“DOLE Classic Romaine is triple washed and ready-to-eat. As a result, it is not necessary to wash the salad prior to eating.” From http://www.dole.com/Products/Products_Detail.jsp?CatGroupID=4&ID=79, Dole Food webpage 2005
On September 22, 2005 the Minnesota Department of Health (MDH) Public Health Laboratory (PHL) received an E. coli O157:H7 isolate for confirmatory testing and Pulse Field Gel Electrophoresis (PFGE) subtyping. PFGE results were reported on September 26 and uploaded to PulseNet, a national database of PFGE patterns or “fingerprints” maintained at the federal Centers for Disease Control and Prevention (CDC). The pattern derived from digestion with the restriction endonuclease Xba I was assigned Pattern number EXHX01.0238. The isolate was soon tested with a second enzyme, Bln I, and the resulting pattern was assigned pattern number EXHA26.1040. Prior to September 19, the Bln I pattern had not been posted on PulseNet.
Isolates obtained from culture of stool submitted by two new ill patients were received at the MDH PHL on September 23, 2005 and subtyped. PFGE results showed that the two new isolates and the isolate received on September 22 were indistinguishable by two enzymes. By September 29, 2005 isolates obtained from seven more patients arrived at the MDH PHL for further analysis. Public health investigators recognized that an E. coli O157:H7 outbreak was underway in Minnesota.
While laboratory testing was performed, MDH epidemiologists conducted preliminary interviews with patients who were laboratory confirmed with E. coli O157:H7. On the morning of September 28 investigators had identified pre-packaged lettuce produced by Dole Food Company, Inc. as the likely vehicle of transmission for infection with E. coli O157:H7. A supplemental questionnaire focusing on the type and brand of lettuce consumed and where it was purchased, was developed and administered to case-patients previously interviewed and newly identified cases. On September 29 Minnesota Department of Agriculture (MDA) staff collected a bag of Dole lettuce at the home of a case patient and began microbiologic testing for the presence of E. coli O157:H7.
On September 30 the MDH issued a press release advising the public that 11 cases of E. coli O157:H7 had been identified in Minnesota residents who had eaten Dole lettuce purchased from at least four different stores in the Twin Cities area. Persons with symptoms of E. coli were told to contact the MDH and their physician. Dr. Chris Braden at the Foodborne and Diarrheal Disease Branch at the CDC announced that no other states were reporting outbreak associated cases.
Meanwhile MDA microbiologists continued to process lettuce specimens obtained from households with cases of confirmed E. coli O157:H7. On Monday, October 3 the agency reported that sample number M-05-2310, Lot Number B250215B received on September 30 had tested positive for E. coli O157:H7. The isolate obtained from the sample was sent to the MDH for PFGE analysis. The resulting pattern was indistinguishable to the pattern identified in case-patients. A second specimen, M-05-2318, lot number unavailable, would also yield positive results.
News of the positive lettuce specimen prompted the Food and Drug Administration (FDA) to issue a nationwide health alert regarding Dole pre-packaged salads on October 2. The FDA announcement reiterated warnings expressed in the MDH press release and further described the Dole products associated with illness, Classic Romaine, American Blend, and Greener Selection. Although cases had only been identified in Minnesota, the product was noted to have been distributed nationwide.
It would not be long before cases of E. coli O157:H7 in Wisconsin and Oregon would be recognized. The Wisconsin case was a 12 year old female with E. coli O157:H7 who had a history of eating Dole pre-packaged lettuce. PFGE subtyping showed that her isolate was indistinguishable to the EXHX01.0238 pattern and one band different on the second enzyme pattern. Despite the one band difference, MDH molecular epidemiologists considered the girl to be part of the outbreak concluding that the difference was not enough to preclude the case from being considered outbreak related.
The Oregon case was indisputably associated with consumption of Dole pre-packaged salad mix. A 60 year old Portland resident was hospitalized and laboratory confirmed with E. coli O157:H7 on September 21, 2005. The patient had experienced onset of symptoms on September 18, four days after purchasing and consuming Dole brand “Classic Romaine” salad mix. Michael Roberson, representative for Albertsons’, the grocery store of purchase, confirmed that the chain’s Portland area distributing center had received Dole Greener Selection and Dole Classic Romaine. A portion of the salad mix was still in the patient’s refrigerator. A photograph taken of the packaging documents that Ms. Scheetz purchased Dole salad mix with a “Best if Used By” date of 9/23/05, lot number was B250215B. PFGE subtyping showed that the Oregon isolate was indistinguishable by two enzymes to other ill Dole lettuce consumers in Minnesota.
Aware of the potential severity of an E. coli O157:H7 outbreak, the FDA and the Food and Drug Branch at the California Department of Health Services initiated an investigation at the Dole processing plant. Preliminary information indicated that 22,321 cases of Dole pre-packaged lettuce with a “Best If Used By” date of 9/23/05 and a production code starting with “B250” were shipped from a single Dole processing facility in central California to 34 states in early September. Investigators estimated that since each case contained between 6 and 12 bags, approximately 244,866 bags of lettuce had made it to market.
On October 11, 2005 the MDH counted 23 laboratory confirmed cases of E. coli O157:H7 and seven epidemiologically linked cases. Illness onset dates ranged from September 16 to September 30. Two cases had developed Hemolytic Uremic Syndrome (HUS). Oregon and Wisconsin reported one case each. Case control study data show a statistically significant association between illness and consuming Dole pre-packaged lettuce with a matched odds ratio of 6.8, 95% confidence interval, 1.4-31.9, and a p-value of 0.01. The California Department of Health Services continues to conduct a trace back investigation to farms implicated in earlier lettuce outbreaks. A final outbreak report and traceback summary has not been provided. Eventually, a total of 32 persons from three states would be linked to the E. coli O157:H7 outbreak.