Listeria monocytogenes is a foodborne pathogen that can cause bacteraemia, meningitis, and complications during pregnancy. In July 2012, molecular subtyping identified indistinguishable L. monocytogenes isolates from six patients and two samples of different cut and repackaged cheeses. A multistate outbreak investigation was initiated. Initial analyses identified an association between eating soft cheese and outbreak-related illness (odds ratio 17·3, 95% confidence interval 2·0-825·7) but no common brand. Cheese inventory data from locations where patients bought cheese and an additional location where repackaged cheese yielded the outbreak strain were compared to identify cheeses for microbiological sampling. Intact packages of imported ricotta salata yielded the outbreak strain. Fourteen jurisdictions reported 22 cases from March-October 2012, including four deaths and a fetal loss. Six patients ultimately reported eating ricotta salata; another reported eating cheese likely cut with equipment also used for contaminated ricotta salata, and nine more reported eating other cheeses that might also have been cross-contaminated. An FDA import alert and US and international recalls followed. Epidemiology-directed microbiological testing of suspect cheeses helped identify the outbreak source. Cross-contamination of cheese highlights the importance of using validated disinfectant protocols and routine cleaning and sanitizing after cutting each block or wheel.

In July 2011, a cluster of Yersinia enterocolitica infections was detected in southwestern Pennsylvania, USA. We investigated the outbreak's source and scope in order to prevent further transmission. Twenty-two persons were diagnosed with yersiniosis; 16 of whom reported consuming pasteurized dairy products from dairy A. Pasteurized milk and food samples were collected from this dairy. Y. enterocolitica was isolated from two products. Isolates from both food samples and available clinical isolates from nine dairy A consumers were indistinguishable by pulsed-field gel electrophoresis. Environmental and microbiological investigations were performed at dairy A and pasteurization deficiencies were noted. Because consumption of pasteurized milk is common and outbreaks have the potential to become large, public health interventions such as consumer advisories or closure of the dairy must be implemented quickly to prevent additional cases if epidemiological or laboratory evidence implicates pasteurized milk as the outbreak source.