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Volume 26-04 |
RESTAURANT-ASSOCIATED OUTBREAK OF SALMONELLA TYPHIMURIUM PHAGE TYPE 1 GASTROENTERITIS - EDMONTON, 1999Background On 9 September 1999, four laboratory-confirmed cases of Salmonella Typhimurium infection in the Capital Health region (metro Edmonton, Alberta) were contacted by Capital Health Environmental Health Officers (EHOs) as part of the Capital Health's enteric disease surveillance system. The only exposure common to all four individuals was the consumption of a meal at an Edmonton, Alberta restaurant between 29 August and 2 September 1999 inclusive. In addition, a common food item, an ice cream pie dessert, was reportedly consumed by all four cases. An investigation into this apparent outbreak was then initiated by Capital Health's Environmental Health program. Methods Because of the strong epidemiologic link to consumption of food at one particular food establishment, Capital Health EHOs visited the implicated premises on 9 and 10 September to conduct inspections and interview staff. Seven food samples from the implicated food facility were submitted to the Northern Alberta Provincial Laboratory of Public Health for microbiologic analysis. Credit card slips for meal purchases at the restaurant between 29 August and 2 September were provided by the restaurant for later use in the case-control study. Controls were obtained from two sources - asymptomatic individuals in the same dining party as symptomatic individuals questioned during the investigation, and a random sample of individuals named on credit card slips from 29 August to 2 September meal purchases (and persons in the same dining party as an individual). Phone numbers for names on credit card slips were obtained from a local telephone directory. Controls (n = 36) were contacted by Capital Health EHOs between 13 and 17 September 1999. Those who reported eating at the restaurant between 29 August and 2 September inclusive were asked what food items they consumed, and whether or not they developed gastroenteritis in the several days following the meal. Further cases of gastroenteritis were added to the outbreak investigation through the notifiable disease system. All laboratory-confirmed cases of Salmonella identified in the Capital Health region and adjacent health regions between 9 and 24 September 1999 were questioned about consumption of a meal at the implicated restaurant, as well as information regarding other symptomatic individuals in their dining party. Those who met the case definition (outlined below) were also administered an outbreak investigation questionnaire. Results Disease presentation The outbreak case definition was restricted to restaurant patrons who reported onset of gastroenteritis (i.e. diarrhea, abdominal cramps, nausea, and/or vomiting) after consumption of food at the implicated restaurant during August and September 1999. In total, 27 individuals met this definition, 16 of whom were laboratory-confirmed positive for Salmonella Typhimurium Phage Type 1 in stool. All cases reported eating at the implicated restaurant between 29 August and 2 September inclusive. Six cases resided outside of the Capital Health region. Reported symptoms included diarrhea (94%), chills (94%), abdominal cramps (89%), fever (89%), head and/or body aches (83%), nausea (67%), and vomiting (56%). The reported incubation period ranged from 6 to 105 hours (mean: 30.1 hours, median: 26 hours). Duration of the gastroenteritis ranged from 1 to 7 days. Eleven individuals reported to hospital emergency, seven of whom were admitted to hospital. Restaurant management reported that four employees had onset of gastroenteritis between 4 and 10 September. These employees were contacted by Capital Health EHOs. None reported consumption of the implicated dessert and none were reportedly involved with its preparation (all four individuals were apparently servers at the restaurant). One clinical sample was obtained from an employee still symptomatic at the time of the investigation: it was negative for Salmonella. Food samples Food samples submitted for microbiologic analysis as part of this investigation included four pieces of the implicated ice cream pie in the premises at the time of the 9 September inspection, as well as three ingredients of this dessert - whole-shell eggs, semisweet chocolate, and commercially prepared ice cream. All seven food samples were negative for Salmonella. Restaurant management reported that these food samples were likely not from the lot served between 29 August to 2 September. Food histories Twenty-six of the 27 individuals who met the case definition reported consumption of one particular variety of ice cream pie. This menu item was served to cases on all 5 days of the implicated time window - 29 August (44%), 30 August (15%), 31 August (7%), 1 September (26%), and 2 September (7%). As well, the 27 symptomatic individuals identified during the investigation were in a total of 13 dining parties, 11 of which had reports of more than one symptomatic individual in the party. In all 11 of these dining parties, it was reported that one piece of the implicated dessert was shared by two or more individuals in the same dining party. Statistical analysis A case-control study was conducted to statistically confirm the association between onset of illness and consumption of the ice cream pie dessert. The analysis revealed a strong statistical relationship between illness and consumption of a particular ice cream pie dessert (odds ratio = 442.00, 95% confidence interval = 30.5 to 16,674.8, p < 0.001). The large odds ratio and the large range of the confidence interval are likely due to the consumption of the ice cream pie being reported by only two asymptomatic controls, and the small sample size. Production of the ice cream pie Because of the strong link between consumption of the ice cream pie and subsequent development of gastroenteritis, restaurant staff were interviewed regarding methods for its preparation. This dessert is prepared on-site in batches every 3 to 4 days; one batch represents 24 slices. The restaurant provided a copy of the recipe for the ice cream pie, which consists of a chocolate cookie crumb and melted butter crust, and a filling of commercially prepared ice cream, egg yolk, egg white, semi-sweet chocolate, margarine, icing sugar, vanilla, toasted almonds, and milk. The product is sliced into large cubes (approximately 5 inches per side). Restaurant management estimated that more than 30 pieces of the implicated dessert were sold during 29 August to 2 September. A deviation from the standard recipe was discovered during the investigation. The recipe calls for pasteurized egg products; however, when bakery department staff were questioned, it was reported that whole shell eggs (egg yolks and whites) were used. Eggs in the implicated restaurant at the time of the investigation were obtained from an approved supplier, and according to package labelling, were graded. Food handling practices Several breaches in proper food handling practice were observed at the restaurant during the investigation. During the 9 September 1999 inspection, perishable food was being kept at temperatures that favor bacterial growth as a result of malfunctioning refrigeration units, several food-preparation areas were unclean, and no soap and paper towels were available at the employee washroom handwash sink. In addition, it was suspected that at least one employee of the restaurant was working while symptomatic with gastroenteritis during the outbreak investigation. Discussion The information obtained during the investigation strongly suggests that the outbreak of Salmonella Typhimurium Phage Type 1 gastroenteritis occurred as a result of consumption of an ice cream pie dessert between 29 August and 2 September 1999 at an Edmonton, Alberta restaurant. This is evidenced by the high proportion of symptomatic individuals who reported consumption of this food, as well as the high odds ratio for the dessert calculated from the case-control study. The way in which the ice cream pie may have been contaminated cannot be confirmed retrospectively. It is hypothesized that only one batch of the ice cream pie was contaminated, and that the 5-day period during which cases reported consumption of this dessert represented the time required to serve the entire batch. It is also probable that several cases were not reported. The total population at risk in this outbreak is unknown; however, because this dessert is served in large pieces and is frequently shared within dining parties, the number of cases involved are likely greater than the number of slices which were served during the 5-day period. The process by which the dessert became contaminated remains unclear. All ingredient samples submitted for analysis during the investigation were negative for Salmonella, and there was no evidence that a Salmonella-infected food handler or a contaminated food surface came into contact with the product during the 5-day period. However, ingredient samples submitted for analysis were likely not used to make the batch that was served from 29 August to 2 September, and it could not be confirmed if a food handler in the restaurant was shedding Salmonella at the time the implicated dessert was prepared. Despite insufficient evidence to confirm how the suspected food product may have become contaminated, improper food handling practices observed at the restaurant during the investigation may have contributed to the outbreak. The use of unpasteurized whole shell eggs i in the implicated food is one potential cause. Salmonella Typhimurium has been found to be a bacterial contaminant of eggs(1), eggs are frequently implicated in outbreaks of Salmonella gastroenteritis(2), and the ice cream pie may have not received sufficient heat treatment to destroy any bacteria which might have been in the ingredients. Evidence of inadequately equipped hand washing facilities in the restaurant coupled with an account of an employee on duty while symptomatic with gastroenteritis may also be important. If such practices were occurring during preparation of the implicated dessert, they could be other potential causes of this outbreak, as symptomatic food handlers have been implicated in foodborne outbreaks of Salmonella Typhimurium(3). Inadequate sanitation in food preparation areas and improper temperature control of perishable foods observed in the premises may also have played a role, as such conditions increase the likelihood of contamination and growth of bacteria in food. Prevention Strategies To minimize the likelihood of similar disease outbreaks in the future, the following recommendations were made by Capital Health, Environmental Health to the restaurant management:
References
Source: Lance Honish, BSc, CPHI(C), Environmental Health Officer-Food Program Specialist, Capital Health - Community Care and Public Health, Edmonton, Alberta.
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