In early June 2009, public health investigators noticed an increase in isolates submitted by multiple state public health laboratories to PulseNet with a two-enzyme pattern combination of EXHX01.0224 and EXHA26.0536. This is a relatively common pattern combination, appearing at least 364 times in the PulseNet database since 2005. Newly submitted isolates were tested by MLVA to further characterize genetic differences among isolates with PFGE pattern EXHX01.0224 and EXHA26.0536. MLVA pattern “A” was common to most of the newly submitted isolates.

Once investigators recognized the cluster, they administered a hypothesis-generating questionnaire to case patients to identify common exposures to the bacteria. Twenty-seven of 30 case-patients reported eating raw cookie dough before symptom onset.

The CDC developed protocol for a case-control study. Cases were defined as patients with E. coli O157:H7 with symptom onset on or after March 1, 2009, sharing an indistinguishable PFGE pattern combination of EXHX01.0224 /EXHA26.0536 and having an indistinguishable MLVA pattern A (or having a MLVA pattern differing at a single locus by one repeat).

One control (not ill) per case-patient was selected. Cases and controls were matched by sex, age group, and state of residence. Study participants were asked about a variety of foods consumed in the week before onset of illness of the matched case patients. As shown in the table below, a variety of food items were considered as the possible vehicle of E. coli O157:H7.


(Note: these data are preliminary and may change slightly)

The only food item statistically associated with illness was raw cookie dough (Odds Ratio 55.6, 95% confidence limits 14.14-282.07). The association was highly significant (p = <0.0001), implying that there is a less than a 1 in 10,000 chance that the finding of an association between eating raw cookie and becoming ill with E. coli O157:H7 occurred by chance.

Strong epidemiologic data showing an association between consuming raw cookie dough and illness and molecular microbiologic data showing a 2-enzyme PFGE match and MLVA match, prompted Nestle USA to issue a recall on June 19, 2009 of Nestle Toll House refrigerated cookie dough products.