Trends in incidence and susceptibility among methicillin-resistant Staphylococcus aureus isolated from intranasal cultures associated with rhinosinusitis

Abstract

Background: Reports regarding the incidence and antibiotic susceptibility of methicillin-resistant Staphylococcus aureus (MRSA) in rhinosinusitis (RS) are limited. This study was designed to identify epidemiology and trends of MRSA incidence and antimicrobial resistance in the sinonasal cavities.

Methods: This is a retrospective case series. All intranasal/sinus cultures obtained by otolaryngologists at Stanford over a 20-year period (1990-2010) were retrospectively reviewed by mining the microbiology database. Nested searches were then made for all S. aureus and MRSA cultures. Patterns of incidence and changes in antibiotic susceptibilities were tabulated and statistical analysis was performed.

Results: Our search retrieved 10,387 positive intranasal culture samples, with S. aureus found in 800 (7.7%), and MRSA comprising 110 (1.06%) of this subset. Between the years of 1990 and 1999, only 2/112 (1.7%) of S. aureus-positive nasal cultures were positive for MRSA, with a sharp rise in incidence to 86/606 (14.2%) from 2000 to 2005, and to 22/82, 26.8% from 2006 to 2010. On a percent basis, using logistic regression modeling, this represents a statistically significant increasing trend (p < 0.0001) for MRSA sinusitis. However, over the 20-year interval studied, the patterns of antibiotic resistance among MRSA remained unaltered, especially with regard to trimethoprim-sulfamethoxazole and vancomycin.

Conclusion: S. aureus and MRSA isolates from intranasal cultures, which were essentially absent before the year 2000, became significantly more common earlier this decade. These data show the increased role of MRSA in sinusitis. MRSA antibiotic susceptibilities have remained, however, largely stable during this time period.

Figure 1.

Absolute incidence and logistic regression of methicillin-resistant Staphylococcus aureus (MRSA) incidence over 20 years. (A) Frequency of positive nasal cultures of methicillin-sensitive S. aureus (MSSA), MRSA, Streptococcus spp., Pseudomonas spp., and Haemophilus influenza (y-axis, denoted the number of cases) (B) Trends in positive, intranasal MRSA culture on a percent basis. Error bars denote 95% confidence interval for each year's proportion, the dashed line marks the calculated trend in incidence using logistic regression analysis, and the dotted line marks the quadratic.