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

Abstract

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.

Keywords: Listeria; Foodborne infections; food safety; infectious disease epidemiology; outbreaks.

Fig. 1.

Number of laboratory-confirmed cases of Listeria monocytogenes infection associated with caramel apples by state of patient residence, United States, 1 October 2014 to 1 February 2015 (n = 35).

Fig. 2.

Dates of Listeria monocytogenes isolation for 35 patients involved in outbreak associated with caramel apples, United States, 1 October 2014 to 1 February 2015.

Fig. 3.

Dendrogram of whole-genome multilocus sequence typing (wgMLST) results of human, environmental, and produce isolates that match the outbreak strains (n = 50 isolates). * Isolates not directly related to the outbreak. § Isolates that are indistinguishable from the outbreak PFGE patterns but excluded from the outbreak via wgMLST. † Isolates from an unrelated caramel apple production facility that are not highly related to outbreak-associated isolates. Not all apple isolates underwent wgMLST.