Decreasing incidence of skin and soft tissue infections with a persistent summer peak at an academic medical center, 2006-2014

Background: The incidence of skin and soft tissue infections (SSTIs) in the United States increased sharply after 2000 with the emergence of the USA300 MRSA strain, but few studies provide data after 2010. We examined recent trends in SSTI incidence at the University of Chicago Medicine (UCM).

Methods: Demographic and clinical data were obtained from the UCM Clinical Data Warehouse for patient encounters with an ICD-9-coded SSTI diagnosis in January 2006 - March 2014. Repeat encounters for a single patient in each year were excluded. Incidence density was calculated per 1,000 encounters by quarter and year and stratified by inpatient, outpatient clinic and emergency department (ED) encounters; and by age group, gender and race. Poisson regression was performed to assess change over time.

Results: In 2006-14, data were analyzed for 38,201 SSTI-associated encounters from 31,869 subjects. Among all patients treated at UCM, there was a decrease of 1% per year in the incidence of SSTIs in 2006-13, with an overall decrease of 16%, from 6.17 per 1,000 encounters in 2006 to 5.13 per 1,000 encounters in 2013 (rate ratio [RR] = 0.97, 95% CI: 0.97 – 0.98). There was a significant decrease in SSTI-related encounters among inpatients (RR = 0.97, 95% CI: 0.96 – 0.98), ED patients (RR = 0.98, 95% CI: 0.97 – 0.98), adults (RR = 0.98, 95% CI: 0.97 – 0.98), children (RR = 0.96, 95% CI: 0.95 – 0.97) and African Americans (RR = 0.99, 95% CI: 0.98 – 0.99). SSTIs increased significantly only among clinic outpatients (RR = 1.01, 95% CI: 1.00 – 1.02). Children, compared with adults, had a significantly higher likelihood of having an encounter related to an SSTI (RR = 1.81, 95% CI: 1.77 – 1.85). Compared with outpatient clinic encounters, there was a higher rate of SSTIs among both inpatient (RR = 9.07, 95% CI: 8.80 – 9.35) and ED encounters (RR = 9.22, 95% CI: 9.01 – 9.42). Both overall pediatric and overall ED SSTI encounters exhibited a strong seasonal pattern, with a peak incidence each year during the third quarter.

Conclusion: The incidence of SSTIs at UCM decreased in children and adults treated as inpatients and in the ED. There was marked seasonal variation, peaking during the summer months. This suggests a reversal of the massive increase in SSTI incidence after 2000 in the U.S.