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Knowledge of seasonal trends in hospital-associated infection incidence may improve surveillance and help guide the design and evaluation of infection prevention interventions. We estimated seasonal variation in the frequencies of inpatient bloodstream infections (BSIs) caused by common bacterial pathogens and examined associations of monthly BSI frequencies with ambient outdoor temperature, precipitation, and humidity levels.


A database containing blood cultures from 132 U.S. hospitals collected between January 1999 and September 2006 was assembled. The database included monthly counts of inpatient blood cultures positive for several clinically important Gram-negative bacteria (Acinetobacter, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa) and Gram-positive bacteria (Enterococcus spp and Staphylococcus aureus). Monthly mean temperature, total precipitation, and mean relative humidity in the postal ZIP codes of participating hospitals were obtained from national meteorological databases.


A total of 211,697 inpatient BSIs were reported during 9,423 hospital-months. Adjusting for long-term trends, BSIs caused by each Gram-negative organism examined were more frequent in summer months compared with winter months, with increases ranging from 12.2% for E. coli (95% CI 9.2–15.4) to 51.8% for Acinetobacter (95% CI 41.1–63.2). Summer season was associated with 8.7% fewer Enterococcus BSIs (95% CI 11.0–5.8) and no significant change in S. aureus BSI frequency relative to winter. Independent of season, monthly humidity, monthly precipitation, and long-term trends, each 5.6°C (10°F) rise in mean monthly temperature corresponded to increases in Gram-negative bacterial BSI frequencies ranging between 3.5% for E. coli (95% CI 2.1–4.9) to 10.8% for Acinetobacter (95% CI 6.9–14.7). The same rise in mean monthly temperature corresponded to an increase of 2.2% in S. aureus BSI frequency (95% CI 1.3–3.2) but no significant change in Enterococcus BSI frequency.



Summer season and higher mean monthly outdoor temperature are associated with substantially increased frequency of BSIs, particularly among clinically important Gram-negative bacteria.


Michael R. Eber
Michelle Shardell
Marin L. Schweizer
Ramanan Laxminarayan
Eli N. Perencevich
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