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BACKGROUND: During September 9-17, 2014, 8 of 35 (23%) Colorado Salmonella cases had the Typhimurium PFGE pattern JPXX01.0286. This pattern had been associated with live poultry exposure during a spring 2014 outbreak in southern Colorado (n=8) and a 2013 multistate outbreak (n=356). However, only 2 of 8 (25%) September 2014 cases reported poultry exposure and the geographic distribution of cases was uncommon for a Colorado poultry-associated outbreak, suggesting a different source of illness. Epidemiologists, laboratorians and environmental health specialists collaborated to determine the source of infection and prevent future cases.
METHODS: Cases were defined as persons with culture-confirmed Salmonella Typhimurium, PFGE pattern JPXX01.0286, with illness onset between August 5 and October 6, 2014. The state public health laboratory analyzed specimens using multiple locus variable-number tandem repeat analysis (MLVA) and cases were interviewed with a supplemental questionnaire (national hypothesis-generating or poultry-specific). A population-based case-control study was conducted among cases reporting no poultry exposure. An environmental assessment was conducted at Agricultural Supply Store B in southern Colorado where some cases reported purchasing poultry.
RESULTS: Eighteen cases with indistinguishable PFGE patterns were identified between August 11 and October 14, 2014. MLVA identified three distinct patterns. Twelve cases had MLVA pattern 1. Of these, four (33%) were female with a median age of 27 years (range 4 months-80 years). These cases predominantly resided in the Denver-metropolitan area. Case-control results indicated consuming ground beef purchased from Grocery Chain A was associated with illness (OR=6, CI 1.17, 30.7). Six cases had MLVA patterns 2 or 3; three (50%) were female with a median age of 9.5 years (range 5 months-51 years). All six cases lived in a southern Colorado city and 5 of 6 purchased poultry or animal-related supplies items at Agricultural Supply Store B. Two environmental samples collected from Store B after the fall poultry supply had been sold and when no poultry were present were positive for Salmonella Typhimurium JPXX01.0286, MLVA pattern 3.
CONCLUSIONS: Involvement of all three disciplines allowed us to separate the two outbreaks, determine the sources of infection, and implement appropriate control measures. MLVA was crucial in detecting distinct outbreaks in the setting of indistinguishable PFGE patterns. Environmental sampling effectively linked exposures for outbreak 2 and epidemiologic investigation identified a food item responsible for outbreak 1. These outbreaks also illustrate the importance of investigating every cluster, even when a specific pattern is highly correlated with a particular vehicle (e.g. “poultry pattern”).