Whole genome sequencing of Salmonella Chester in Norway has uncovered a possible outbreak during summer 2016 related to Greece.

At the time, Norway saw an increase in Salmonella Chester cases among travelers to Greece. No infections were reported in Europe outside Norway. Results indicated an outbreak related to travel to Rhodes, Greece, which was different from a simultaneous multi-country outbreak associated with travel to Morocco.

Seven of eight Salmonella Chester patients with a history of travel to Greece in summer 2016 were interviewed about travel destination, dates of stay, accommodation, travel agency and foods consumed but the only common exposure was staying on the island of Rhodes.

In the summers of 2014 and 2015, several European countries had an increase in cases caused by Salmonella Chester. This multi-country outbreak was associated with travel to Morocco and linked to multiple food sources. 

Previously, cases of Salmonella Chester had rarely been reported but after this outbreak, it was included among the 20 most common Salmonella serovars causing infections in humans in Europe in 2014. The serovar has since accounted for 0.4 percent of the annually reported salmonellosis cases in Europe.

Salmonellosis is notifiable to the Norwegian Surveillance System for Communicable Diseases (MSIS) and corresponding isolates are sent to the National Reference Laboratory for Enteropathogenic Bacteria at the Norwegian Institute of Public Health.

Whole genome sequencing of Salmonella Chester in Norway showed four geographically distinct clusters associated with patient travel history. The study included isolates from 51 cases between 2000 and 2016. During this time, the number of Salmonella Chester cases by year in Norway ranged between zero and 18.

Researchers investigated genetic relatedness of Salmonella Chester for surveillance and outbreak detection by core genome multilocus sequence typing (cgMLST) and compared with results by single nucleotide polymorphism (SNP)-based reference genome mapping.

The aim was to identify if cases with a history of travel to Greece were part of the Moroccan outbreak cluster and to examine molecular epidemiology of isolates from patients with a history of travel to other regions.

“Although interviews were unable to confirm a source or common exposure for the cases travel-related to Rhodes, Greece, the WGS results of both the SNP-based and the cgMLST analysis give reason to believe that these cases constituted an outbreak,” said researchers.

Confirmed travel history was available for 80 percent of included cases, to Europe (n = 13), Asia (12) and Africa (16) with nine having no travel history.

Sequence type (ST) ST411 and a single-locus variant ST5260 (n = 17) were primarily acquired in southern Europe, ST1954 (15) in Africa, ST343 (11) and ST2063 (eight) primarily in Asia. The European one was further divided into a Greek (10) and a Cypriot (four) cluster.

Two clusters were related to travel to Asia, one with just over half of the cases reporting travel to Thailand, and the other with travel history to Sri Lanka or Thailand. The European and African clusters were similar to each other but more distant from the two Asian clusters.

Travel history allowed researchers to identify geographical clusters and results showed unrelated clusters, describing probable outbreaks, were overlapping in time.

“For example, the isolates from the African cluster, identified in the period between 2012 and 2016, were unrelated to the isolates associated with travel to Rhodes, Greece, which were identified in the summer of 2016,” added researchers.