Seasonal variation in Escherichia coli bloodstream infections in northern Israel

Seasonal variation in the infection rate with certain Gram-negative organisms has been previously described, but few studies have been published regarding Escherichia coli. The aim of this study was to investigate the incidence rate of E. coli bloodstream infection (BSI) and the association with temperature in different seasons in the Yizrael Valley. Positive blood cultures sent to the microbiology laboratory of Ha'Emek Medical Centre over a period of 8 years (January 2001 to December 2008) were included. The mean monthly temperature in the Yizrael Valley in the same period was compared with the monthly E. coli BSI rate. We divided the year into three periods: winter (December to February: mean temperature < 15 degrees C), transitional (March, April and November: mean temperature 15-19 degrees C) and summer (May to October: mean temperature >= 20 degrees C). In addition, we correlated the mean monthly antibiotic use in the same period measured as total defined daily doses for the whole regional population with E. coli BSI. During the study period, 2810 BSIs were recorded (35% E. coli). In 67.4% of the cases of E. coli bacteraemia, the source was urinary tract infection. The crude incidence of E. coli BSI was 4.1/1000 admissions. There was no difference in the number of cultures/month (mean: 29 +/- 6). However, E. coli BSI was 19% and 21% more frequent in summer than in the transitional and winter seasons, respectively (p 0.01). The antibiotic consumption was significantly higher in the winter period. We found significantly higher rates of E. coli BSI in the summer period. Host, bacterial and ecological factors, together with high consumption of antibiotics during the winter season, could partially explain these findings.