Multi-country outbreak of Salmonella Enteritidis infections linked to Polish eggs

First published in EFSA Supporting Publications
15 December 2017
11 December 2017
Technical Report

A multi-country outbreak of Salmonella Enteritidis linked to eggs from Poland is ongoing in the EU/EEA. Testing through whole genome sequencing (WGS) shows the isolates are part of four distinct but related genetic clusters. ECDC and EFSA are liaising with relevant authorities in the affected EU/EEA countries and the European Commission to facilitate the coordination of investigation and response measures. From 1 February 2017 to 28 November 2017, eight EU/EEA countries have reported 196 confirmed cases of Salmonella Enteritidis belonging to four distinct WGS clusters, and 72 probable cases sharing one of the six S. Enteritidis multiple locus variable-number tandem repeat analysis (MLVA) outbreak profiles. Outbreak cases, both confirmed and probable, have been reported by Belgium, the Czech Republic, France, Luxembourg, the Netherlands, Norway, Sweden and the United Kingdom. In addition, 340 historical confirmed cases and 374 historical probable cases were reported before February 2017 by 16 countries (the previously listed eight countries plus Croatia, Denmark, Finland, Greece, Hungary, Ireland, Italy and Slovenia). The United Kingdom has reported the most cases (166 confirmed and three probable). Additional S. Enteritidis cases have recently been reported by some of the above-mentioned EU countries but molecular typing results are pending. The European outbreak case definition is based on a limited number of S. Enteritidis genotypes (four WGS clusters for the definition of confirmed cases and six MLVA profiles for the definition of probable cases), and is therefore extremely specific. In countries where MLVA or WGS is not undertaken routinely, additional cases are unlikely to be recognised as associated with this outbreak. An analysis of confirmed cases with a travel history to Bulgaria and Portugal in spring and summer 2017 shows that these two countries were probably affected by the outbreak. Available evidence from epidemiological, microbiological and tracing investigations linked the cases in Sweden to consumption of eggs originating from one packing centre and two laying hen farms in Poland. The Norwegian egg product isolate was traced back to another packing centre and another laying hen farm in Poland. Information from investigations in the United Kingdom to date was not sufficient to identify the vehicle of infection for all cases, but for some cases the eggs identified as the likely source were traced back to a Polish laying hen farm. So far, no epidemiological connection has been identified between the four Polish farms and the two Polish packing centres involved in Swedish, Norwegian and British investigations. However, some food business operators involved in the event investigated in Norway, and one possibly involved in the investigations in the United Kingdom, were also implicated in the 2016 outbreak linked to Polish eggs [1]. The collection of more epidemiological information would support a more precise assessment. All S. Enteritidis isolates from food sampled in Sweden and Norway are closely genetically related (i.e. belonging to the same five single nucleotide polymorphism (SNP) single linkage clusters) to the strains identified in one of the human clusters included in the case definition. The isolates detected in food and environmental samples in the United Kingdom are closely related (i.e. clustered at t5 level) to the strains identified in another human cluster strain included in the case definition. No detections of S. Enteritidis have been reported in any farms identified in the present outbreak as the possible sources of contaminated eggs or in the holdings supplying the mentioned farms. However, some positive farms identified in the 2016 outbreak are epidemiologically or microbiologically linked to positive food sampled in Norway and the United Kingdom. This supports the hypothesis that there is persistent contamination of two or more premises with the Salmonella strains detected in the 2016 outbreak still circulating at the farm level in Poland. EU/EEA countries should consider interviewing patients infected with outbreak strains of S. Enteritidis, following up on clusters associated with food outlets and performing appropriate investigations in food and related supply chains at those food outlets. Isolates with outbreak MLVA profiles should be sequenced. ECDC offers sequencing services for human isolates. A cluster analysis of the sequences should be performed using the Public Health England (PHE) single nucleotide polymorphisms (SNP) pipeline, as per the European outbreak case definition. New human cases and developments on public health investigations should be reported to EPIS-FWD (Epidemic Intelligence Information System for Food- and Waterborne Diseases and Zoonoses). Data gaps regarding food investigations in some countries have hampered the proper assessment of the epidemiological connections and microbiological contamination at primary production level and at other food business operators downstream in the food chain. Epidemiological data, especially for food/veterinary isolates, are essential to confirm the outcome of microbiological investigations. It is recommended that the involved countries carry out further epidemiological studies and tracing investigations to identify the food business operators at the source of the outbreak. Characterisation of food/veterinary isolates by WGS methods is encouraged to support the multi-country analyses. EURL Salmonella can provide technical support to countries’ laboratories with regard to subtyping isolates from food samples. ECDC and EFSA encourage the competent authorities in the food safety and the public health sectors in the affected EU/EEA countries and at the European level to continue sharing information on the epidemiological, microbiological and environmental investigations, including issuing relevant notifications using the Rapid Alert System for Food and Feed (RASFF) and the Early Warning and Response System (EWRS), the latter of which represents the official channel to notify cross-border threats. RASFF notifications should be completed with information on the exposure to food of related human cases, traceability information on the suspected food vehicles and WGS/MLVA results to support traceability investigations

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