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Outbreak of Escherichia coli O157:H7 Infection Associated With Eating Fresh Cheese Curds --- Wisconsin, June 1998

On June 15, 1998, the Division of Public Health, Wisconsin Department of Health and Family Services, was notified of eight laboratory-confirmed and four suspected Escherichia coli O157:H7 infections among west-central Wisconsin residents who became ill during June 8--12. This report summarizes the outbreak investigation, which implicated fresh (held <60 days) cheese curds from a dairy plant as the source of infection.

A primary case was defined as the first laboratory-confirmed case in a household; a secondary case was one that occurred 3--8 days after a primary case in the same household. A matched case-control study was conducted to assess potential sources of infection. For the purposes of the case-control study, a case was defined as culture-confirmed illness among residents of Chippewa and Eau Claire counties with illness onset during June 7--18. For each case-patient, two community controls matched by sex and age group (range: from <10 years within 2 years to >10 years within 5 years) were interviewed by telephone. Case-patients and controls were interviewed about food exposures and potential risk factors for E. coli O157:H7 infection within 7 days before onset of illness.

In response to the case-control study, the Wisconsin Department of Agriculture, Trade, and Consumer Protection visited dairy plant A to collect cheese samples, raw ingredients, and packaging materials; to review employee food handling and hygienic practices; and to assess potential sources of contamination from raw milk. Product and environmental samples (e.g., vat surfaces and floor drains) from the dairy plant were screened for phosphatase activity to identify evidence of raw milk.

Fifty-five laboratory-confirmed case-patients were identified, including two from secondary households. Case-patients were from seven Wisconsin counties (27 from Chippewa and 16 from Eau Claire counties); two case-patients were visiting from out of state. Median age was 27 years (range: 15 months--90 years) and 37 (67%) were female. The most frequently reported symptoms included bloody diarrhea (55 [100%]), cramps (50 [91%]), fatigue (39 [71%]), and nausea (38 [69%]). Mean duration of diarrhea was 5.1 and 4.5 days for 25 hospitalized and 30 nonhospitalized case-patients, respectively.

Eating fresh cheese curds during June 1--17 was reported by all 24 case-patients in Chippewa and Eau Claire counties and eight (18%) of 45 controls (matched odds ratio=undefined; 95% confidence interval=20.6--infinity). Illness was not linked to eating other cheese products (e.g., shredded, sliced, block, or string cheese). Of the 43 laboratory-confirmed case-patients whose cheese curd source could be identified, all had eaten fresh cheese curds produced at dairy plant A; 19 had purchased the curds from an unrefrigerated display at plant A, and 24 had purchased them refrigerated from retail stores that received shipments from plant A. Fifteen (50%) of 30 case-patients who recalled the purchase date had bought the curds on June 5 or 6. The median number of curds eaten was eight (range: one--28), the equivalent of approximately 1.6 oz of cheese.

Thirty-five specimens from plant A that were produced during the outbreak were tested: nine environmental samples, 18 unopened cheese samples, six opened retail packages of curds, and two unopened retail packages of curds. Five of the six opened retail packages of curds and four of the 18 unopened cheese samples were positive for nonbacterial phosphatase (Scharer method). E. coli O157:H7 was isolated from an opened package of curds that had been served at a party attended by nine persons with culture-confirmed illness. The contents of this package tested positive for nonbacterial phosphatase. Among 44 E. coli O157:H7 case-patient isolates available for pulsed-field gel electrophoresis, 42 were indistinguishable from each other and from the curd isolate.

Dairy plant A had produced four or five vats of pasteurized cheddar and Colby cheese products 5 days a week since 1977. Each vat yielded approximately 1500 pounds of cheese that was pressed into 40-lb blocks, daisies (rounds of cheese), or was packaged as fresh cheese curds. Dairy plant A also produced unpasteurized (raw milk) cheddar cheese daisies every June as part of Dairy Month. Certain raw milk cheese products can be produced and sold legally as long as the cheese is held at >35 F (>1.7 C) for at least 60 days before it is sold*. Curds are sold fresh (held <60 days); therefore, curds must be made with pasteurized milk. At least one 1500-lbs vat of raw milk cheddar cheese was made on May 27 and June 2--5. These vats were used inadvertently to make fresh curds, which were incorrectly labeled "pasteurized" cheddar cheese curds, and distributed and sold in six Wisconsin counties.

Reported by: J Durch, MPH, T Ringhand, MPH, Chippewa County Dept of Public Health, Chippewa Falls; K Manner, M Barnett, Wisconsin Dept of Agriculture, Trade, and Consumer Protection; M Proctor, PhD, S Ahrabi-Fard, MS, Communicable Disease Epidemiology Section; J Davis, MD, State Epidemiologist for Communicable Disease, Wisconsin Div of Public Health. D Boxrud, Minnesota Health Dept. Foodborne and Diarrheal Diseases Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note: 

Cheese is made in vats by coagulating milk with enzymes and/or acids. After whey is drained, the large cheese clumps are removed and milled into curds, salted, and packaged in small plastic bags for sale. Raw milk consumption has been associated with campylobacteriosis, salmonellosis, E. coli O157:H7, yersiniosis, listeriosis, tuberculosis, brucellosis, cryptosporidiosis, and staphylococcal enterotoxin poisoning (1). In 1950, the U.S. Food and Drug Administration (FDA) required manufacturers of soft and fresh cheeses to use pasteurized milk and allowed raw milk to be used only for certain aged cheeses (2). In 1986, E. coli O157:H7 illness was associated with consuming raw milk (3). In 1987, FDA banned the interstate sale of raw milk in retail packages. During 1973--1992, 40 (87%) of 46 raw milk-associated outbreaks occurred in the 28 states that permitted the intrastate sale of raw milk (4). During the same period, 11 of 32 cheese-associated outbreaks were attributed to contamination before distribution (5).

This outbreak investigation illustrates the hazards of using raw milk to produce commercial products that may lead to mislabeling or contaminating pasteurized product by equipment or ingredients. This practice can result in pasteurized products contaminated by equipment or ingredients and in product mislabeling. States that allow the sale of unpasteurized milk or dairy products made from unpasteurized milk should take appropriate steps to reduce the risk for contamination and mislabeling to prevent similar outbreaks.


  1. Potter ME, Kaufmann AF, Blake PA, Feldman RA. Unpasteurized milk: the hazards of a health fetish. JAMA 1984;252:2048--52.
  2. US Food and Drug Administration. Cheeses; processed cheeses; cheese food; cheese spreads, and related foods: definitions and standards of identity; final rule. Federal Register 1950;19:5656--90.
  3. Martin ML, Shipman LD, Wells JG, et al. Isolation of Escherichia coli O157:H7 from dairy cattle associated with two cases of haemolytic uraemic syndrome [Letter]. Lancet 1986;8514:1043.
  4. Headrick ML, Korangy S, Bean NH, et al. The epidemiology of raw milk=associated foodborne disease outbreaks reported in the United States, 1973 through 1992. Am J Public Health 1998;88:1219--21.
  5. Altekruse SF, Timbo BB, Mowbray JC, Bean NH, Potter ME. Cheese-associated outbreaks of human illness in the United States, 1973 to 1992: sanitary manufacturing practices protect consumers. Journal of Food Protection 1998;61:1405--7.

* Code of Federal Regulations Title 21, Part 133.

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