OBJECT: The relationship between time of year and surgical site infection (SSI) 
following neurosurgical procedures is poorly understood. Authors of previous 
reports have demonstrated that rates of SSI following neurosurgical procedures 
performed during the summer months were higher compared with rates during other 
seasons. It is unclear, however, if this difference was related to 
climatological changes or inexperienced medical trainees (the July effect). The 
aim of this study was to evaluate for seasonal variation of SSI following spine 
surgery in a network of nonteaching community hospitals.
METHODS: The authors analyzed 6 years of prospectively collected surveillance 
data (January 1, 2007, to December 31, 2012) from all laminectomies and spinal 
fusions from 20 hospitals in the Duke Infection Control Outreach Network of 
community hospitals. Surgical site infections were defined using National 
Healthcare Safety Network criteria and identified using standardized methods 
across study hospitals. Regression models were then constructed using Poisson 
distribution to evaluate for seasonal trends by month. Each analysis was first 
performed for all SSIs and then for SSIs caused by specific organisms or classes 
of organisms. Categorical analysis was performed using two separate definitions 
of summer: June through September (definition 1), and July through September 
(definition 2). The prevalence rate of SSIs during the summer was compared with 
the prevalence rate during the remainder of the year by calculating prevalence 
rate ratios and 95% confidence intervals.
RESULTS: The authors identified 642 SSIs following 57,559 neurosurgical 
procedures (overall prevalence rate = 1.11/100 procedures); 215 occurred 
following 24,466 laminectomies (prevalence rate = 0.88/100 procedures), and 427 
following 33,093 spinal fusions (prevalence rate = 1.29/100 procedures). Common 
causes of SSI were Staphylococcus aureus (n = 380; 59%), coagulase-negative 
staphylococci (n = 90; 14%), and Escherichia coli (n = 41; 6.4%). Poisson 
regression models demonstrated increases in the rates of SSI during each of the 
summer months for all SSIs and SSIs due to gram-positive cocci, S. aureus, and 
methicillin-sensitive S. aureus. Categorical analysis confirmed that the rate of 
SSI during the 4-month summer period was higher than the rate during the 
remainder of the year, regardless of which definition for summer was used 
(definition 1, p = 0.008; definition 2, p = 0.003). Similarly, the rates of SSI 
due to grampositive cocci and S. aureus were higher during the summer months 
than the remainder of the year regardless of which definition of summer was 
used. However, the rate of SSI due to gram-negative bacilli was not.
CONCLUSIONS: The rate of SSI following fusion or spinal laminectomy/laminoplasty 
was higher during the summer in this network of community hospitals. The 
increase appears to be related to increases in SSIs caused by gram-positive 
cocci and, more specifically, S. aureus. Given the nonteaching nature of these 
hospitals, the findings demonstrate that increases in the rate of SSI during the 
summer are more likely related to ecological and/or environmental factors than 
the July effect.