Epidemiology of Methicillin-Resistant Staphylococcus Aureus Bloodstream Infections from 2013-2017 at Chiang Mai University: What Changes Occurred from 2007-2011?

Abstract Background: Methicillin-resistant Staphylococcus aureus (MRSA) is an established pathogen that causes hospital- acquired infections worldwide. Bloodstream infection is associated with significant morbidity and mortality. We conducted a study aimed at describing the epidemiology of MRSA bloodstream infections, to determine the minimal inhibitory concentration (MIC) of the antibiotic vancomycin among MRSA isolates, and to determine the rate and risk factors of mortality.Methods: A retrospective study was conducted among patients aged ≥ 18 years whose blood culture grew MRSA at Chiang Mai University from January 2013 to December 2017Results: The annual prevalence of MRSA in S.aureus bloodstream infections from 2013 to 2017 were 32.8, 23.1, 26.8, 19.2 and 15.4%, respectively. This prevalence showed a non-significant decrease (p = 0.086). Eighty-four patients with 84 episodes of MRSA bloodstream infections were enrolled. Fifty-three patients (63.1%) were male, and the median age was 68.5 years (IQR 56, 79). Fifty-eight patients (69%) had bloodstream infections with other sites of infection: pneumonia (28 episodes, 43.1%), skin and soft tissue infections (16 episodes, 24.6%), osteomyelitis (7 episodes, 10.8%), infective endocarditis (4 episodes, 6.2%), septic arthritis (4 episodes, 6.2%), arterial graft infections (4 episodes, 6.2%), and urinary tract infections (2 episodes, 3.1%). Percentage of patients with vancomycin MICs ≥ 1.5 mg/L were 68.2%, 62.5%, 47.4%, 26.7%, and 75% from 2013 to 2017, respectively. (p = 0.325). The mortality rate was 64.3%. There was no significant difference in mortality rate between those infected with MRSA with a MIC of vancomycin < 1.5 and ≥ 1.5 mg/L (p = 0.172). Factors associated with mortality included age ≥ 40 years old (OR 11.35; 95% CI: 1.35–95.78, p = 0.026), presence of alteration of consciousness (OR 11.19; 95% CI: 2.83–44.18, p = 0.001) and concurrent pneumonia (OR 4.44; 95% CI: 1.09–18.14, p = 0.038).Conclusions: Methicillin-resistance among Staphylococcus aureus bloodstream infections showed a non-significant decrease of 50%, from 32.88% and 15.4%, between 2013 and 2017. Concurrent infection with pneumonia increased mortality. Although the vancomycin MIC was unchanged from 2013 to 2017, the mean MICs were > 1.0 mg/L. Careful monitoring of vancomycin MIC creep is crucial for the selection of the appropriate antibiotic dosage to prevent treatment failure.