On March 23, 2012 the Centers for Disease Control and Prevention (CDC) issued its final update of the multistate outbreak of E. coli O157:H7 infections linked to romaine lettuce.[1] The outbreak investigation was assigned outbreak code 1110MOEXH-2. Genetically, the outbreak strain was identified by Pulsenet pattern designations EXHX01.0047/EXHA26.0015. At the completion of the investigation on March 21, 2012, 58 persons residing in 9 states were infected with the outbreak strain of E. coli O157:H7. The number of ill persons identified in each state is as follows: Arizona (1), Arkansas (2), Illinois (9), Indiana (2), Kansas (3), Kentucky (1), Minnesota (3), Missouri (38), and Nebraska (1).
Among persons for whom information was available, illnesses began from October 10, 2011 to November 4, 2011. Ill persons ranged in age from one to 94 years, with a median age of 29 years old. Sixty-three percent (63%) were female. Among persons for whom information was available, illnesses began from October 9, 2011 to November 7, 2011. Ill persons ranged in age from 1 to 94 years, with a median age of 28 years. Among the 49 ill persons with available information, 33 (67%) were hospitalized, and 3 developed hemolytic uremic syndrome (HUS). No deaths were reported.
Collaborative investigative efforts of state, local, and federal public health agencies indicate that romaine lettuce sold primarily at several locations of a single grocery store chain, Schnucks, was the likely source of illnesses in this outbreak. Contamination likely occurred before the product reached Schnucks stores.
During October 10 to November 4, 2011, public health officials in several states and CDC conducted an epidemiologic study by comparing foods eaten by 22 ill and 82 well persons, including 45 well persons who shopped at a Schnucks grocery store during the week of October 17, 2011. Analysis of this study indicated that eating romaine lettuce was associated with illness. Ill persons (85%) were significantly more likely than well persons (46%) to report eating romaine lettuce in the week before illness. Ill persons (85%) were also significantly more likely than well persons (46%) to report shopping at a Schnucks store. Among ill and well persons who shopped at Schnucks, ill persons (89%) were significantly more likely than well persons (9%) to report eating a salad from the salad bar at Schnucks. Several different types of lettuce were offered on the Schnucks salad bars. Of 18 ill persons who reported the type of lettuce eaten, 94% reported eating romaine lettuce. No other type of lettuce or other item offered on the salad bar was reported to be eaten by more than 55% of ill persons.
Ill persons reported purchasing salads from salad bars at Schnucks between October 5 and October 24, 2011. A total of nine (9) store locations were identified where more than one (1) ill person reported purchasing a salad from the salad bar in the week before becoming ill. This included two (2) separate locations where four (4) ill persons reported purchasing a salad at each location. For locations where more than one (1) ill person reported purchasing a salad from the salad bar and the date of purchase was known, dates of purchase were all within four (4) days of other ill persons purchasing a salad at that same location. Schnucks voluntarily removed suspected food items from the salad bar on October 26, 2011. Romaine lettuce served on salad bars at all locations of Schnucks had come from a single lettuce processing facility owned and operated by Vaughan Foods, Inc., located in Moore, Oklahoma. Vaughan Foods was also the sole distributor of processed romaine lettuce to Schnucks stores. This implies that contamination of romaine lettuce likely occurred before the product reached Schnucks locations.
The FDA and several state agencies conducted traceback investigations for romaine lettuce to try to identify the source of contamination. Traceback investigations focused on ill persons who had eaten at salad bars at several locations of Schnucks and on ill persons at two college campuses, in Minnesota (1 ill person) and Missouri (2 ill persons). Traceback analysis determined that a single common lot of romaine lettuce harvested from Farm A was used to supply Schnucks locations as well as the Centennial Dining Hall at the University of Minnesota during the time of the illnesses. This lot was also provided to a distributor that supplied lettuce to the university campus in Missouri, but records were not sufficient to determine if this lot was sent to this university campus. Preliminary findings of investigation at Farm A did not identify the source of the contamination. Farm A was no longer in production during the time of the investigation.