Outbreak of listerosis due to imported cooked ham, Switzerland 2011

H Hächler,P Giannini,G Marti,A Baumgartner,A Lehner,R Stephan,F Weiss,M Jermini,M Jost,B Bally,J Beck

doi:10.2807/ese.18.18.20469-en

Abstract

From 24 April to 31 July 2011, nine cases of listeriosis were registered in the cantons of Aargau, Basel-Land and Zurich, Switzerland. In six of the cases, infection with Listeria monocytogenes was laboratory confirmed, while three remained suspected cases. The suspected cases were family members of confirmed cases with identical or similar symptoms. All confirmed cases were infected with a L. monocytogenes strain belonging to serovar 1/2a: all had an indistinguishable pulsotype by pulsed-field gel electrophoresis (PFGE). The same strain was detected in samples of cooked ham that were on sale from a particular retailer. Two samples of ham tested contained 470 and 4,800 colony-forming units (CFU) L. monocytogenes per gram respectively. Data of shopper cards from two confirmed cases could be evaluated: both cases had purchased the contaminated ham. The outbreak initiated a product recall and alert actions at national and European level, through the Rapid Alert System for Food and Feed (RASFF). Following the RASFF alert, the company producing the contaminated ham was inspected by the responsible authorities. Their investigations showed that the ham was not contaminated in the production plant, but in the premises of a company to which slicing and packing was outsourced.

Highlights

Infections with Listeria monocytogenes in animals have been known since the first studies in this field by Murray et al in 1926 [1]
At the beginning of the 1980s, the first outbreaks due to foods contaminated with L. monocytogenes were recognised in the United States: the foods concerned were coleslaw, pasteurised milk and Mexican-style soft cheese [1]
A confirmed case was defined as person whose infection – reported to the Federal Office of Public Health – was laboratory confirmed as due to L. monocytogenes serotype 1/2a matching the outbreak pulsed-field gel electrophoresis (PFGE) pattern, with a test date between 24 April and 31 July 2011 in the cantons of Aargau, Basel-Land and Zurich, Switzerland

Summary

Introduction

Infections with Listeria monocytogenes in animals have been known since the first studies in this field by Murray et al in 1926 [1]. The first sporadic infections in humans were detected [2]. In Switzerland, they have been reported mandatorily since 1975. At the beginning of the 1980s, the first outbreaks due to foods contaminated with L. monocytogenes were recognised in the United States: the foods concerned were coleslaw, pasteurised milk and Mexican-style soft cheese [1]. In Switzerland, beginning in 1983, an increase in the number of listeriosis cases was observed. An artisanal soft cheese (Vacherin Mont d’Or), produced in the winter months in the western part of the country, was identified as the source of infection [3]

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Conclusion