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Multistate outbreak of Listeria monocytogenes infections linked to whole apples used in commercially produced, prepackaged caramel apples: United States, 2014–2015

  • K. M. ANGELO (a1), A. R. CONRAD (a1), A. SAUPE (a2), H. DRAGOO (a3), N. WEST (a4), A. SORENSON (a5), A. BARNES (a6), M. DOYLE (a7), J. BEAL (a7), K. A. JACKSON (a1), S. STROIKA (a1), C. TARR (a1), Z. KUCEROVA (a1), S. LANCE (a1) (a7), L. H. GOULD (a1), M. WISE (a1) and B. R. JACKSON (a1)...

Whole apples have not been previously implicated in outbreaks of foodborne bacterial illness. We investigated a nationwide listeriosis outbreak associated with caramel apples. We defined an outbreak-associated case as an infection with one or both of two outbreak strains of Listeria monocytogenes highly related by whole-genome multilocus sequence typing (wgMLST) from 1 October 2014 to 1 February 2015. Single-interviewer open-ended interviews identified the source. Outbreak-associated cases were compared with non-outbreak-associated cases and traceback and environmental investigations were performed. We identified 35 outbreak-associated cases in 12 states; 34 (97%) were hospitalized and seven (20%) died. Outbreak-associated ill persons were more likely to have eaten commercially produced, prepackaged caramel apples (odds ratio 326·7, 95% confidence interval 32·2–3314). Environmental samples from the grower's packing facility and distribution-chain whole apples yielded isolates highly related to outbreak isolates by wgMLST. This outbreak highlights the importance of minimizing produce contamination with L. monocytogenes. Investigators should perform single-interviewer open-ended interviews when a food is not readily identified.

Corresponding author
*Author for correspondence: K. M. Angelo, DO, MPH-TM, 1600 Clifton Road NE, Mailstop A-38, Atlanta, GA, USA 30329-4027. (Email:
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† These authors contributed equally to this work.
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Epidemiology & Infection
  • ISSN: 0950-2688
  • EISSN: 1469-4409
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