Romaine lettuce grown on an unnamed farm has been blamed for an outbreak of E. coli O157:H7 illnesses that sickened 60 people in 10 states between October 10th and December 4th. CDC believes that the outbreak is now over.

Missouri reported the largest number of illnesses (37), but outbreak cases also occurred in Arizona (1), Arkansas (2), Georgia (1), Illinois (9), Indiana (2), Kansas (3), Kentucky (1), Minnesota (3) and Nebraska (1). Approximately two-thirds of the outbreak victims were hospitalized, and two people developed hemolytic uremic syndrome. Most of the illnesses occurred during the second half of October 2011.

The outbreak puzzled Missouri public health authorities for weeks. Illnesses appeared to be linked epidemiologically to salad bars located in supermarkets – mainly in the St. Louis area. Schnucks, the supermarket chain that appeared to be linked to the outbreak, acted quickly to remove any potentially offending produce items from its salad bars.

The Missouri Department of Health and Senior Services analyzed 55 food samples in connection with the St. Louis area outbreak cases, but was unable to find a single E. coli-positive sample. Patient interviews and a comparison of eating patterns with those of healthy individuals in Missouri and other affected states pointed to romaine lettuce as a probable source of the illnesses.

Traceback studies determined that the romaine lettuce served at all of the Schnucks stores came from a single lettuce processing facility via a single distributor. Lettuce from the same farm also was supplied to university campuses in Minnesota and Missouri during the time of the outbreak illnesses.

An investigation was carried out at the implicated farm (identified by CDC simply as “Farm A”), but preliminary findings did not point to a source for the contamination. The farm was not in production during the investigation.

CDC considers this outbreak to be over, and has not issued outbreak-specific advice to consumers.