Spinach-associated Escherichia coli O157:H7 Outbreak, Utah and New Mexico, 2006

In 2006, Utah and New Mexico health departments investigated a multistate cluster of Escherichia coli O157:H7. A case–control study of 22 case-patients found that consuming bagged spinach was significantly associated with illness (p<0.01). The outbreak strain was isolated from 3 bags of 1 brand of spinach. Nationally, 205 persons were ill with the outbreak strain.


The Study
The UDOH Enteric Disease Case Report Form and a supplemental CDC questionnaire on spinach consumption were administered by local or state public health offi cials to all participants by telephone. Information collected included date of disease onset, symptoms, treatment, communitybased exposures, and a food-item history. Questions referred to the 8-10 days before case-patient symptom onset. Casepatients were fi rst interviewed 3-23 days after illness onset (mean = 11.6 days); follow-up interviews for the questionnaire were completed within 23 days of illness onset.
Two controls per case-patient were matched by sex and age group to prevent confounding from potential differences in diet (3); age groups were <4 years, 5-12 years, 13-18 years, 19-64 years, and >65 years. Controls were selected by using sequential-digit telephone dialing based on the matched case-patient's telephone number. Controls reported no gastrointestinal illness 3 days before and after symptom onset date of their matched case-patient.
Exact matched odds ratios (mOR) and confi dence intervals (CI) were calculated by using conditional logistic regression in SAS 9.1 (SAS Institute, Cary, NC, USA). An α of 0.05 was used. No statistical analyses were performed for the categories "spinach brand" or "location spinach was eaten" because of insuffi cient data and inability to generate point estimates; we provide only descriptive evaluations of these variables. Only persons who indicated defi nite exposure to a single brand of spinach were included in the evaluation of brand.
The Utah Public Health Laboratories (UPHL) and New Mexico Scientifi c Laboratory Division (NMSLD) provided analytic testing services for all clinical and spinach samples. Public health offi cials collected spinach samples in their original packaging from confi rmed casepatients. One spinach sample in New Mexico was frozen; all other samples were refrigerated. Modifi ed FDA Bacterial and Analytical Manual methods were used to recover E. coli O157:H7 from both clinical and spinach samples (4  UK) and DIFCO H7 antiserum (DIFCO Laboratories, Detroit, MI, USA). All clinical isolates were tested for Shiga toxin genes (stx1 and stx2) by PCR. Clinical and spinach isolates were tested for Shiga toxin expression with the Premier EHEC enzymatic immunoassay Shiga-toxin test kit (Meridian Diagnostics, Inc., Cincinnati, OH, USA). PFGE was performed on E. coli O157:H7 cultures from clinical and spinach samples by using standard CDC PulseNet operating procedures (5). PCR testing reagents and protocols were provided through the Laboratory Response Network (LRN) and FERN.
Eighteen cases were confi rmed in Utah and 5 in New Mexico. Onset dates ranged from August 22 to September 11 (Figure). Shiga toxin 2 was detected in stool samples from all patients. Demographic, clinical, and food exposure information was available for all patients ( No patients, versus 6 controls, reported only eating spinach at a restaurant; 16 patients and 10 controls reported only eating spinach in a private home ( Table 2). Washing spinach before eating did not signifi cantly change the odds of being a case-patient (Table 2). No study participants reported eating only cooked spinach; therefore, cooking was not analyzed. Ten case-patients and 3 controls reported definite exposure to single brands of spinach; only brands A and B were reportedly consumed by these patients. Seven case-patients and 1 control reported defi nite exposure to brand A; 0 case-patients and 2 controls reported defi nite exposure to brand B (Table 2).
Seven bags of spinach (5 from Utah, 2 from New Mexico) were provided by 7 case-patients. Five bags had been opened and their contents partially consumed. E. coli O157:H7, matching the national outbreak strain, was detected by PCR and culture in 2 Utah spinach bags and the New Mexico unfrozen bag. All 3 were open bags of brand A baby spinach. Lot codes were available on 2 bags; both were packed on August 15, 2006, at the same plant, on the same shift, but on different machines. The 4 bags from which E. coli O157:H7 was not detected were also brand A baby spinach and were packed on August 15, 22, 23, and 28, 2006. Two bags that tested negative were eaten by case-patients who reported eating from multiple bags before illness onset.

Conclusions
Consumption of fresh brand A spinach was associated with E. coli O157:H7 infection with both epidemiologic and laboratory data. Washing spinach before consumption did not affect odds of being a case-patient. Possible reasons for this include 1) E. coli could be internalized into the plant structure by entering through the roots (6), and 2) bacteria are more likely to adhere to cut surfaces of leafy greens (e.g., prepackaged spinach) (7). That no case-patients reported only eating spinach in a restaurant suggests that the source of the contaminated spinach did not supply commercial establishments.
The percentage of case-patients in whom HUS developed (29%) was high when compared with that in prior E. coli O157:H7 outbreaks (15%-20%) (8,9). This fi nding is consistent with studies that associate Shiga toxin 2-expressing E. coli with a higher incidence of HUS (10,11).
Our study was subject to certain limitations. One is potential recall bias since controls had a longer time lag between consumption and interview and less motivation  to accurately recall what foods they had eaten. However, overestimation of the association between spinach consumption and illness is unlikely because more controls reported having eaten spinach than were previously identifi ed in surveys of the general population (12). Analysis of brand was limited because of poor recall among all participants. The sample size was small, resulting in imprecise effect estimates and, in certain cases, an inability to calculate a measure of association. Exact ORs were used to partially counteract this limitation. This investigation was conducted in response to a national outbreak of E. coli O157:H7 infections with matching PFGE patterns among 205 persons in 26 states (13). Less than 2 weeks after its initiation, this investigation provided laboratory and epidemiologic evidence implicating spinach. The FDA used these data to focus its fi eld investigation and interventions and linked the contaminated spinach to samples taken from a stream, cattle manure, and feces from wild pigs on ranches in Salinas Valley, California (13). In August 2006, FDA launched a lettuce safety initiative to address recurring outbreaks of E. coli O157 infections (14). After this outbreak, the initiative was expanded to include all leafy greens.