The investigation of a multistate outbreak of Shiga toxin-producing Escherichia coli O157:H7 (STEC) that sickened 77 people and hospitalized 35 was traced back to ready-to-bake cookie dough, prompting infectious disease specialists to ask for stronger pasteurization and more consumer warnings.
A report in Clinical Infectious Diseases outlined the outbreak and the work done by national and local health officials to track down the source.
No single source could be identified for certain for the outbreak. But one brand of dough was present in 94% of cases, and three nonoutbreak STEC strains were isolated from it, leading to a recall of 3.6 million packages of the cookie dough.
The detective work began with May 19, 2009, through PulseNet, the network of public health and food regulatory agency laboratories coordinated by the CDC. It identified a cluster of 17 identical cases of E. coli O157:H7 from 13 states. This exceeded the baseline average of six to seven isolates typically submitted each month.
National, state and local public health officials launched a matched case-control study between patients sickened by E. coli and from other enteric illnesses such as Salmonella and Campylobacter. A questionnaire focused on 20 food items, including ready-to-bake cookie dough, and product information, including lot numbers and use-by dates from packages of suspected products.
Seventy-seven cases from 30 states met the case definition between March 16, 2009 and July 8, 2009. Patients aged 5 to 19 years comprised 55% of patients; the median age was 15 (range, 2 to 65); 71% were female. Thirty-five of 64 (55%) patients with available information were hospitalized and 10 of 57 (18%) developed hemolytic-uremic syndrome; none died.
Health officials entered 36 case patients and 36 matched controls from 18 states and questioned them about 20 foods. Cookie dough was the only exposure significantly associated with illness; 33 of 35 case patients (94%) versus 4 of 36 controls (11%) reported consuming cookie dough during the 7 days before becoming sick, and one brand of cookie dough was eaten by 31 of the 33 case patients (94%).
The Food and Drug Administration inspected two facilities that made the implicated dough but they came up empty. Unopened packages of the brand of dough were clear of E. coli, and four opened packages linked to illnesses were free of the specific strain that had caused the outbreak. Despite the lack of a link to the outbreak and the lack of any safety violations, the other contaminations found in the product led to the recall.
Researchers concluded that possible means of contamination include a contaminated ingredient during processing, a lapse in plant biosecurity measures, intentional contamination, or cross contamination with another food processed in the plant.
Flour, which isn’t pasteurized or treated with a “kill step” like other ingredients, is the prime suspect in the outbreak, the authors concluded. Because flour is frequently bought in large quantities, a single purchase of contaminated flour might have been used to make multiple lots and varieties of dough over a long time. This was consistent with the use-by dates on packages obtained from patients, which suggested that product contamination occurred over several weeks.
If flour is the culprit, it got off scot-free. Still, the researchers noted, foods containing raw flour should be considered as possible vehicles of infection of future STEC outbreaks. After the recall, the product manufacturer switched to heat-treated flour and enhanced warning labels about eating raw dough.
All manufacturers should consider using heat-treated or pasteurized flour, researchers said. Manufacturers should consider making ready-to-bake prepackaged cookie dough as safe as a ready-to-eat food. They noted that several patients bought the raw dough with no intention of baking it, leading researchers to conclude that making the snacks safer may be the best possible outcome.
*This blog post was originally published at ACP Internist*