Risk factors for sporadic infection with Escherichia coli O157:H7

Little is known about risk factors for sporadic infection with Escherichia coli O157:H7. In response to a sharp increase in reported cases in New Jersey during July 1994, we conducted a case-control study to identify principal sources of infection and contributing practices.

Standardized questionnaires were used to evaluate (1) potential exposures of case patients and matched controls and (2) knowledge, attitudes, and practices of food preparers in case and control households. Patient isolates were subtyped by pulsed-field gel electrophoresis.

Patients with E coli O157:H7 infection (N = 23; median age, 9 years; 55% female) were more likely than healthy controls to have eaten a hamburger in the week preceding illness (matched odds ratio, undefined; P < .001); 80% of the hamburgers eaten by ill persons were prepared at home. Food preparers in case households were less likely than those in control households to report washing their hands (odds ratio, 8.5; P < .005) and work surfaces (odds ratio, 10.5; P < .05) after handling raw ground beef. Pulsed-field gel electrophoresis yielded 17 unique subtypes among the 23 patient isolates, indicating multiple sources of infection.

Hamburgers prepared at home are an important source of sporadic E coli O157:H7 infections. We estimate that adequate hand washing by food preparers could have prevented 34% of E coli O157:H7 infections in the study population.