On September 15, 2008, Ingham County Health Department (ICHD) was notified that nine students of Michigan State University (MSU) were seen in the emergency department over the weekend with gastrointestinal symptoms of abdominal pain, diarrhea, and bloody diarrhea. Lab cultures had confirmed that at least two of them were positive for E. coli O157:H7. The ICHD then launched an investigation with help from the Michigan Department of Community Health (MDCH), and both the United States & Michigan Department of Agriculture (MDA).

Over the ensuing days it became clear that the outbreak was not limited to MSU. While at MSU, the reported number of E. coli O157:H7 cases had risen to 18 (3 confirmed, 15 probable), there were also a reported 12 cases at Lenawee County Jail (5 confirmed, 7 probable). In fact, by September 29, a total of 26 confirmed cases of E. coli O157:H7 with the same genetic fingerprint had been reported to MDCH, from eight Michigan counties. Additionally, nine individuals in Illinois and three from the Province of Ontario had also been identified with the same genetic strain of E. coli O157:H7.

By this point, there was also strong epidemiological evidence linking the outbreak to institutional size, bagged iceberg lettuce. Two separate case-control studies had been conducted by MDCH at MSU and the Illinois Department of Public Health, and both implicated iceberg lettuce as the source of contamination. As a result, the MDA coordinated a traceback investigation of iceberg lettuce and found that the common supplier of all iceberg lettuce to MSU, the Lenawee County Jail, a restaurant in Illinois, as well as other foodservice locations identified by ill individuals, was Fresh-Pak Inc., distributed under the name, “Aunt Mid’s.”

The MDA subsequently conducted product and environmental sample testing at Aunt Mid’s. Though the tests did not find E. coli, testing was on current products, not on products from the outbreak timeframe. Lettuce from the outbreak timeframe was not available for testing during the investigation due to the perishable nature of the product.

Meanwhile, the toll of people affected by the E. coli O157:H7 outbreak had increased. By October 3rd, Michigan had identified 34 cases in nine counties with the same PFGE pattern by two enzymes. This included: nine students from MSU (Ingham County), five inmates at the Lenawee County Jail, three students at the University of Michigan and one in Washtenaw County, five in Macomb County, five in Wayne County, three in Kent County, and one each in St. Clair, Oakland, and Genesee Counties. The onset dates of symptoms of these confirmed genetically linked E. coli O157:H7 patients ranged from September 8 to 19.

The epidemiological investigation by MDA, which had already identified Aunt Mid’s as the common supplier of iceberg lettuce, soon revealed the likely origin of the contamination Using illness dates, ship dates, and delivery dates, the MDA was able to narrow the origin to California. The California Department of Public Health then assisted the investigation by surveying 15 possible supplier farms. By October 10, Michigan and California had both traced the lettuce supplied to the initial cases to Santa Barbara Farms in Santa Barbara, California.