On Friday, May 6, 2011 the Illinois Department of Public Health (IDPH) notified Kane County Health Department (KCHD) that the IDPH Public Health Laboratory (PHL) had recently isolated Salmonella serotype Typhimurium (S. Typhimurium) in six County residents. That same day KCHD received five more reports of S. Typhimurium from other laboratories via the Illinois National Electronic Database. Diagnosis of S. Typhimurium in 11 Kane County residents in a short period of time was unusual. IDPH epidemiologists advised KCHD investigators gather patient information about all foods consumed within 72 hours of symptom onset. The typical incubation period for infection with Salmonella ranges between 12 and 72 hours. Laboratory results of molecular subtyping by Pulsed Field Gel Electrophoresis (PFGE) were pending at the IDPH PHL. If genetic tests showed that patients were infected with an indistinguishable strain of S. Typhimurium, it was likely that patients had become infected through a common exposure.

KCHD staff reviewed answers to case investigation questionnaires administered earlier in the week to case patients with S. Typhimurium. Two patients reported eating at Portillo’s Restaurant located at 3895 East Main Street in St. Charles. An initial inspection at the restaurant was conducted that day (May 6) and no critical issues were found. There were no reports of ill employees.

KCHD investigators conducted interviews of newly reported case patients throughout the weekend. Seven patients reported eating at the Portillo’s Restaurant located in St. Charles within 72 hours of symptom onset. IDPH PHL reported that these 7 patients were among 10 patients who were infected with an indistinguishable strain of S. Typhimurium as determined by PFGE. The strain was identified as JPXX01.0006/JPXA26.0026. Investigators learned that a DeKalb County resident had been diagnosed with strain JPXX01.0006/JPXA26.0026. On April 19, he and about 19 other co-workers at Strata Tac had eaten food catered by the Portillio’s located in St. Charles. There was no longer any doubt that the restaurant was the source of a foodborne outbreak. IDPH assigned identification number 2011-098 to the outbreak.

Public health investigators conducted a case-control study among Portillo’s customers. Cases were Portillo’s customers who were laboratory confirmed with S. Typhimurium. Controls were non-ill meal companions. Investigators also conducted a cohort study of Strata Tac employees who had eaten the food catered by Portillo’s. Results of both studies revealed a total of 36 case patients linked to food prepared at the St. Charles location of Portillo’s. Nineteen Portillo’s customers were laboratory confirmed with the outbreak strain (JPXX01.0006/JPXA26.0026) of S. Typhimurium. The remaining 17 customers were classified as “probable” cases meaning they were not laboratory confirmed but had symptoms consistent with salmonellosis. Most patients lived in Kane County although patients lived in DuPage County, Dekalb County, Will County, Cook County, Kendall County and in Minnesota. Regardless of place of residence, they all consumed food prepared at the Portillo’s restaurant located in St. Charles.

A total of 81 employees were tested for Salmonella. Two employees tested positive for the outbreak strain of S. Typhimurium. Both experienced onset of symptoms after the outbreak was underway suggesting they were likely victims and not the source of the outbreak. Analysis of data collected as part of the case-control study implicated salad as the contaminated vehicle of transmission. None of the leftover food items tested positive for S. Typhimurium.