BACKGROUND: In March 2016, CDC PulseNet identified a cluster of Listeria monocytogenes infections with a rare pulsed-field gel electrophoresis (PFGE) pattern combination. The clinical isolates were highly related to one another by whole genome multi-locus sequence type (wgMLST) analysis and to two environmental isolates collected by FDA from a frozen fruit and vegetable processing facility in the same month. CDC, state public health and regulatory officials, and FDA initiated an investigation into the source of the illnesses.
METHODS: We defined a case as infection with an outbreak strain of Listeria monocytogenes, isolated from a normally sterile site from September 1, 2013 to May 15, 2016. The outbreak included two rare and distinct strains. Strain 1 included isolates with PFGE pattern combination GX6A16.0397/ GX6A12.1067 and highly related by wgMLST (range: 0-16 alleles, median: 6). Strain 2 included isolates with PFGE pattern combination GX6A16.0101/ GX6A12.0874 and highly related by wgMLST (range: 0-9 alleles, median: 7.5). We collected shopper card data when available and interviewed ill people using the Listeria Initiative case report form and a supplemental questionnaire focused on frozen foods, including vegetables and fruits.
RESULTS: We identified nine cases in four states: California (6), Connecticut (1), Maryland (1), and Washington (1). Isolation dates ranged from September 13, 2013 to May 3, 2016. Ill people ranged in age from 56 to 91 years (median: 76); 78% were female, and all nine were hospitalized; three died. Eight ill people were infected with Strain 1, and one with Strain 2. Of four ill people interviewed with the supplemental questionnaire and providing shopper card data, three ate frozen vegetables in the month prior to becoming ill. Open and unopened bags of one of the reported brands of frozen vegetables from one ill person’s home and retail samples of another brand of frozen vegetables yielded the outbreak strains. Both brands of frozen vegetables were produced by a firm that received frozen vegetables from a processing facility in which an outbreak strain of Listeria was identified in the environment. An estimated 456 products sold under 42 brands were ultimately recalled as a result of the investigation.
CONCLUSIONS: This is the first outbreak of listeriosis linked to frozen vegetables identified in the United States. Further research is warranted on the prevalence and risk of Listeria monocytogenes contamination in frozen vegetables and fruits to guide food regulatory policy.