In this outbreak, 401 cases of Salmonella I 4,5,12:i:- infection were reported. However, for every reported case of Salmonella infection, an estimated 38 additional cases are not detected or reported. Therefore, many more persons might have been ill as a result of this outbreak. Consumption of Banquet pot pies was associated with illness, and testing of Banquet turkey pot pies collected from patients’ homes yielded the outbreak strain. Mass food distribution can lead to widely distributed outbreaks, underscoring the importance of coupling laboratory-based surveillance of foodborne infections at the molecular subtype level with interviewing of patients to detect, solve, and truncate outbreaks.
The initial evidence that Banquet pot pies were the outbreak vehicle was acquired by MDH through the routine practice of combining data from PFGE subtyping of all Salmonella isolates and rapid interviewing of all patients. At MDH, these interviews used detailed food exposure questions to obtain open-ended histories, brand names, and purchase locations. Cross-referencing exposures identified in initial interviews and using an iterative approach to reinterview patients about suspicious exposures led to rapid identification of the possible outbreak vehicle.
Frozen, not-ready-to-eat microwavable meals have been reported previously as vehicles in salmonellosis outbreaks. Raw chicken nuggets and chicken strips were associated with Salmonella infections in a 1998 Australian outbreak and in Canadian studies of sporadic infections performed in 2003. Stuffed chicken products were implicated in five outbreaks in Minnesota during 1998, 2005, 2006, and 2008 (MDH, unpublished data, 2008). Consumer confusion regarding the raw or cooked nature of these products was documented in these reports; products were not clearly labeled as containing raw poultry ingredients, and they were breaded and prebrowned, leading to the perception that they were precooked (MDH, unpublished data, 2008).
This outbreak differs from previously reported outbreaks with frozen, not-ready-to-eat food vehicles in that all meat ingredients were intended to be precooked before leaving plant A. However, the pot pies associated with this outbreak had a raw flour crust and were not-ready-to-eat, which allows consumers to prepare the food item to the level of doneness they prefer but also requires consumers to ensure that minimum cooking temperatures are reached to control microbiologic hazards. Furthermore, because raw frozen poultry pastes used to make the liquid portion of the chicken and turkey pie fillings enter plant A, pies might have contained undercooked poultry or been cross-contaminated from these raw poultry pastes, which often harbor Salmonella. Despite an intensive investigation of plant A and its ingredient suppliers, the source of contamination remains unknown.
This outbreak identified labeling concerns. Specifically, recommended microwave cooking times on the pot pie packaging were based on wattage categories, but most patients were unaware of their microwave wattage. Because of the small size of the case control study, the investigation could not determine whether microwaving pot pies rather than cooking them in a conventional oven was a risk factor for illness. Twenty-three percent of case-patients who ate a pot pie reported cooking the pies in conventional ovens, so improper microwave cooking could not account for the entire outbreak. However, given the observed limited knowledge about microwave wattage and the frequency of deviating from microwaving instructions, microwaving likely did lead to inadequate cooking.
Inadequate microwave cooking was thought to be partly responsible for two previous outbreaks of Salmonella infections. Industry and regulators should consider examining the manufacturing processes for frozen not-ready-to-eat foods to determine the extent to which microwave cooking is safe for these products. Labeling and cooking instructions on not-ready-to-eat frozen foods should be clear to ensure that consumers are aware of health risks and to facilitate compliance with validated cooking methods. Clear and prominent listing of output wattage on microwave appliances might improve consumer adherence to manufacturer’s cooking instructions. Consumers should follow cooking instructions specific for an oven’s wattage.
