Abstract
In September 2017, a cluster of monophasic Salmonella Typhimurium isolates was identified at the National Reference Laboratory for Enteropathogenic Bacteria in Norway. We investigated the cluster to identify the source and implement control measures. We defined a case as a person with laboratory-confirmed salmonellosis with the outbreak strain multiple locus variable-number tandem repeat analysis type. We conducted descriptive epidemiological and environmental investigations and performed whole genome sequencing (WGS) with core and accessory genome multilocus sequence typing of all isolates from cases or the environment connected with this outbreak. We identified 21 cases, residing in 10 geographically dispersed counties, all of whom had consumed food or drinks from a café at Oslo Airport. Case distribution by date of symptom onset suggested that a point source was introduced in mid-August followed by continued environmental contamination. The incubation periods ranged 0–16 days and increased as the outbreak progressed, likely due to increasingly low-dose exposure as control measures were implemented. WGS confirmed an identical cluster type-944 in all cases and six environmental specimens from the café. Control measures, including temporary closure and kitchen refurbishment, failed to eliminate the environmental source. We recommend strengthened hygiene measures for established environmental contamination during an outbreak.
Highlights
Non-typhoidal Salmonella infection is the second most commonly reported gastrointestinal infection in the European Union/European Economic Area (EU/ EEA) [1]
Description of the cases As at 1 February 2018, 21 confirmed cases were reported to the Norwegian Institute for Public Health (NIPH)
Typhimurium isolates were of the same multiple locus variable-number tandem repeat analysis (MLVA) type and clustered together closely in whole genome sequencing (WGS) analysis, which pointed to a café at Oslo Airport as the common source of the outbreak
Summary
Non-typhoidal Salmonella infection is the second most commonly reported gastrointestinal infection in the European Union/European Economic Area (EU/ EEA) [1]. Since 2005, between 866 and 1,942 cases of salmonellosis have been reported annually. 60–80% of notified cases are acquired abroad, as Norway has few established domestic Salmonella reservoirs. S. Typhimurium is one of the most commonly encountered Salmonella serovars in Norway accounting for 33% of domestically acquired salmonellosis cases in 2000-15 [2]. Typhimurium with antigenic formula 4,[5],12:i.- has been identified in several countries, both in production animals and human cases [2,3,4]; monophasic S. Typhimurium was first identified in 2007 and in 2016, it accounted for 12% of all salmonellosis cases and 17% of all domestic salmonellosis cases reported to the Norwegian Institute for Public Health (NIPH) [8]
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