Seasonal Variation in the Incidences of Bacterial Infection among Hospitalized Patients: Summer Peaks in Gram-Negative Infections

Background: Knowledge of seasonal trends in community-acquired infections facilitates infection prevention by focusing public health efforts, as it has, for example, in the case of influenza. Similarly, knowledge of seasonal trends in hospital infections may improve diagnosis and treatment and help guide both the design and evaluation of infection control interventions.  

Objective: To identify and describe seasonal variation in bacterial infections caused by common hospital pathogens.

Methods: Clinical culture data from 132 clinical microbiology laboratories passively reporting to The Surveillance Network Database--USA were obtained from 1999 through 2006. Inpatient cultures positive for Acinetobacter species, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Staphylococcus aureus, or enterococci were included in the study. Inpatient isolates of Acinetobacter spp. and P. aeruginosa that tested positive for imipenem resistance were also examined separately as indicator multidrug-resistant organisms. Patients could only provide one culture per organism per year to the database. Mixed-effects Poisson regression analyses examined the associations of season with monthly number of infections caused by each pathogen while controlling for long-term trends and US Census region.

Results: Analysis of 1,758,581 unique clinical cultures from 9,671 laboratory-months identified higher summer incidences of infection for each of the gram-negative organisms under study compared to winter months (P<0.001 for each organism). Particularly high summer infection rates were observed for Acinetobacter spp. (rate ratio [RR], 1.27) and K. pneumoniae (RR, 1.15). In Acinetobacter spp., summer rates were increased in all regions ranging from RR=2.39 (P<0.001) in New England to RR=1.13 (P<0.001) in the South Atlantic. Elevated summer incidences of infections were also observed for imipenem-resistant Acinetobacter spp. (RR, 1.10; P <0.001) and imipenem-resistant P. aeruginosa (RR, 1.03; P =0.003). By contrast, lower summer incidences of infections were observed for S. aureus (RR, 0.97; P<0.001) and enterococci (RR, 0.92; P<0.001) compared to winter months.

Conclusions: Substantial seasonal variation was observed in the incidences of bacterial infection caused by common hospital pathogens. Among all of the Gram-negative pathogens studied, higher rates of infections were observed during the summer months compared to winter months. Among the Gram-positive pathogens studied, infection rates were slightly lower during summer months compared to winter months. These findings can be used to inform infection prevention strategies, particularly for Acinetobacter. Furthermore, these results highlight the need to control for seasonal trends when completing longitudinal quasi-experimental studies of infection prevention interventions.