October 1996
NEW ORLEANS — Investigators have implicated lettuce as a vehicle for Escherichia coli 0157:H7 infection in two outbreaks, according to recent reports.
One outbreak occurred in Maine and affected people who had attended a Boy Scout retreat. The other outbreak occurred in Montana. Investigators presented details here at the 36th Interscience Conference on Antimicrobial Agents and Chemotherapy.
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The outbreaks were unusual be cause most E. coli 157:H7 outbreaks result from people eating undercooked beef, especially ground beef, tainted with the pathogen.
In September 1995, a physician in Maine reported E. coli 0157:H7 infections in three patients who had participated in the retreat. Investigators interviewed retreat attendees using a standard questionnaire.
They defined a case as three or more loose stools in 24 hours or gastrointestinal illness and evidence of E. coli 157:H7 infection by culture or serology.
Investigators contacted 103 (94%) of the 109 people who attended the retreat. Of these, 30 (29%) met the case definition. Patients had a median age of 15 years and ranged in age from 12 to 79 years. Ninety-three percent were male. Of the 30 patients, 93% had stomach cramps, 90% had diarrhea, and 30% had bloody stool. Two patients were hospitalized, but hemolytic uremic syndrome (HUS), a potential serious complication of E. coli 157:H7 infection, did not develop in any patient.
Thirty people (38%) who ate either a sandwich containing iceberg lettuce at lunch or a salad at dinner became ill, compared with none of 14 people who did not eat either item. The same lettuce was served in salad and sandwiches.
Lettuce was stored in the same refrigerator as ground and stew beef, but investigators could not identify any incident of contamination during storage or food preparation. Investigators reported that they traced no other E. coli 0157:H7 infections to lettuce sold by a distributor to five schools and a hospital. Farm inspections revealed no obvious source of contamination.
Investigators concluded that the lettuce mostly became contaminated by beef at the retreat. They noted that the lack of obvious errors in handling the food "provides support for improving the microbiological quality of meat."
In July 1995, an outbreak of E. coli 0157:H7 infections in western Montana prompted a case-control study there to identify risk factors. Investigators identified a case as diarrhea or abdominal cramps in July in a western Montana resident, with either a stool culture yielding the pathogen or positive serologic result for the pathogen.
Investigators identified 40 patients, who ranged in age from three to 86 years. Thirteen patients were hospitalized, and HUS developed in one. Among 30 case patients and 60 controls matched for age and telephone exchange, consumption of leaf lettuce was the only exposure independently associated with illness. Case patients were 30.5 times more likely to have eaten lettuce than controls.
Lettuce eaten by patients came from at least three stores and either of two distributors, according to investigators. At one store, lettuce from difference sources was soaked together, suggesting that the pathogen on lettuce from a single source may have cross-contaminated lettuce from other sources.
This E. coli 0157:H7 outbreak associated with leaf lettuce highlights the importance of produce as a vehicle in foodborne illness, investigators stated. Sanitary growing and handling procedures are necessary to prevent such infections, they stated.
For more information:
- Presented at the 36th Interscience Conference on Antimicrobial Agents and Chemotherapy, New Orleans:
- Ackers M, Mahon B, Leahy E, et al. An outbreak of Escherichia coli 0157:H7 infections associated with leaf lettuce consumption, Western Montana. Abstract K43.
- Mermin J, Mead P, Gensheimer K, Griffin. Outbreak of E. coli 0157:H7 infections among Boy Scouts in Maine. Abstract K44.
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