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Outbreak of Escherichia coli O157:H7 Infection -- Georgia and Tennessee, June 1995

On June 26, 1995, the Division of Public Health, Georgia Department of Human Resources (GDPH), was notified of three cases of Escherichia coli O157:H7 infection among residents of a community in north Georgia who had onsets of illness within a 24-hour period (onset during June 19-20); in comparison, during 1993-1994, only two cases of this infection had been reported in the same community. Because of the proximity of this community to the Tennessee border, on June 28 GDPH notified the Tennessee Department of Health (TDH) about these cases. TDH subsequently identified two confirmed cases with onsets of illness during June 23-24. Both of these cases were among persons residing in eastern Tennessee approximately 100 miles from the community in Georgia, and one occurred in an 11-year-old boy who was hospitalized with hemolytic uremic syndrome (HUS). This report summarizes the investigation of this outbreak, which implicated eating hamburgers purchased at a fast-food restaurant chain (i.e., chain A) as the source of infection.

Active surveillance for additional cases was initiated in hospitals in both states. Cases were defined as laboratory-confirmed E. coli O157:H7 infection among persons who became ill during June 11-25, or abdominal cramps and bloody diarrhea of at least 72 hours' duration among persons residing in the same household as a person with a culture-confirmed case. A matched case-control study was conducted to assess potential sources of the outbreak. Only the first case (index case) in each household was included in the study. For each case, two neighborhood controls matched by age range were selected. Laboratory analyses included O157 and H7 agglutination tests and pulsed-field gel electrophoresis for DNA analysis of E. coli O157:H7 isolated from stool. Case-patients and controls were interviewed to collect information about food exposures and potential risk behaviors within 7 days before onset of illness.

GDPH and TDH identified 10 case-patients with onset of illness during June 13-23. Patients ranged in age from 7 to 89 years (mean: 32 years), and seven were male. Excluding the HUS case, the median duration of illness was 7 days. All case-patients had had grossly bloody diarrhea and severe abdominal cramps for greater than 72 hours.

Eight of the 10 case-patients were included in the case-control study. One was excluded because his parents declined participation and another because a spouse was the index patient in the household. Eating hamburgers purchased at one of three chain A restaurants (two in Tennessee and one in Georgia) during June 13-21 was reported by seven of the eight patients and one of the 16 controls (matched odds ratio=infinity, 95% confidence interval=2.5-infinity). No other exposures were significantly associated with E. coli O157:H7 infection. All three restaurants obtained unfrozen ground beef patties from the same meat processing plant and reported complete turnover of stock, generally within 3 days.

Seven of the eight cases were confirmed by isolation of E. coli O157:H7 from stool specimens; DNA patterns were identical for six of these patients. The single case-patient for whom the isolate had a different DNA pattern did not recall eating at a chain A restaurant and had onset of illness on June 13. Inspections of chain A restaurants in Georgia and Tennessee did not identify deficiencies in cooking temperature or procedures, but did identify potential opportunities for cross-contamination from the ground beef. Meat samples obtained at least 4 days after the case-patients visited the restaurants were negative for E. coli O157:H7.

Based on the epidemiologic and laboratory findings, GDPH and TDH concluded that hamburgers served at chain A restaurants were the source of this outbreak, most likely as a result of undercooking of or cross-contamination from the ground beef to the buns or other items on the hamburger. GDPH and TDH recommended a thorough assessment of food-handling and cooking procedures at chain A restaurants. In addition, chain A restaurants instituted a training program for workers in proper food-handling practices.

Reported by: M Cannon, H Thomas, Catoosa County Health Dept, Ringgold; W Sellers, MD, Rome District Health Office, Rome; M Bates, Georgia State Public Health Laboratory, P Blake, MD, H Stetler, MD, K Toomey, MD, State Epidemiologist, Div of Public Health, Georgia Dept of Human Resources. J Fowler, S Halford, Knox County Health Dept, Knoxville; G Young, Hamilton County Health Dept, Chattanooga; S Hall, MD, Knox County Regional Office; P Erwin, MD, East Tennessee Region; V Boaz, MD, Chattanooga-Hamilton County Regional Office; G Swinger, DVM, Tennessee Dept of Health. Foodborne and Diarrheal Diseases Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases; Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: E. coli O157:H7 was first recognized as a human pathogen in 1982. Infection with this pathogen may be asymptomatic or associated with a range of manifestations including mild diarrhea, severe hemorrhagic colitis, HUS, thrombotic thrombocytopenic purpura, and death (1,2). From January 1, 1993, through September 14, 1995, a total of 63 clusters or outbreaks of E. coli O157:H7 infection were reported to CDC by 32 states; these outbreaks accounted for 1734 cases (CDC, unpublished data). In three U.S. studies conducted during 1985-1990, E. coli O157:H7 was the third or fourth most common bacterial pathogen isolated from stool specimens (2) and, among stool cultures at 10 hospitals throughout the United States, E. coli O157:H7 was isolated from 8% of visibly bloody stools (3).

Ground beef is the most common vehicle for E. coli O157:H7 transmission in investigated outbreaks. Since January 1993, ground beef has been identified as the primary vehicle of infection in 25 (40%) of the 63 reported outbreaks of this infection. E. coli O157:H7 can be recovered from the intestines of approximately 1% of cattle; because of processing practices, meat from many animals may comprise one hamburger (2). Although current U.S. Department of Agriculture regulations specify only gross inspection of carcasses, more comprehensive regulations -- including process controls that incorporate guidelines for microbiologic testing of meat -- have been proposed and already have been implemented by some producers. Complete implementation of these production practices should decrease E. coli O157:H7 contamination of the meat supply.

Ground beef contaminated with E. coli O157:H7 can cause illness when the meat is not thoroughly cooked (to an internal temperature of at least 155 F {68 C}) or when raw or undercooked meat cross-contaminates other food items. Because the infectious dose is low, even limited deficiencies in food preparation or handling can result in exposure and infection (2). Although this investigation did not identify deficiencies in hamburger cooking temperatures, opportunities for cross-contamination were detected. Measures for preventing cross-contamination include washing hands and surfaces after contact with raw ground beef, storing raw ground beef to ensure that drippings do not contaminate other foods, and using different utensils to handle raw and cooked meat.

As of January 1996, reporting of E. coli O157:H7 infection was required by 38 states (W. Keene, Oregon Department of Human Resources, personal communication, 1996), including Georgia and Tennessee; neither state had required reporting of E. coli O157:H7 at the time of this outbreak. The outbreak described in this report underscores the need for clinical laboratories to screen stool specimens for E. coli O157:H7 on sorbitol-MacConkey (SMAC) agar. In this outbreak, E. coli O157:H7 was detected by a laboratory in Georgia that routinely screened for this pathogen. In a recent survey of clinical microbiology laboratories in the United states, only 54% screened all bloody stool specimens on SMAC agar (4). CDC recommends that laboratories in all states screen at least all bloody stools for E. coli O157:H7.


  1. Boyce TG, Swerdow DL, Griffin PM. Escherichia coli O157:H7 and the hemolytic uremic syndrome. N Engl J Med 1995;333:364-8.

  2. Griffin PM. Escherichia coli O157:H7 and other enterohemorrhagic Escherichia coli. In: Blaser MJ, Smith PD, Ravdin JI, Greenberg HB, Guerrant RL, eds. Infections of the gastrointestinal tract. New York: Raven Press, Ltd., 1995:739-61.

  3. Ries A, Griffin P, Greene K. Escherichia coli O157:H7 diarrhea in the United States: a 10 center surveillance study. In: Program and abstracts of the 33rd Interscience Conference on Antimicrobial Agents and Chemotherapy, New Orleans 1993:385.

  4. Boyce TG, Pemberton AG, Wells JG, Griffin PM. Screening for Escherichia coli O157:H7: a nationwide survey of clinical laboratories. J Clin Microbiol 1995;33:3275-7.

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