Seasonal variation in Escherichia coli bloodstream infection: a population-based study

Seasonal variation in the rates of infection with certain Gram-negative organisms has been previously examined in tertiary-care centres. We performed a population-based investigation to evaluate the seasonal variation in Escherichia coli bloodstream infection (BSI). We identified 461 unique patients in Olmsted County, Minnesota, from 1 January 1998 to 31 December 2007, with E. coli BSI. Incidence rates (IR) and IR ratios were calculated using Rochester Epidemiology Project tools. Multivariable Poisson regression was used to examine the association between the IR of E. coli BSI and average temperature. The age- and gender-adjusted IR of E. coli BSI per 100 000 person-years was 50.2 (95% CI 42.9–57.5) during the warmest 4 months (June through September) compared with 37.1 (95% CI 32.7–41.5) during the remainder of the year, resulting in a 35% (95% CI 12–66%) increase in IR during the warmest 4 months. The average temperature was predictive of increasing IR of E. coli BSI (p 0.004); there was a 7% (95% CI 2–12%) increase in the IR for each 10-degree Fahrenheit (c. 5.5°C) increase in average temperature. To our knowledge, this is the first study to demonstrate seasonal variation in E. coli BSI, with a higher IR during the warmest 4 months than during the remainder of the year.