Outbreak investigation

Most foodborne outbreaks are detected and investigated at the local level. Other illness clusters, especially those involving individuals in multiple states, rely on PulseNet, a network of over 80 public health and food regulatory laboratories located throughout the United States, to identify cases and to help investigate potential sources of the infection. PulseNet laboratories use a standardized method, pulsed-field gel electro-phoresis (PFGE), to determine the DNA fingerprint of bacterial foodborne pathogens (6, 19). PulseNet database managers group isolates with indistinguishable PFGE patterns into clusters and notify CDC epidemiologists when the frequency of illnesses with any PFGE pattern exceeds a seasonally adjusted baseline. Among food isolates collected by FSIS during inspection activities, those that are indistinguishable from the clinical isolates by PFGE also are included in the cluster investigation. When a cluster is identified, CDC epidemiologists create a case definition and work with local and state health departments to identify and interview case patients. In this 2014 investigation, a case was defined as infection with the Salmonella Heidelberg strain with the PFGE XbaI pattern JF6X01.0051 isolated on or after 15 May 2014. This pattern accounts for only 4% of Salmonella Heidelberg clinical isolates uploaded to PulseNet each year.

During the hypothesis generation phase of this outbreak investigation, local and state public health officials interviewed case patients about foods and environmental exposures occurring in the 7 days before illness onset. Based on the results of preliminary interviews, a focused questionnaire was developed that included detailed questions about consumption of poultry, egg, tomato, pepper, onion, and fresh herbs, how these foods were packaged, and where and when they were purchased. The proportion of case patients reporting exposure to specific foods was compared with the proportion of healthy persons reporting consumption of the same foods in interviews from the Foodborne Diseases Active Surveillance Network (FoodNet) population survey of healthy adults in the United States conducted during 2006 and 2007 (5). A binomial probability distribution was used to determine which food exposures reported by case patients were significantly higher than those reported by FoodNet population survey respondents.

To provide additional clues regarding the source of this outbreak, health departments also investigated an illness subcluster by conducting a cohort study of persons who were exposed to foods served at a birthday party. Illness subclusters consist of two or more unrelated ill persons who ate or shopped at the same venue around the same time.

When suspected foods were identified during the course of the investigation, state government officials and FSIS conducted traceback investigations into the source of the foods. Product information, such as type of food, brand, and date and location of purchase, was collected. During this investigation, local health department staff also inspected a restaurant implicated in the illness subcluster to (i) identify restaurant practices that may have resulted in cross-contamination or undercooking of food and (ii) identify sources of Salmonella in the facility or its food.