BACKGROUND: Salmonella Enteritidis pattern JEGX01.0004 (SE 4) is one of the most common pulsed-field gel electrophoresis (PFGE) patterns in Tennessee (TN) and the United States. SE 4 clonality using PFGE methodology has limited the ability to detect outbreaks. In 2016, TN began conducting Whole Genome Sequencing (WGS) on selected isolates of SE to assist with cluster detection. We investigated a localized outbreak at Restaurant X in June 2016 and subsequently enhanced surveillance efforts using WGS throughout the summer.
METHODS: During the Restaurant X outbreak, a cohort study was conducted. Relative risks and chi-square p-values were calculated to identify foods associated with illness. Isolates or specimens from patients diagnosed with Salmonella infection were submitted to the TN Public Health Laboratory for confirmation, serotyping, PFGE, and WGS. Bioinformatic analysis was conducted at the Wadsworth Center (NYSDOH). Enhanced real-time surveillance for SE 4 cases with key demographic and geographic features and/or egg exposures was conducted. Inspections were conducted at an egg farm in TN by the Department of Agriculture and the US Food and Drug Administration.
RESULTS: The Restaurant X outbreak investigation identified an epidemiologically significant exposure of eating steak with raw egg Béarnaise sauce. Environmental Assessment at Restaurant X indicated that eggs were sourced from a local producer leading to enhanced surveillance using WGS for egg-associated cases. Two subsequent outbreaks at distinct restaurants were identified during the summer of 2016. Both sourced shell eggs from the previously implicated farm in TN. A total of 15 confirmed cases of SE 4 were identified between the two outbreaks. WGS conducted on all 15 SE 4 patient isolates were within 0-3 single nucleotide polymorphism’s (SNPs) of each other. Two separate inspections of the farm yielded 6 Salmonella positive specimens including SE, although not within 0-3 SNPs of the human cases. Facilities who received eggs from this farm were provided education on safe egg handling and preparation.
CONCLUSIONS: An outbreak investigation implicating raw egg consumption led to enhanced surveillance for egg-associated SE 4 cases from a single egg producer. WGS was critical in the identification of a long-term SE 4 outbreak affecting multiple restaurants. While unable to identify the outbreak strain at the farm, the investigation led to identification of SE on the farm and implementation of public health actions to prevent additional illnesses. These outbreaks suggest that real-time sequencing of SE 4 in TN will identify outbreaks that previously were undetected using PFGE alone.