On July 17, 2002, Spokane Regional Health District (SRHD) contacted the Washington State Department of Health (WDOH) to report a cluster of diarrheal illnesses among a group of teenaged girls who had recently attended a drill team dance camp at Eastern Washington University (EWU). Laboratory tests conducted the WDOH Public Health Laboratory would later confirm the illnesses to be E. coli O157:H7 with an indistinguishable PFGE pattern. Subsequently, SRHD became aware of additional cases of E. coli O157:H7 with the same PFGE pattern that had no association with dance camp and EWU. This led to a broader investigation by a number of public health agencies.
The epidemiological investigation revealed two groups or “clusters” of persons with E. coli O157:H7. Infected persons in both groups shared an indistinguishable PFGE pattern. The first group consisted of 55 cases of E. coli O157:H7, 34 of which were culture confirmed, i.e. they produced stool samples that grew out the E. coli O157:H7 bacteria in a laboratory culture. These 55 persons were among those who worked at or attended the dance camp at EWU. A second group who attended a church camp consisted of 14 persons, including one culture confirmed case of E. coli O157:H7. It was quickly determined that illnesses at the church camp were secondary to the dance camp outbreak. Several of the young women who had attended the dance camp went on to attend the church camp immediately thereafter.
In addition there were several other cases of culture confirmed E. coli O157:H7 with a PFGE pattern indistinguishable to the dance team outbreak pattern. None of these cases had any connection whatsoever to either the dance camp or church camp. Collectively, these cases were classified by investigators to be “outliers” because they were not part of the core group of associated outbreak cases.
With respect to the first group at the drill team camp at EWU, the WDOH and SRHD epidemiological investigation identified the probable source of the E. coli infections to be romaine lettuce. This conclusion was based on statistical results of a cohort study of workers and attendees at the camp regarding their food consumption, activities, and possible environmental exposures. The statistical association between the illnesses caused by E. coli O157:H7 and the consumption of romaine lettuce was established to a 95% confidence interval. The trace back investigation conducted by WDOH revealed that the romaine lettuce served at the camp in the relevant time frame was shredded, bagged, and sold by defendant Spokane Produce.
As described previously, the second group of illnesses was determined to be secondary infections that resulted from exposure to one or more persons who had developed E. coli O157:H7 infections after consuming romaine lettuce supplied by Spokane Produce while attending the dance camp at the EWU campus. The later dates of onset of these infections reflected the additional time necessary to complete the second exposure and incubation periods .
In addition to the two main clusters, health investigators were able to link all of the individuals considered to be “outliers” to consumption of Spokane Produce romaine lettuce. All of the outliers considered to be primary cases consumed romaine lettuce during the five days prior to the onset of illness. Two of the outlier cases were persons from Walla Walla County. These two unrelated persons shared no connection other than the fact that they both consumed salad containing Spokane Produce romaine lettuce at Paisano’s Restaurant in Walla Walla during the same week in mid-July 2002. Trace back investigations indicated that the restaurant had received the same Spokane Produce bagged lettuce that had been delivered to the dance camp at EWU.
An additional outlier in Spokane County ate a garden salad made with romaine lettuce at O’Dougherty’s, a Spokane restaurant, during the same week as the campers at EWU. O’Dougherty’s also received the same Spokane Produce bagged romaine product delivered to the camp at EWU, identified as item number 197386. Two other cases from Spokane County also ate salad in the proper time frame, although the source of the lettuce in their salads could not be determined after the fact.
Lastly, three members of the same family from Coeur d’Alene, Idaho, developed E. coli O157:H7 infections with a PFGE pattern indistinguishable to ill attendees at the EWU dance camp. The two year-old in the family, consumed lettuce at Borah Elementary School, which received Spokane Produce romaine lettuce in the relevant time frame. Four other members became subsequently became ill with laboratory confirmed E. coli O157:H7, following onset of the initial infection. In addition, prior to their illnesses, family members had consumed salad mix purchased by Albertsons grocery store in Coeur d’Alene which had likewise received the Spokane Produce product.
At the conclusion of the investigation, WDOH concluded that the source of the outbreak was romaine lettuce prepared and sold by Spokane Produce:
During the same time period, illnesses caused by E. coli O157:H7 with an indistinguishable PFGE pattern occurred among unrelated individuals in the several states. The only link between these additional cases was the consumption of romaine lettuce during the five days before their illness onset.
This romaine-associated outbreak of E. coli O157:H7 included cases from a cafeteria dinner on a Spokane campus, lunch the following day from the same cafeteria, a restaurant salad in Spokane County, romaine purchased at several Spokane area grocery stores, two restaurant salads in Walla Walla County, and romaine served at a restaurant in a Midwestern State. Cases were clustered in time, and no outbreak associated isolates were subsequently identified through PulseNet.
Thus, WDOH concluded that the outbreak had a common source: romaine lettuce prepared and sold by Spokane Produce. This conclusion was reached not only for attendees of the EWU camp, but for those who consumed the lettuce in other settings outside the camp who were determined to have been infected with the same strain of E. coli O157:H7 bacteria as dance team members.