Nov 26, 2008 (CIDRAP News) – A widely publicized Salmonella outbreak that was linked to frozen pot pies last year involved 401 cases in 41 states and put more than 100 patients in hospitals, the Centers for Disease Control and Prevention (CDC) said today in a final report on the episode.
The outbreak prompted changes in the label instructions for Banquet pot pies and warnings about the importance of thoroughly cooking frozen, not-ready-to-eat foods. And in today's report, the CDC says the food industry and regulators should examine manufacturing processes for such foods to determine how safe it is to cook them in microwave ovens.
Cases in the outbreak began in February and continued until December, peaking in September, according to the article in the Nov 28 issue of Morbidity and Mortality Weekly Report. Of patients for whom the information was available, 144 of 289 (50%) had bloody diarrhea, and 108 of 338 (32%) were hospitalized, the CDC says. The outbreak strain is known as Salmonella enterica serotype I 4,5,12:i:-.
Rajal Mody, MD, a CDC Epidemic Intelligence Service officer, said a hospitalization rate of 32% is "close to average" for Salmonella outbreaks. He said a recent study that compared the hospitalization rates for many different Salmonella serotypes found that the average is 22.8%. The study was published in the Journal of Infectious Diseases (see link below).
"There are definitely some strains that are lower, some in the 16% range, and there's one as high as 67%, but that is a fairly rare serotype," Mody told CIDRAP News. He said the same study indicated that the average hospitalization rate for the I 4,5,12:i:- strain is about 25%.
The first cluster of cases involving the outbreak strain with matching DNA fingerprints was detected by Pennsylvania disease detectives in June, the CDC report says. But the source of infection was not discovered until a case-control study was launched in October.
As part of that effort, epidemiologists in the Minnesota Department of Health determined that four case-patients had eaten Banquet pot pies in the week before they got sick. Further investigation of cases and neighborhood matched controls pointed to Banquet turkey pot pies as the only food associated with the outbreak.
In subsequent interviews, 174 of 236 case-patients reported they had eaten frozen pot pies in the week before they fell ill, and more than 90% of these were Banquet or other brands made in the same plant. In addition, the outbreak strain was found in 13 unopened Banquet turkey pot pies collected from patients, all of them produced on Jul 13 or Jul 31, 2007.
ConAgra Foods on Oct 8, 2007, suspended production of pot pies at the plant linked to the outbreak, and a few days later the company recalled all pot pies made there. Previous reports listed the plant location as Marshall, Mo.
The CDC report indicates that many of the outbreak cases might have been related to undercooking of the pot pies in microwave ovens. "Banquet pot pie microwave instructions might have been confusing because different parts of the package recommended different preparation times," the article says. Also, the microwave instructions varied by wattage, but few of the patients who were interviewed knew the wattage of their microwave. The report notes that ConAgra revised the labeling and instructions on the pot pies before resuming production.
However, improper microwave cooking could not explain the entire outbreak, because 23% of case-patients who ate a pot pie reported cooking the pies in conventional ovens, the CDC says. The case-control study was not large enough to determine whether using a microwave rather than a conventional oven was a risk factor for illness.
Several previous salmonellosis outbreaks have been linked to frozen, not-ready-to-eat foods, including several tied to frozen chicken entrees, the report notes. The pot pie outbreak differed from the previous ones in that all the meat ingredients in the pies were supposed to be precooked, with the crust being the only raw part, it says. The report suggests that contamination could have come from "raw frozen poultry pastes" used in making the pies. However, an intensive investigation of the ConAgra plant and its suppliers failed to pinpoint any source of contamination.
In view of the likely role of microwave cooking in the outbreak, the CDC says, "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."
Besides calling for clear instructions and warnings on frozen microwavable foods, the agency says that clear and prominent listing of the wattage on microwave ovens might improve consumer compliance with the cooking instructions.
CDC. Multistate outbreak of Salmonella infections associated with frozen pot pies—United States, 2007. MMWR 2008 Nov 28;57(47):1277-80 [Full text]
Oct 12, 2007, CIDRAP News story "ConAgra recalls pot pies as Salmonella cases rise"
Study from Jul 1, 2008, Journal of Infectious Diseases: "Salmonellosis outcomes differ substantially by serotype"