Widespread Listeriosis Outbreak Attributable to Pasteurized Cheese, Which Led to Extensive Cross-Contamination Affecting Cheese Retailers, Quebec, Canada, 2008

Abstract

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.

Whole-Genome Sequencing Allows for Improved Identification of Persistent Listeria monocytogenes in Food-Associated Environments

“…Scenario i is supported by a previous study that showed transfer of a persistent L. monocytogenes strain with dicing equipment transferred from one processing plant to another (56). Scenario ii is supported by a previously reported multistate U.S. outbreak of listeriosis linked to turkey deli meat (54) and a widespread outbreak of listeriosis in Quebec, Canada, linked to pasteurized cheese (57). In the former report, L. monocytogenes identical, as determined by PFGE, to the outbreak strain were isolated from three delis and subsequently the source of contamination was traced back to persistent L. monocytogenes contamination (over up to 12 years) at a deli meat processor that supplied each of these delis (54).…”

“…These cases suggest persistence of a strain representing a given clade at an external site, which serves as a source of introduction into multiple delis. A strain could persist at a supplier facility and be introduced to multiple delis through product shipments, as suggested in a 2008-2009 listeriosis outbreak from pasteurized Mexican-style cheese (58), and consistent with the studies cited above (54,57). Alternatively, a given strain with a recent MRCA could be a recently emerged, evolutionary successful strain found across multiple source environments.…”

“…In the former report, L. monocytogenes identical, as determined by PFGE, to the outbreak strain were isolated from three delis and subsequently the source of contamination was traced back to persistent L. monocytogenes contamination (over up to 12 years) at a deli meat processor that supplied each of these delis (54). In the latter report, L. monocytogenes identical, as determined by PFGE, to the outbreak strain were isolated from environmental or product samples from 22 retailers, and the source of contamination was subsequently traced back to a single upstream producer (57).…”

Examination of Food Chain-Derived Listeria monocytogenes Strains of Different Serotypes Reveals Considerable Diversity in inlA Genotypes, Mutability, and Adaptation to Cold Temperatures

“…Historically, lineage I 4b strains have been overrepresented in clinical listeriosis cases and are less frequently recovered from foods (8)(9)(10). However, over the past decade, lineage II 1/2a strains typically overrepresented in food and environmental samples (3,9,11) have been frequently linked to human disease, causing notable outbreaks in Switzerland (12) and the United Kingdom and two 2008 outbreaks in Canada (7,13). With regard to the Canadian outbreaks, 1/2a strains comprise the majority of Canadian clinical isolates, followed by 4b (14).…”

Foodborne bacteria in dairy products: Detection by molecular techniques

“…This is the most commonly used molecular subtyping method for surveillance and outbreak detection because of its high discriminatory power and reproducibility (Deng et al, 2016). PFGE was used for the molecular subtyping of Listeria monocytogenes isolates in a commercial cheese made from pasteurized milk that caused an outbreak in Germany from October 2006 through February 2007 (Koch et al, 2010) and in Quebec, Canada, in 2008(Gaulin et al, 2012. Additionally, in 2010, this technique was useful in determining the genotypic diversity of Listeria monocytogenes in the acid curd cheese that caused a multinational outbreak between 2009 and 2010 (Schoder et al, 2014).…”