Early Detection of Large Deli-Associated Salmonella Enteritidis Outbreak Aided By the Use of Culture-Independent Diagnostic Tests — Idaho, 2015

BACKGROUND:  Between June 8 and 9, 2015, 2 gastrointestinal illness complaints and 7 electronic laboratory reports (ELR) of Salmonella were received from the same community.  Culture independent diagnostic test (CIDT) results accounted for 86% (6/7) of ELRs.  All cases reported recent consumption of items from grocery store deli (Deli A), located in southwest Idaho.  An investigation was initiated to determine the magnitude of the outbreak, source of infection, and mitigate further spread.

METHODS:  Standard epidemiologic and environmental investigation methods were used. Cases were outbreak associated if they had a clinically compatible illness and ate a product from Deli A or their airport outlet kiosk between June 3 and 15, 2015, or were epi-linked to a confirmed case.  Patient samples were analyzed in clinical laboratories and when possible, submitted to the Idaho Bureau of Laboratories (IBL) for culture and pulsed-field gel electrophoresis.  Acceptable laboratory criteria for outbreak-associated probable cases included CIDT methods.  An EpiX call for cases was posted June 16, 2015. Food from the deli and a case household were tested by IBL.  The average time from sample collection date to first report-to-state date was compared for culture and CIDT.

RESULTS:  275 outbreak-related cases were identified from 10 states, including Idaho; of those 147 were culture-confirmed S. Enteritidis (SE). XbaI PFGE pattern JEGX01.0004 was the outbreak pattern. 39% (106/275) of cases were initially reported via ELR based on CIDT results; sample collection date to report-to-state date averaged two days (range: 1 – 4 days). 15.6% (43/275) of cases were first reported based on culture results an average of 5 days after sample collection (range 1-21 days).  86.7% (n=92) of CIDT tests were confirmed as SE by IBL. The onset dates for primary cases ranged from June 4 – 13, 2015. The average age was 41.3 years (range: 1-89y) and 59% of cases were female. Fifteen cases were hospitalized and there were no deaths.  SE was isolated from multiple foods collected from the deli and an open container from a case household.  Multiple Deli A food handlers were diagnosed with salmonellosis; illness onsets were after the first community-associated case. Deli A closed on 6/15/2015 for five days. No new deli product-associated cases were reported after reopening.

CONCLUSIONS: CIDTs facilitated earlier outbreak detection and response to this multistate outbreak than reliance on culture-based tests. Although the exact cause of this outbreak remained elusive, cross-contamination in the deli prep area likely played a significant role.