Outbreak Characteristics and Epidemic Curves for Multistate Outbreaks of Salmonella Infections Associated with Produce: United States, 2009-2015

Produce is recognized as a source of Salmonella -related foodborne outbreaks in the United States. Identifying produce as a source of foodborne outbreaks is challenging given short product shelf lives and durations of many produce-associated outbreaks. Investigators consider produce a plausible source when illnesses occur over a short time period and disproportionately affect middle-aged or female individuals. We reviewed characteristics of past Salmonella produce outbreaks and their consistency with principles used by epidemiologists when generating hypotheses about an outbreak source. We queried the Foodborne Disease Outbreak Surveillance System for multistate, produce-associated Salmonella outbreaks reported to the Centers for Disease Control and Prevention from 2009 to 2015. All produce-associated outbreaks were classified as fruit outbreaks or vegetable outbreaks using an established classification scheme. We then compared fruit and vegetable outbreaks by characteristics of size, gender, age, age groups, geographic spread, duration, and velocity measures using Wilcoxon rank-sum tests. Epidemic curves were created to display visual representations of outbreak duration and velocity. We identified 14 fruit outbreaks and 24 vegetable outbreaks. The median number of illnesses for all produce-associated outbreaks was 30 and a high median percentage of illnesses were in females (61.9%). Median age was 34 years, with a median of 53.2% of illnesses affecting the 18-59 age group. For all outbreaks, median duration was 77 d and median time to the 50th percentile of illnesses was 32.5 d. Fruit and vegetable outbreaks differed only in the age groups affected. We used outbreak data to verify common indicators of produce-associated Salmonella outbreaks. Outbreaks affected females and middle-aged individuals more commonly, while fruit and vegetable outbreaks impacted different age groups. Although median outbreak duration was less than 12 weeks for both fruit and vegetable outbreaks, there was considerable variation, decreasing its utility as an indicator of produce as a source of the outbreak.