Escherichia coli 0157.H7 outbreak linked to home-cooked hamburgers.

Although outbreaks of Escherichia coli O157.H7 have been linked to consumption of contaminated ground beef, the organism is rarely isolated from the implicated meat. In addition, most epidemiologic investigations of illness associated with E. coli O157.H7 infections have been directed at restaurant-associated outbreaks, and the sources of infection for sporadic cases rarely have been identified. In July 1993, three cases of culture-confirmed E. coli O157.H7 was isolated from that meat. This report summarizes the investigation of these cases by local and state public health officials.

On July 12, 1993, a hospital laboratory in Fort Bragg, California, reported a case of E. coli O157.H7 infection in a 13-year old girl to the Mendocino County Public Health Department (MCPHD). The patient had had onset of bloody diarrhea on July 7 and recovered. Members of her family reported having eaten home-cooked hamburgers on July 5 made from meat purchased from a local market (market A) on July 3; the hamburgers had been cooked "medium rare." All five family members who ate the hamburgers reported diarrhea; the index patient and her mother had bloody diarrhea. E. coli O157.H7 was isolated from leftover ground beef from the same package used to make the hamburgers.

Two additional cases of culture-confirmed E. coli O157.H7 infection occurred in persons residing in the same community: an 18-year old man who had onset of bloody diarrhea on July 18 and an 84-year old woman with diabetes mellitus and chronic uremia who developed nonbloody diarrhea after eating the hamburger. Although no patients developed hemolytic uremic syndrome (HUS), the elderly woman died three weeks after hospitalization; her death attributed to her chronic renal disease.

Media announcements from MCPHD requested persons who had experienced bloody diarrhea during July to contact the department. Of five persons who reported having had bloody diarrhea, four submitted stool for culture. Although all were negative, the cultures had been obtained 11-26 days after onset of diarrhea. Reviews of the emergency department log of the district hospital for July 1-22 did not identify additional cases of bloody diarrhea.

Environmental health staff from MCPHD and staff from the U.S. Department of Agriculture (USDA) inspected market A and the other two markets in the community that sold ground meat (markets B and C) but did not identify violations in meat storage or grinding procedures. Shelf samples of ground beef from all three markets were obtained for testing. The owner of market A also initiated a voluntary recall of all ground beef purchased at that market during June 25-July 19; as a result, 91 packages of ground beef were returned.

Of the 15 samples of ground beef obtained from market A and tested, four were positive for E. coli O157.H7. All positive samples had been placed on the shelves on July 3. Of 16 samples from market B, one was positive for E. coli O157.H7. None of seven samples obtained from market C were positive. The packages placed on the shelf of market A on July 3 were obtained from "chubs," which are large tubes of ground beef purchased from an outside supplier. The market often reground the meat in its own grinder and sometimes added "trim meat" from other sources. A traceback of the meat was not performed.

Because the isolates produced an uninterpretable pattern by pulsed-field gel electrophoresis, selected isolates were further characterized by phage typing at the National Laboratory for Enteric Pathogens, Laboratory Center for Disease Control, in Ottawa (Ontario, Canada). Phage type 31 was identified in the three patient isolates, the leftover ground beef obtained from the freezer of the index patient's family, and the two isolates selected for testing from market A. The sample from market B (which was not implicated in the outbreak) was phage type 4.

Following the investigation, MCPHD provided information to all county meat markets about optimal meat-grinding procedures and issued a press release advising consumers to cook ground beef thoroughly.

Editorial Note

E. coli O157.H7 was first described as a pathogen in humans in 1982 following the investigation of two outbreaks of illness that were associated with consumption of hamburger from a fast-food restaurant chain (1). Since then, more than 12 outbreaks have been reported in the United States (2). Although other investigations have implicated consumption of undercooked ground beef, less commonly identified sources of E. coli O157.H7 infection have included roast beef, unpasteurized milk, apple cider, and municipal water (2,3). Person-to-person transmission in child day care centers also has been documented (4).

E. coli O157.H7 infection causes diarrhea (often bloody) and abdominal cramps; fever is infrequent. Infection with E. coli O157.H7 is a relatively common cause of sporadic diarrheal illness: in prospective studies of patients with diarrhea, E. coli O157:H7 has been isolated more frequently than Shigella (2). Children and the elderly are at higher risk for clinical manifestations and complications. Although illness usually resolves within one week, 5% to 10% of patients develop HUS, which is characterized by hemolytic anemia, thrombocytopenia, and renal failure. HUS is a common cause of acute renal failure in children, and the case-fatality rate is 3% to 5%.

Sporadic cases and small outbreaks of E. coli O157.H7 infection similar to the cluster described in this report probably occur throughout the United States but are not recognized. Many clinical healthcare providers do not routinely order stool cultures for patients with diarrhea. Even when stool cultures are ordered, clinicians may to be aware that most laboratories do not culture stools for E. coli O157.H7 using sorbitol-MacConkey medium unless specifically requested (5).

The findings in this report illustrate the usefulness of subtyping in distinguishing outbreak strains of E. coli O157.H7 from those present in the community but unassociated with an outbreak. There are at least 62 known phage types of E. coli O157.H7. In Canada, where phage typing is the predominant subtyping method used, phage type 31 accounts for 9% of isolates tested (6).

E. coli O157.H7 may be present in the intestines of healthy cattle and may contaminate meat during slaughter. The process of grinding beef may then transfer pathogens from the surface of the meat to the interior.

Because of the publicity generated by large restaurant-associated outbreaks, many persons associate infections caused by E. coli O157. H7 with restaurant-served ground beef. However, the outbreak in Mendocino County emphasizes that home-cooked hamburgers can be a source of infection and underscores the need to cook ground beef until the interior is no longer pink and juices run clear; thorough cooking kills E. coli O157.H7. On March 28, 1994, the USDA Food Safety and Inspection Service published regulations mandating that safe handling instructions be included on all raw meat and poultry product labeling (59 FR 14528-40). These regulations include instructions to cook meat thoroughly.

In June 1993, the Council of State and Territorial Epidemiologists (CSTE) passed a resolution that reporting of E. coli O157.H7 infections should be required in all states. As of October 1, 1993, however, only 17 states required E. coli O157.H7 infection to be reported to state health departments (G.S. Birkhead, M.D., CSTE, personal communications, 1994). CDC is working with state health departments to establish national surveillance for E. coli O157.H7 infections. National surveillance and increased laboratory testing E. coli O157.H7 will assist in defining the public health impact of this emerging pathogen.


1. Riley L.W., R.S. Remis, S.D. Helgerson, et al. (1993), "Hemorrhagic colitis associated with a rare Escherichia coli serotype," N Engl J Med, 308:681-5.

2. Griffin, P.M., R.V. Tauxe (1991), "The epidemiology of infections caused by Escherichia coli O157.H7, other enterhemorrhagic E. coli, and the associated hemolytic uremic syndrome," Epidemiol Rev, 13:60-98.

3. Besser, R.E., S.M. Lett, J.T. Weber, et al. (1993), "An outbreak of diarrhea and hemolytic uremic syndrome from Escherichia coli O157.H7 in fresh-pressed apple cider," JAMA, 269:2217-20.

4. Belongia, E.A., M.T. Osterholm, J.T. Soler, D.A. Ammend, J.E. Braun, K.L. MacDonald (1993), "Transmission of Escherichia coli O157.H7 infection in Minnesota child day-care facilities," JAMA, 269:883-8.

5. CDC (1994), "Laboratory screening for Escherichia coli O157.H7-Connecticut," MMWR, 43:192-4.

6. Khakhria, R., D. Duck, H. Lior (1990), "Extended phage-typing scheme for Escherichia coli O157.H7," Epidemiol Infect, 105:511-20.

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