Background: Little is known about risk factors for sporadic infection
with Escherichia coli O157:H7. In response to a sharp increase in repo
rted cases in New Jersey during July 1994, we conducted a case-control
study to identify principal sources of infection and contributing pra
ctices. Methods: Standardized questionnaires were used to evaluate (1)
potential exposures of case patients and matched controls and (2) kno
wledge, attitudes, and practices of food preparers in case and control
households. Patient isolates were subtyped by pulsed-field gel electr
ophoresis. Results: Patients with E coli O157:H7 infection (N=23; medi
an age, 9 years; 55% female) ss ere 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 lik
ely than those in control households to report washing their hands (od
ds ratio, 8.5; P<.005) and work surfaces (odds ratio, 10.5; P<.05) aft
er handling raw ground beef. Pulsed-field gel electrophoresis yielded
17 unique subtypes among the 23 patient isolates, indicating multiple
sources of infection. Conclusions: Hamburgers prepared at home are an
important source of sporadic E coli O157:H7 infection. We estimate tha
t adequate hand washing by food preparers could have prevented 34% of
E coli O157:H7 infections in the study population.