Listeria monocytogenes Associated with Pasteurized Chocolate Milk, Ontario, Canada

In an investigation of a listeriosis outbreak in Ontario, Canada, during November 2015-June 2016, pasteurized chocolate milk was identified as the source.Because listeriosis outbreaks associated with pasteurized milk are rare in North America, these findings highlight that dairy products can be contaminated after pasteurization.L isteria monocytogenes is a formidable pathogen ac- quired primarily through contaminated food.Invasive listeriosis is a reportable disease in Ontario, Canada; ≈50 case-patients (0.4 cases/100,000 persons) have been reported annually since 2005 (1).Recent outbreaks of listeriosis in North America have been associated with delicatessen meats, soft cheeses, raw produce, and unpasteurized dairy products (2-4).However, listeriosis outbreaks linked to pasteurized fluid milk are rare.A study in the United States reviewed 83 fluid milkassociated disease outbreaks during 1990-2006; however, only 1 outbreak was attributed to L. monocytogenes (5).We report an outbreak of listeriosis associated with pasteurized chocolate milk in Ontario, Canada. The StudyWe defined an outbreak case-patient as a person in Ontario with listeriosis symptom onset after November 1, 2015, who had pulsed-field gel electrophoresis (PFGE) pattern combinations LMACI.0015/LMAAI.0024or LMA-CI.0015/LMAAI.0069.Thirty-four case-patients met the outbreak definition; only Ontario residents were identified.Eleven case-patients had an onset date during November 14, 2015-February 14, 2016.Onset dates ranged from April 11 to June 20, 2016, for 21 case-patients in the second wave; the remaining 2 case-patients were outliers (Figure 1).Median age was 73 years (range <1-90 years).More than half of the case-patients were female (20/34, 59%).Hospitalizations occurred for 32 (94%) case-patients, and 4 deaths (12%) were reported.In Ontario, local public health professionals complete the national invasive listeriosis questionnaire and collect food samples.We conducted a case-case analysis by using Ontario case-patients listed in the national listeriosis database as controls.We used a variety of methods to support hypothesis generation, including supplemental questionnaires, centralized interviewing, and reviewing purchase records collected through shoppers' loyalty card programs.A meeting was also held with representatives from a grocery chain that was common for case-patients (retail chain A) for insights into possible sources.PFGE and whole-genome sequencing were performed at the Public Health Ontario Laboratory, in accordance with PulseNet Canada protocols (Table ; Appendix Figure, https:// wwwnc.cdc.gov/EID/article/25/3/18-0742-App1.pdf).Food safety investigations, including targeted retail sampling, were conducted by the Canadian Food Inspection Agency and Ontario Ministry of Agriculture, Food and Rural Affairs.Laboratory analyses of food samples were conducted by the Canadian Food Inspection Agency and the Public Health Ontario Laboratory.Several hypotheses were generated during the course of this outbreak.In the first wave, a concurrent listeriosis outbreak associated with leafy greens was ongoing in the United States and Canada.However, product testing did not establish a relationship between the 2 outbreaks.Cheddar cheese was also suspected, but a food safety investigation, including sampling at the manufacturer, did not support a link to this outbreak (6,7).Although leafy greens and cheddar cheese were ruled out, 1 commonality remained; shopping at retail chain A was reported frequently by case-patients. Listeria monocytogenes Associated with Pasteurized Chocolate Milk, Ontario, Canada