Outbreak of Salmonella Paratyphi B Variant L Tartrate + among elementary school students: Discordant results for interviews vs e-ticket

Background: During September 2010, an outbreak of Salmonella was identified in an Oklahoma school system. An outbreak investigation and case-control study were initiated to determine the source.

Methods: The larger student body was questioned regarding consumption of cafeteria meals. Cases and classmate controls were interviewed in-depth regarding illness, food history, and social activities from August 31 to September 13. Confirmed cases had laboratory confirmed Salmonella Paratyphi B Variant L Tartrate + with Xbal pattern JG6X01.0007. Probable cases had three or more days of diarrhea occurring between August 31 and September 13. In-person and telephone interviews were conducted to determine food exposures. Absentee and electronic meal records were obtained to verify questionnaire responses. Individuals were re-interviewed if discrepancies were identified.

Results: Of 513 responses, there was ten percent (n=52) discordance between e-ticket meals and attendance records. Fourteen cases and 54 controls were interviewed. E-ticket records revealed 32 (47%) of 68 provided at least one discordant response regarding meals purchased from school; discordance was differential between cases and controls. All cases consumed school lunch on September 2, compared with 64% of controls, however, no specific food item was significantly associated with illness.

Conclusions: E-ticket and absentee records are useful to verify interview responses. If available, these resources should be obtained early in school-based outbreak investigations and used to identify incorrect responses. These records may be helpful in limiting recall bias during interviews, especially if an extended period of time exists between the exposure period and interview date.