The Missouri Department of Health and Senior Services (DHSS) has been leading a team of local, federal, and state public health experts in investigating the cause of the E. coli outbreak that has hit the St. Louis region. To date, the Missouri State Public Health Laboratory is testing or has tested 41 samples from individuals suspected to have the illness. So far, 28 of those samples have tested positive for E. coli O157:H7. Because some of these samples are duplicate samples from individuals, DHSS can report that 26 individuals have laboratory-confirmed E. coli illness. The State Lab has also tested 17 food samples taken from individuals’ homes and local Schnuck’s salad bars. That testing indicated that none of the samples contained E. coli or shiga toxin, its harmful byproduct. Testing of additional human and food samples continues, along with the other components that are involved in investigating an outbreak of this type.
Upon confirmation of the suspected outbreak on October 26, DHSS dispatched a team of medical and scientific experts to begin the investigation. This team includes epidemiologists, disease investigators, and food safety inspectors. At the same time, DHSS requested assistance from the Centers for Disease Control and Prevention (CDC). CDC dispatched a team of scientists who arrived in St. Louis on Friday, October 29. All of these resources joined colleagues at the local level who had already begun the investigation.
Conducting an investigation such as this requires a multi-pronged approach, including patient interviews, facility inspections, and laboratory testing. At the same time, investigators provide information to hospitals and health care professionals in the affected area to assist in the treatment of patients suspected to have E. coli illness.
Investigators interview patients to create a “dietary history,” documenting what the patient has consumed in the days before the onset of illness. Investigators also attempt to identify individuals who have consumed the same product or products, such as a family member, but who have not taken ill. From these interviews, investigators try to identify a common thread among patients’ food consumption. If possible, investigators obtain food samples from patients for testing. But the reliability of food samples from homes decreases with the passage of time, as the food ages and opportunities for cross-contamination increase. To date, investigators have discovered that 85% of patients (17 of 20) who reported shopping at Schnuck’s stores also reported obtaining food from the salad bar. While this suggests a source, investigators cannot at this point say with certainty that they have identified the source, as they continue to search for an explanation for the remaining cases. As has been reported, all suspected items were voluntarily removed as a precautionary measure on October 26 from Schnuck’s salad bars in the region and replaced with items from different sources.
Food safety inspectors conduct inspections of stores/facilities that may be possible sources of contamination. To date, these inspectors have been to several Schnuck’s stores and warehouses, and have not found conditions in those locations that would allow E. coli to contaminate food held there for sale or distribution. The investigators have therefore enlisted the assistance of the Federal Food and Drug Administration as they move further up the distribution chain, which could include distributors and/or producers located outside of Missouri.
Laboratory testing is required to confirm the existence of E. coli, and to identify the specific strain and “genetic fingerprint” involved, so as to establish whether the suspected cases are part of the same outbreak. This testing, which includes the culturing of bacteria, requires several days. Thus far, 28 of 41 human stool samples have tested positive for a strain of E. coli known as E. coli O157:H7. In addition 26 of the samples have completed the process down to “fingerprint” identification. All 26 match, suggesting that all are part of this outbreak and that only one specific organism is involved. The completion of testing on remaining samples continues, with results expected this week.
Laboratory testing also determines whether E. coli is present in any food samples submitted. As with human samples, this testing takes several days. Thus far, 17 samples have been submitted, and all have come back negative for E. coli. These samples included two strawberries retrieved from sick individuals’ homes. The remaining samples consist of lettuce, strawberries, and Caesar dressing taken from several Schnuck’s stores in the region. Testing of additional food samples continues.