Secular trends in outbreaks caused by Salmonella spp. serotypes, United States, 1973-2012 Open Access

Jackson, Brendan R. (2014)

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Salmonella infections cause an estimated 94 million illnesses worldwide annually and are the leading cause of hospitalization and death from US foodborne illness. Serotyping yields important information for Salmonella public health surveillance. Although most illnesses represent sporadic infections, surveillance of salmonellosis outbreaks provides valuable insight into transmission routes of Salmonella infection if a source can be implicated. The serotype distribution among all Salmonella laboratory isolates reflects a variety of sources and not just foodborne transmission (estimated 55-95% of illnesses). We used data from the US Foodborne Disease Outbreak Surveillance System to examine secular trends in serotype distribution specifically within foodborne salmonellosis. We calculated proportions of foodborne salmonellosis outbreaks caused by each serotype within 9 time periods from 1973-2012 with bootstrap 95% confidence intervals (CIs) and compared these with proportions of serotypes among laboratory isolates from the National Salmonella Surveillance System, which reflect all transmission routes. Among 3,326 salmonellosis outbreaks with a single, known serotype, 72% were caused by the 4 most common serotypes (Enteritidis, Typhimurium, Heidelberg, and Newport). Of 90 serotypes reported, 28 caused 10 outbreaks or more (94% of outbreaks). Serotype Enteritidis caused 9% (95% CI 4-13%) of outbreaks from 1973-1977, 68% (95% CI 63-73%) from 1993-1997, and 30% (95% CI 26-34%) from 2008-2012. From 1993-1997 to 2008-2012, serotypes Typhimurium (11 percentage points), Newport (9), and Javiana (3) exhibited the largest increases in proportions of outbreaks. The proportion of outbreaks caused by serotype Enteritidis was higher than the proportion of isolates that were Enteritidis in each time period (maximum difference: 70% of outbreaks vs. 26% of isolates in 1993-1997), whereas proportions of outbreaks were lower than proportions of isolates in nearly all time periods for serotypes Typhimurium, Newport, and Javiana, suggesting that non-foodborne transmission might be more common for these serotypes since they were underrepresented among foodborne disease outbreaks. Given that the overall incidence of salmonellosis has not declined in the past decade despite intensive efforts, information about these secular trends in serotypes and foodborne disease outbreaks may be useful in guiding future control and prevention strategies.

Table of Contents

Section, Page Background/Literature Review, 1 Methods, 4 Results, 8 Discussion, 11 Future Directions, 18 References, 19 Figures, 24 Tables, 28 Supplemental Figures, 30

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