A major Listeria monocytogenes outbreak occurred in the province of Quebec, Canada, in 2008, involving a strain of L. monocytogenes (LM P93) characterized by pulsed-field gel electrophoresis (PFGE) and associated with the consumption of pasteurized milk cheese. This report describes the results of the ensuing investigation. All individuals affected with LM P93 across the province were interviewed with a standardized questionnaire. Microbiological and environmental investigations were conducted by the Quebec's Food Inspection Branch of Ministère de l'Agriculture, des Pêcheries et de l'Alimentation du Québec among retailers and cheese plants involved in the outbreak. Between 8 June and 31 December 2008, 38 confirmed cases of LM P93 were reported to public health authorities, including 16 maternal-neonatal cases (14 pregnant women, and two babies born to asymptomatic mothers). The traceback of many brands of cheese that tested positive for LM P93 collected from retailers identified two cheese plants contaminated by L. monocytogenes strains on 3 and 4 September. PFGE profiles became available for both plants on 8 September, and confirmed that a single plant was associated with the outbreak. Products from these two plants were distributed to more than 300 retailers in the province, leading to extensive cross-contamination of retail stock. L. monocytogenes is ubiquitous, and contamination can occur subsequent to heat treatment, which usually precedes cheese production. Contaminated soft-textured cheese is particularly prone to bacterial growth. Ongoing regulatory and industry efforts are needed to decrease the presence of Listeria in foods, including pasteurized products. Retailers should be instructed about the risk of cross-contamination, even with soft pasteurized cheese and apply methods to avoid it.

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