Introduction: Certain pathogens are more prevalent at certain times of the year [1,3]. While resistance patterns are currently incorporated into antibiotic rotations, seasonality, or variation in pathogen incidence by time of year, is not. Furthermore, relationships between virulence, antibiotic resistance and seasonality are not well understood. We sought to characterize seasonal variation in a dataset of inpatient microbiology culture results derived from the day-to-day operation of a surgical ICU.

Methods: All final microbiology culture results from surgical intensive care (SICU) patients over a complete 7-year period were obtained (2002-01-01 – 2008-12-31). Cultures were identified by site, organism, and date of collection. Incidence of infectious organisms was stratified by month regardless of year.

Results:
1. Out of an estimated 12800 total SICU patients, 1622 had one or more positive cultures with 141 different pathogens or pathogen classes represented.
2. Variations over the year from the overall mean behavior are present in many species. One interesting example is Acinetobacter baumannii (Figure) where the spring months present a significant deficit of positive cultures [2].
3. Our data rejects the null hypothesis that that the spring months (Feb-May) are no different from the overall mean (binomial test, p<0.05).
4. Grouping the data in chunks of two years allows the 12-month periodicity to be marginally detected by an autocorrelation (p<0.05).
5. Certain pathogens were not present in sufficient numbers to make strong inferences about the seasonality, or lack thereof.

Conclusion: The seasonal variation observed may be consistent with the nature of Acinetobacter as a soil pathogen. Increased vigilance for certain pathogens, especially those which may be highly resistant to antibiotic therapy, may be warranted at certain times of the year.

References: [1] Freeman, J; Anderson, D; Sexton, DJ. 2009. Emerging Evidence for Seasonality of Gram-Negative Bacterial Infections. Infection control and hospital epidemiology 30 (8): 813-814. [2] McDonald, LC; Banerjee, SN; Jarvis, WR. Natl Nosocomial Infections Survey. 1999. Seasonal variation of Acinetobacter infections: 1987-1996. Clinical infectious diseases 29 (5): 1133-1137. [3] Retailliau HF, Hightower AW, Dixon RE, Allen JR. 1979. Acinetobacter calcoaceticus: a nosocomial pathogen with an unusual seasonal pattern. J Infect Dis. Mar;139(3):371-5.

Figure: Positive cultures of A. baumannii per month in the SICU 2002-2008 (dark grey bars). As a comparison, the MacDonald et al. (1999) data set is show in the lighter bars. The fitted curve is a 3-component Fourier description of the latter set.