Multistate Outbreak of Listeria monocytogenes Associated with Mexican-Style Cheese Made from Pasteurized Milk among Pregnant, Hispanic Women

Listeriosis is a severe infection caused by Listeria monocytogenes. Since 2004, the Centers for Disease Control and Prevention has requested that listeriosis patients be interviewed using a standardized Listeria Initiative (LI) questionnaire. In January 2009, states and the Centers for Disease Control and Prevention began investigating a multistate outbreak of listeriosis among pregnant, Hispanic women. We defined a case as an illness occurring between October 2008 and March 2009 with art L. monocytogenes isolate indistinguishable from the outbreak strain by pulsed-field gel electrophoresis. We conducted a multistate case-control study using controls that were selected from L. monocytogenes illnesses in non-outbreak-related pregnant, Hispanic women that were reported to the LI during 2004 to 2008. Eight cases in five states were identified. Seven of these were pregnant, Hispanic females aged 21 to 43 years, and one was a 3-year-old Hispanic girl, who was excluded from the study. Seven (100%) cases but only 26 (60%) of 43 controls had consumed Mexican-style cheese in the month before illness (odds ratio, 5.89; 95% confidence interval, 1.07 to infinity; P = 0.04). Cultures of asadero cheese made from pasteurized milk collected at a manufacturing facility during routine sampling by the Michigan Department of Agriculture on 23 February 2009 yielded the outbreak strain, leading to a recall of cheeses produced in the plant. Recalled product was traced to stores where at least three of the women had purchased cheese. This investigation highlights the usefulness of routine product sampling for identifying contaminated foods, of pulsed-field gel electrophoresis analysis to detect multistate outbreaks, and of the LI for providing timely exposure information for case-control analyses. Recalls of contaminated cheeses likely prevented additional illnesses.