Salmonella Potsdam Outbreak at a Restaurant in Connecticut

BACKGROUND : In January 2016, the Connecticut (CT) Department of Public Health (DPH) identified five laboratory-confirmed Salmonella Potsdam cases, an unusual serotype in CT. Initial interviews identified eating at the same food service establishment (FSE) during December 2015 – January 2016 as a common exposure. Epidemiologic, laboratory, and environmental investigations were initiated to further categorize the extent of illness and pinpoint the source of exposure.

METHODS : Cases were first interviewed using a standard questionnaire. Once an epidemiologic link was made to a common FSE, cases were re-interviewed and asked specifically about the FSE and food items consumed. Cases were defined as individuals with laboratory-confirmed S. Potsdam, with onset of symptoms within 72 hours after dining at the FSE during December 2015 – January 2016. Probable cases were defined as dining companions of confirmed cases with onset of diarrhea within 72 hours after dining at the FSE who did not seek medical attention. Isolates of S. Potsdam were tested using Pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS) at the Katherine A. Kelley State Public Health Laboratory (PHL). Samples collected during the environmental investigation at the FSE were submitted to the PHL for testing and included food worker stool specimens, food, and environmental samples.

RESULTS : Four S. Potsdam isolates were indistinguishable by PFGE at the PHL; the fifth isolate differed by one band. Results from WGS indicated that when compared to each other, all five S. Potsdam isolates were genetically indistinguishable from one another, including the isolate that differed on PFGE by one band. All five cases reported eating a tongue and tripe appetizer at the same FSE during December 2015 – January 2016. An additional four probable cases were identified; all consumed the tongue and tripe appetizer. Nine food worker stool specimens, 7 food samples, and 9 environmental swabs tested negative for Salmonella at the PHL.

CONCLUSIONS: An outbreak of Salmonella Potsdam occurred at a FSE in New Haven County in late December 2015 and early January 2016. All five cases were epidemiologically linked not only to the same FSE but also to one common menu item. Serotyping and PFGE testing was critical in rapid identification of the outbreak. In addition, WGS results were able to definitively link to the outbreak one case that had differed based on PFGE. Routine use of WGS will enhance outbreak detection and investigation, in particular, with isolates for which PFGE is less discriminating.