Abstract
BackgroundKnowledge 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.MethodsA 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 spp, 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.ResultsA 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.ConclusionsSummer season and higher mean monthly outdoor temperature are associated with substantially increased frequency of BSIs, particularly among clinically important Gram-negative bacteria.
Highlights
Seasonal variation in the incidence of human infection can influence diagnosis and empiric treatment and has the potential to guide both the design and the evaluation of infection prevention interventions
The objective of this study was to evaluate seasonal changes in the frequencies of bloodstream infections (BSIs) caused by Gramnegative and Gram-positive bacterial pathogens using a large, nationally representative database of clinical cultures collected from inpatients in the United States
Inpatient blood cultures positive for Gram-positive bacteria (Enterococcus spp and Staphylococcus aureus) or Gram-negative bacteria (Acinetobacter spp, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa) that were reported between January 1999 and September 2006 were included in the analysis
Summary
Seasonal variation in the incidence of human infection can influence diagnosis and empiric treatment and has the potential to guide both the design and the evaluation of infection prevention interventions. The importance of seasonal variation in community-associated infection incidence is well recognized, for influenza [1]. Certain hospital-associated pathogens, Gram-negative bacteria, are increasingly recognized as exhibiting seasonal trends in infection incidence [2,3,4,5]. Existing studies have been limited by their inability to assess independent associations of infections with meteorological elements that may be driving these seasonal trends [2,5]. Knowledge of seasonal trends will allow for improved designs of quasi-experimental (before-after) studies, which are frequently used when assessing infection prevention interventions. 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
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