Outbreak of Salmonella enterica serotype I 4,5,12:i:- infections: the challenges of hypothesis generation and microwave cooking

Abstract

We investigated an outbreak of 396 Salmonella enterica serotype I 4,5,12:i:- infections to determine the source. After 7 weeks of extensive hypothesis-generation interviews, no refined hypothesis was formed. Nevertheless, a case-control study was initiated. Subsequently, an iterative hypothesis-generation approach used by a single interviewing team identified brand A not-ready-to-eat frozen pot pies as a likely vehicle. The case-control study, modified to assess this new hypothesis, along with product testing indicated that the turkey variety of pot pies was responsible. Review of product labels identified inconsistent language regarding preparation, and the cooking instructions included undefined microwave wattage categories. Surveys found that most patients did not follow the product's cooking instructions and did not know their oven's wattage. The manufacturer voluntarily recalled pot pies and improved the product's cooking instructions. This investigation highlights the value of careful hypothesis-generation and the risks posed by frozen not-ready-to-eat microwavable foods.

Fig. 1.

Outbreak cases of Salmonella I 4,5,12:i:- infection by week of illness onset, MLVA pattern, and proportion of patients interviewed who reported consumption of any frozen microwavable pot pie and any such pot pie produced in establishment A by phase of outbreak – USA, 2007 (n = 396). The denominator for consumption of establishment A pot pies is restricted to patients who reported any pot pie and reported brand information. For 336 cases with complete information, the median duration from illness onset to date of culture was 3 days. For 65 cases with missing data, we estimated the date of illness onset as being 3 days before the date of culture.

Fig. 2.

Timeline of hypothesis-generation activities (shown in black outlined boxes). Because of reporting delays, the number of cases identified at the start of each activity was less than is indicated in this complete epidemic curve. The three case-patients whose interviews led to the brand A pot pie hypothesis are indicated by the black bars. Other key time points are indicated in the grey outlined boxes.

Fig. 3.

Brand A pot pie package label and microwave cooking instructions. Panel (a) displays the text on the product box before and during the outbreak. Panel (b) displays the text on the product box after packaging was revised as a result of this outbreak.

Fig. 4.

Culture results of 89 brand A pot pies by date of pie production and type of meat. Y-axis indicates the number of pies positive or negative for Salmonella with the outbreak PFGE patterns. The figure does not include two brand A pot pies of unknown meat variety and two brand A pot pies produced before 2007.