Public health investigators in one California county, in October, 2008, investigated the illnesses of two girls infected with the same strain of E. coli O157:H7 infections. The girls had eaten at the same chain restaurant; the name of the restaurant was never disclosed. In a neighboring California county, another case was identified. This case had eaten at another outlet of the same restaurant chain, which was located in a neighboring California county. Genetic testing by pulsed field gel electrophoresis (PFGE) showed that the two girls and the man were sickened with an indistinguishable strain of E. coli O157:H7, designated by PFGE pattern numbers EXHX01.4626/EXHA26.2558. The strain was so unusual that it triggered a cluster investigation. Federal officials at the Centers for Disease Control and Prevention (CDC) assigned Cluster Identification Number 08100NEXH-1mlc to the investigation.

Through OutbreakNet, a national outbreak response unit staffed at the CDC, a fourth case-patient, a woman, in the cluster was identified - a resident of South Dakota. This person had eaten at the same California restaurant chain outlet as the two girls while on vacation in California. The two girls and the South Dakota woman became ill within five days of eating at the restaurant.

Within a matter of days the outbreak grew beyond the confines of the California counties and South Dakota. Public health laboratories in other states (Illinois, Florida, New Jersey, Ohio) identified other matching E. coli O157:H7 isolates. The subsequent investigation revealed that these case-patients had eaten at restaurants, however these restaurants were also never disclosed. None of these cases had eaten at the California restaurants implicated by the initial cases. Investigators suspected that a contaminated ingredient was in the marketplace.

In October, 2008, Canadian investigators identified three E. coli O157:H7 outbreaks of the same E. coli O157:H7 strain in Ontario. The majority of cases were linked to three restaurants. (For more information about these outbreaks, search in this outbreak database for Little Red Rooster Restaurant, M.T. Bellies Restaurant, and Johnathan's Family Restaurant.) Canadian investigators conducted a case-control study, and lettuce was statistically associated with illness. Product traceback showed that there had been a common produce supplier; an undisclosed brand of lettuce common to all of the implicated restaurants. The same lettuce had been supplied to the undisclosed restaurants in the California counties. The National Outbreak Reporting System confirms that there was a multistate outbreak of E. coli O157:H7 involving iceberg lettuce beginning September, 2008.