The two-month investigation into the St. Louis E. coli O157:H7 outbreak is over. Although there’s not enough proof to nail down the definitive source, investigators say the evidence points to one lot of romaine lettuce from a single California farm.

First announcement that the investigation was over came Tuesday when Missouri Health Director Margaret Donnelly broke the news in remarks to her state’s House Appropriations Committee. She explained that it can be difficult to pinpoint the cause of foodborne illness, especially when the apparent source is perishable fresh produce:  

“The food that caused the outbreak is identified in less than 50 percent of the foodborne outbreaks, and the reason for that is because of the amount of time that passes from when the person is exposed to the pathogen until the time public health receives a report,” she told state lawmakers. “This incubation period can be up to 10 days. In addition, after that period of time, food products are often no longer available for analysis.”

The Centers for Disease Control and Prevention (CDC) followed up on Wednesday with its first and final report on the St. Louis E. coli outbreak. CDC said the outbreak was linked to romaine lettuce and involved 60 confirmed cases in 10 states.

Previously, the St. Louis outbreak was thought to include only 37 cases in Missouri and perhaps a handful of scattered cases in surrounding states.

Testing of 55 food samples by Missouri’s state lab failed to turn up any of the outbreak E. coli bacteria.

And Missouri’s health director Donnelly said that while romaine was the apparent cause, records kept by the unidentified California grower who may have supplied the lettuce were “insufficient to complete the picture.”

CDC, however, said romaine lettuce is the “likely source of the illnesses in this outbreak, and contamination likely occurred before the product reached retail stores.”

Lettuce has been a frequent cause of foodborne illness outbreaks. Since 2000 there have been more than 80 foodborne illness outbreaks linked to lettuce and, since 1993 lettuce has been responsible for more than two dozen E. coli outbreaks. 

In the St. Louis outbreak, investigators were able to create DNA “fingerprints” of the E. coli O157:H7 bacteria involved in the outbreak through diagnostic testing with pulse-field gel electrophoresis (PFGE)  to identify all the victims involved. 

As of Dec. 4, 60 persons had been infected with bacteria with that genetic fingerprint in 10 states including 37 in Missouri, nine in Illinois, three in Kansas and in Minnesota, two in Indiana and one in Arizona, Arkansas, Georgia, Kentucky and Nebraska.

The CDC report says the infections occurred between Oct. 10 and Nov. 4, 2011.  The ill people ranged in age from 1 to 94 years old. The median age was 29 and 63 percent were female.  

At least 30 of those infected required hospitalization. Two developed the kidney-damaging hemolytic uremic syndrome (HUS). No deaths were reported.

CDC says it might not be including illnesses that have occurred since Nov. 22 because of the time lag in reporting.

It was widely reported that the popular St. Louis grocery store chain Schnucks was at the center of the investigation, and items from its salad bars were collected and tested. However, the CDC report refers to the retailer only as “Chain A.”

“During October 10 to November 4, 2011, public health officials in several states and CDC conducted an epidemiologic study by comparing foods eaten by 22 ill and 82 well persons, including 45 well persons who shopped at grocery store Chain A during the week of October 17, 2011,” says the report.

“Ill persons (85 percent) were significantly more likely than well persons (46 percent) to report eating romaine lettuce in the week before illness. Ill persons (86 percent) were also significantly more likely than well persons (55 percent) to report shopping at grocery store Chain A.”

The report continued: “Among ill and well persons who shopped at grocery store Chain A, ill persons (89 percent) were significantly more likely than well persons (9 percent) to report eating a salad from the salad bar at grocery store Chain A.”

Although several different types of lettuce were offered on the salad bar, “of 18 ill persons who reported the type of lettuce eaten, 94 percent reported eating romaine lettuce. No other type of lettuce or other item offered on the salad bar was reported to be eaten by more than 55 percent of ill persons.”

The grocery chain fully cooperated with the investigation, the CDC noted, and voluntarily removed suspected food items from all its salad bars on Oct, 26, 2011.

Romaine lettuce served at the grocery chain’s salad bars had come from a single lettuce processing facility via a single distributor, who the CDC did not identify.  Investigators then tried to determine if the same lettuce had gone to university campuses in Minnesota and Missouri, where there were outbreak patients.

While they found that the lettuce went to the Minnesota campus, records were inadequate to show whether romaine from the same lot was sent to the Missouri campus.

According to Donnelly, the investigation fell apart because of poor record keeping by the California grower. And by the time investigators could conduct a preliminary inspection at the California farm, the romaine lettuce harvest was over and production there had ceased.

“The practice of holding back names of distributors and producers is not altogether uncommon, but nonetheless troubling,” said food safety attorney Bill Marler, the publisher of Food Safety News. 

Marler called on Schnucks to do two things — “release the names of both the farm and distributor and make sure the medical bills of their customers are covered.”  

CDC Outbreak Map: