Listeria WGS typing group.

ECDC’s chief scientist Mike Catchpole said the study was a milestone on the way to tackling listeriosis in Europe.

“With this new collaborative effort with the Member States, we have revealed the related nature of many cases of severe listeriosis. We are now strengthening routine surveillance by introducing the collection and analysis of whole genome sequencing data from all reported human listeriosis cases,” he said.

“Improving our surveillance on Listeria cases will save lives, particularly among vulnerable population groups such as the elderly and also pregnant women, who may pass on the bacteria to the fetus if they consume contaminated food.”

Listeriosis is a potentially deadly foodborne disease that has been increasing in EU/EEA countries since 2008. In 2016, there were 2,536 cases reported, including 247 deaths. The incubation period is usually three days to three weeks, but can be as long as 70 days.

An example of a multi-country outbreak is the one linked to frozen vegetables produced by Greenyard in Hungary. It has sickened 54 people in five EU countries and Australia. Ten people infected with the outbreak strain have died. Implicated products were imported in 116 countries. The outbreak is thought to have begun in 2015.

About a third of the clusters found in the study involved more than one country, and for clusters of more than five isolates this increased to about half.

“This can be expected given the international nature of food trade, and indicates that there is potential for a substantial added public health value of introducing EU/EEA-wide, WGS-enhanced surveillance of listeriosis,” according to the study.

Researchers said molecular typing results must be combined with epidemiological and food exposure investigations.

“Given the sometimes-long incubation period of listeriosis, the low number of cases and the severity of the disease, food exposure data should ideally be collected for all cases, without additional waiting for typing results, as presently done in, for example, Denmark, France and the U.S.,” they said.

“It should also be complemented by WGS typing of officially sampled food isolates, and be combined with a joint analysis of the microbiological data to detect potential links between human cases and food items.”

About half of the cases in the study were described as sporadic.

“It is likely that some proportion of cases is truly sporadic in the sense that they are isolated cases related to individual- or household-level food preparation or storage practices, rather than to the microbiological quality of the food at the time of purchase,” according to the researchers. “In general, these cases can only be addressed through preventive measures such as public education, rather than control measures for food business operators.”


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