title: Escherichia coli O157:H7 outbreak linked to raw milk cheese in Quebec, Canada: use of exact probability calculation and casecase study approaches to foodborne outbreak investigation.
abstract: The analytical studies used to investigate foodborne outbreak are mostly case-control or retrospective cohort studies. However, these studies can be complex to perform and susceptible to biases. This article addresses basic principles of epidemiology, probability, and the use of case-case  design to identify the source of an Escherichia coli O157:H7 outbreak linked to raw milk cheese consumption in Quebec, Canada; a small number of cases with the same pulsed-field gel electrophoresis (PFGE) profile were involved. Between 4 December 2008 and 15 January 2009, a cumulative  total of 16 E. coli O157:H7 cases with the same PFGE profile were reported to Quebec public health authorities. Among the first six cases reported, three had consumed raw milk cheese from the same producer (cheese A). Raw milk cheese is consumed by about 2 % of the Quebec population.  By using the exact probability calculation, it was found that a significantly higher proportion of E. coli O157:H7 cases (with the specific PFGE profile) than expected had consumed cheese A (P < 0.001). These computations were updated during the course of the investigation  to include subsequent cases and gave the same results. A case-case study corroborated this result. This article considers alternative statistical and epidemiological approaches to investigate a foodborne outbreak-in particular with an exact probability calculation and case-case comparisons.  This approach could offer a fast and inexpensive alternative to regular case-control studies to target public health actions, particularly during a foodborne outbreak.
authors.affiliation: Ministère de la Santé et des Services sociaux, 1075 chemin Ste-Foy, Québec, Province de Québec, Canada G1S 2M1. colette.gaulin@msss.gouv.qc.ca
authors.forename: Colette
authors.initials: C
authors.lastname: Gaulin
authors.forename: Eric
authors.initials: E
authors.lastname: Levac
authors.forename: Danielle
authors.initials: D
authors.lastname: Ramsay
authors.forename: Réjean
authors.initials: R
authors.lastname: Dion
authors.forename: Johanne
authors.initials: J
authors.lastname: Ismaïl
authors.forename: Suzanne
authors.initials: S
authors.lastname: Gingras
authors.forename: Christine
authors.initials: C
authors.lastname: Lacroix
mesh_terms: D000818:Animals; D015373:Bacterial Typing Techniques; D002417:Cattle; D002611:Cheese / Q000382:microbiology*; D004196:Disease Outbreaks; D016521:Electrophoresis, Gel, Pulsed-Field; D004927:Escherichia coli Infections / Q000453:epidemiology*; D019453:Escherichia coli O157 / Q000302:isolation & purification*; D005260:Female; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D006801:Humans; D008892:Milk / Q000382:microbiology; D011336:Probability; D011792:Quebec / Q000453:epidemiology