Incidence and Effects of Seasonality on Nonpurulent Lower Extremity Cellulitis After the Emergence of Community-Acquired Methicillin-Resistant Staphylococcus aureus

Abstract

Nonpurulent lower extremity cellulitis (NLEC) is a common clinical diagnosis, with β-hemolytic streptococci and Staphylococcus aureus considered to be the most frequent causes. In 1999, the US Public Health Service alerted clinicians to the presence of community-acquired methicillin-resistant S aureus (CA-MRSA) infections in 4 children in the upper Midwest. Since then, it has become a well-recognized cause of skin and soft-tissue infections, in particular, skin abscess. A previous population-based study of NLEC in Olmsted County, Minnesota, reported an unadjusted incidence rate of 199 per 100,000 person-years in 1999, but it is unknown whether CA-MRSA subsequently has affected NLEC incidence. We, therefore, sought to determine the population-based incidence of NLEC since the emergence of CA-MRSA. Age- and sex-adjusted incidence (per 100,000 persons) of NLEC was 176.6 (95% CI, 151.5-201.7). Incidence differed significantly between sexes with age-adjusted sex-specific rates of 133.3 (95% CI, 104.1-162.5) and 225.8 (95% CI, 183.5-268.0) in females and males, respectively. Seasonal incidence differed, with rates of 224.6 (95% CI, 180.9-268.4) in warmer months (May-September) compared with 142.3 (95% CI, 112.8-171.9) in colder months (January-April and October-December). Despite emergence and nationwide spread of CA-MRSA since 1999 in the United States, the incidence of NLEC in Olmsted County was lower in 2013 than in 1999, particularly in females. This suggests that CA-MRSA is not a significant cause of NLEC and that NLEC cases are seasonally distributed. These findings may be important in formulation of empirical therapy for NLEC and in patient education because many patients with NLEC are prone to recurrent bouts of this infection.

Figure 1

Relationship of Age and Sex with Incidence of Non-purulent Lower Extremity Cellulitis in Olmsted County (2013) Note: Incidence data specifying counts of NLEC cases (numerator) and counts of total at-risk adults (denominator) were organized in strata specific to sex and each single year of age (18–100 years). Using this granular form of data, smoothing techniques (loess algorithm) were used to estimate incidence in relation to age and sex. Symbols represent sex-specific incidence rates estimated according to age groups (those shown in Table 2) to provide a crude verification of the smoothed trends. In a multivariable Poisson regression model, rates of incidence were higher in males and increased with age for both sexes (P<.001 for both age and sex effects).