Six cases were identified; four persons had culture-confirmed infections, one had a culture-confirmed infection and HUS, and one had HUS only. The median age of patients was 8 years (range: 6–18 years), and four of the patients (67%) were boys. The six cases identified during this investigation were geographically dispersed throughout California. All six patients reported bloody diarrhea; three (50%) were hospitalized. Illness onset occurred during September 6–24, 2006. Isolates from the five patients with culture-confirmed infections had indistinguishable pulsed-field gel electrophoresis (PFGE) patterns. The PFGE pattern was new to the PulseNet (the National Molecular Subtyping Network for Foodborne Disease) database and differed markedly from the pattern of the E. coli O157:H7 strain associated with a concurrent multistate outbreak linked to spinach consumption (1). Four of the five E. coli O157:H7 isolates were subtyped by multiple-locus variable-number tandem repeat analysis (MLVA) according to a protocol used by CDPH laboratory and were found to have closely related MLVA patterns (2).

Five of six patients reported they had consumed brand A raw dairy products in the week before their illness onset; the sixth patient denied drinking brand A raw milk, although his family routinely purchased it. Among the five patients who consumed brand A dairy products, two consumed raw whole milk, two consumed raw skim milk, and one consumed raw chocolate-flavored colostrum. Four of the five patients routinely drank raw milk from dairy A. One patient was exposed to brand A dairy product only once; he was served raw chocolate colostrum as a snack when visiting a friend. No other food item was commonly consumed by all six patients. No other illness was reported among household members who consumed brand A dairy products.

Using purchase information supplied by the patients’ families, investigators determined that the patients consumed raw milk from lots produced at dairy A during September 3–13, 2006. Milk samples from these production dates were not available for testing. Fifty-six product samples from several lots with code dates of September 17, 2006, or later were retrieved from retails stores and dairy A and were tested for aerobic microflora, total coliform, fecal coliform, and E. coli O157:H7. The outbreak strain of E. coli O157:H7 was not found in any product samples. However, standard aerobic plate counts and coliform counts of collected samples with code dates of September 17 through October 9, 2006, were indicative of contamination. Colostrum samples had high standard plate counts and total coliform counts, and fecal coliform counts of 210–46,000 MPN/g. California standards limit standard plate counts for raw and pasteurized milk to 15,000 CFU/mL and total coliform counts for pasteurized milk to 10 coliform bacteria/mL. At the time of this outbreak, California did not have a coliform standard for milk sold raw to consumers. California also classifies colostrum as a dietary supplement, for which it has no microbiologic standards, rather than a milk product.

Raw milk from dairy A was the likely vehicle of transmission, but the exact mode of milk contamination in this outbreak was not determined.

Asymptomatic cows can harbor pathogens and cause human illness by shedding pathogens in untreated milk or milk products. These findings suggest that if raw milk had been subject to the same coliform standard as pasteurized milk in California, milk from dairy A might have been excluded from sale and this outbreak might have been averted.