Molecular tracing used to find source of Gemany's four-year Listeriosis outbreak

The report concluded the source was most likely a to be a contaminated conveyor belt in a meat processing plant but was unable to prove this

Some 543 L. monocytogenes isolates from food were analysed

A new report in Emerging Infectious Diseases1 has linked the source of a long-occurring outbreak of Listeria monocytogenes in southern Germany to one manufacturer, although it could not be proved conclusively.

Listeriosis is a serious, life-threatening infectious disease caused by L. monocytogenes, sometimes through ready-to-eat (RTE) foods such as packed and pre-sliced cheese and boiled sausages. It mainly affects elderly and immunocompromised persons, pregnant women, and neonates.

Dr Kleta, a microbiologist in the Department of Biological Safety at the German Federal Institute for Risk Assessment in Berlin, Germany, and head of the National Reference Laboratory for L. monocytogenes, was one of the authors of the report. Her research interests focus on food safety and molecular typing methods, especially on those for L. monocytogenes. One of the latest techniques, pulsed-field gel electrophoresis (PFGE) may be used for genotyping or genetic fingerprinting pathogenic organisms. Subtyping has made it easier to discriminate among strains of L. monocytogenes and thus to link environmental or food isolates with clinical infections.

The researchers investigated 543 L. monocytogenes isolates from food taken from the region of those affected by listeriosis during 2012–2016. Using forensic microbiology, the researchers managed to identify several products from one manufacturer that were contaminated with the outbreak genotype, but could not find the required isolates in the factory itself to conclusively prove it was the cause.

Protracted outbreak

In 2012, the number of L. monocytogenes cases in Germany started to rise and by 2016, 707 cases with case-fatality rate of 7% were reported.

The outbreak became apparent through analysis of the L. monocytogenes isolates from patients residing in the federal states Bavaria, Baden-Württemberg and Hesse revealed the same novel pulsed-field gel electrophoresis (PFGE) pattern.

Staff of official food control laboratories had acquired these isolates from food matrices and food processing plants in the affected federal states and then performed PFGE analysis.

A follow-up investigations of a meat processing plant revealed identical isolates of a specific type in another batch of smoked pork belly, in vegetarian sausages and two types of boiled pork sausages. A conveyor belt in a meat processing plant, moving product before packaging, was most likely the source of the contamination. All food products from this meat producer were banned from sale and those already on the market were withdrawn.

The meat processing plant predominantly supplied grocery stores of a single company. Patients and their relatives often shopped at these grocery stores and frequently ate pork products. Altogether, the food consumption histories of patients were compatible with the molecular typing results, but the researchers were unable to prove the producer was the source of the infections. The report says that once the production plant was shut down, the outbreak strain was isolated from only three more people who may have already consumed or consumed pork products from this company.

It concludes that the isolation of the L. monocytogenes outbreak strain in various food products from the same manufacturer, compared to the absence of the outbreak strain in a large number of food products collected during the outbreak from the same region, along with subsequent epidemiologic findings, suggested that the source of outbreak had been identified.

However epidemiologic analysis did not provide the information needed to determine the outbreak source; thus, forensic microbiology based on whole genome sequencing of L. monocytogenes isolates from patients and food became essential to take the appropriate countermeasures.

The report suggests that public health could benefit from continuous molecular surveillance of isolates from humans and food, which could allow for infectious disease outbreaks to be stopped before emergence.

EFSA Consultation

As a result of a rising trend of human listeriosis cases in Europe between 2009-2013, the European Food Safety Authority (EFSA) recently opened a public consultation on its scientific opinion looking at L. monocytogenes contamination of RTE foods and related risks for human health in the European Union.

Experts of the Panel on Biological Hazards have investigated the trend of human listeriosis in different age-gender groups and reviewed recent information available on L. monocytogenes in RTE foods and identified factors that may have impacted on the trend.

1. Kleta S, Hammerl J, Dieckmann R, Malorny B, Borowiak M, Halbedel S, et al. Molecular Tracing to Find Source of Protracted Invasive Listeriosis Outbreak, Southern Germany, 2012–2016. Emerg Infect Dis. 2017;23(10):1680-1683.