The CDC reports as of March 21, 2012, 58 persons infected with the outbreak strain of E. coli O157:H7 were reported from 9 states. The number of ill persons identified in each state was as follows: Arizona (1), Arkansas (2), Illinois (9), Indiana (2), Kansas (2), Kentucky (1), Minnesota (2), Missouri (38), and Nebraska (1). Two cases were removed from the case count because advanced molecular testing determined that they were not related to this outbreak strain. 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. Fifty-nine percent were female. Among the 49 ill persons with available information, 33 (67%) were hospitalized, and 3 developed hemolytic uremic syndrome (HUS). No deaths were reported.
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 grocery store Chain A 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 (86%) were also significantly more likely than well persons (55%) to report shopping at grocery store Chain A. Among ill and well persons who shopped at grocery store Chain A, ill persons (89%) were significantly more likely than well persons (9%) to report eating a salad from the salad bar at grocery store Chain A. Several different types of lettuce were offered on the salad bar at grocery store Chain A. 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.
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 grocery store Chain A and ill persons at university 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 the grocery store Chain A locations as well as the university campus in 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.