Over the past decade, MRSA infections have become increasingly problematic in some community-based settings, most notably among MSM (1
). This study reviewed the treatment and clinical outcomes of culture-confirmed SSTI due to S. aureus
among outpatients at a community health care center serving a large population of MSM. The percentage of S. aureus
infections due to MRSA increased significantly, with MRSA accounting for 75% of SSTI by 2005. Interestingly, MRSA infections have increasingly become the presenting condition that leads to a diagnosis of HIV+
among FCH patients. Although patients with HIV infection/AIDS are known to be at risk for serious bacterial infections (22
), this association has been reemphasized with the emergence of community-associated MRSA (21
In the late 1990s, most MRSA cases were initially treated with oral beta-lactam antibiotics, to which MRSA isolates are insensitive. By 2005, with greater recognition of MRSA as a major cause of SSTI at FCH, clinicians tended to choose TMP-SMX for empirical treatment for suspected S. aureus SSTI. In parallel, a greater percentage of clinical S. aureus isolates were sensitive to the empirical antibiotic and a greater proportion of cases resolved clinically on the empirical antibiotic. The nonsignificant decrease in the proportion of MSSA cases that clinically resolved on empirical therapy over the study period was due to clinicians changing to beta-lactam antibiotics once wound culture data were available, not to a lack of susceptibility of MSSA to TMP-SMX (Table ).
The increase in the frequency of obtaining clinical cultures for patients with skin and soft tissue infections likely reflects growing awareness by FCH clinicians of the high prevalence of MRSA among the patient population and the importance of antimicrobial susceptibility testing for these infections. It is also possible that the increase in clinical cultures was due to an increase in deep-seated infections from which culturable material could be collected. The classification of infections as deep seated or not was not always possible with certainty based on descriptions provided in the medical record. While the observation that the percentage of S. aureus skin and soft tissue infections undergoing incision and drainage significantly increased over the study period suggests a possible increased prevalence of deep-seated infections, such a change may simply reflect the evolution of clinical management of SSTI among FCH clinicians. A prospective study would be needed to answer this question definitively.
Awareness that MRSA is an important cause of community-onset SSTI has not spread to all centers, and patients may still receive monotherapy with agents lacking activity against MRSA, such as cephalexin or dicloxacillin (9
). The clinical importance of inactive empirical therapy has not always been clear, as one group found no significant association between inactive empirical therapy and clinical outcomes (4
). However, these researchers interviewed only a subset of their total MRSA cases, and outcomes were assessed via patient interviews, which may be less reliable than medical-record review. In the current study, information regarding the clinical response to therapy was available for the vast majority of patients in the sample. Furthermore, the prevalence of HIV was only 9% among persons over 18 in the study by Fridkin et al. compared to more than 40% of MRSA patients in our investigation (4
). It is possible that treatment of S. aureus
SSTI with the antibiotics showing the most in vitro efficacy has a more important role in immunocompromised individuals; however, the data from the present study do not support this hypothesis. While incision and drainage are likely to be important for resolution of deep S. aureus
), we found that use of active empirical therapy was significantly associated with improved odds of clinical resolution after incision and drainage were controlled for.
Our data suggest that TMP-SMX is an appropriate empirical oral antibiotic for the outpatient treatment of MRSA SSTI. FCH clinicians have tended to choose TMP-SMX for empirical therapy of MRSA SSTI because it is inexpensive, few isolates at FCH have shown in vitro resistance to it, and clinical response has been satisfactory.
The incidence of intolerance or adverse effects severe enough to discontinue TMP-SMX therapy was low, even though high doses were routinely prescribed (one or two double-strength tablets twice daily). Moreover, TMP-SMX resistance was uncommon, consistent with previous characterizations of community-acquired MRSA resistance patterns (2
). The role for TMP-SMX in the treatment of MRSA infections has recently been reviewed in detail (6
). Although data regarding antimicrobials for community-onset MRSA are limited (3
), Markowitz et al. found that TMP-SMX had clinical efficacy similar to that of vancomycin for treatment of nonendocarditis S. aureus
infections, 47% of which were MRSA in their study (13
). TMP-SMX may be an attractive option for outpatient treatment of MRSA SSTI due to its oral formulation and low cost (6
). Clinicians should bear in mind, however, that additional antibiotic coverage is necessary if infection with group A Streptococcus
is suspected (17
The high prevalence of clindamycin resistance in this study limits clindamycin's usefulness as empirical therapy for suspected MRSA SSTI at Fenway. Data regarding inducible clindamycin resistance in isolates that appeared sensitive to clindamycin would be useful, considering the high prevalence among MRSA isolates of erythromycin resistance (Table ), which is associated with inducible clindamycin resistance (11
). One limitation of this study is the absence of additional supporting laboratory data, such as pulsed field gel electrophoresis analysis to classify strains (16
Furthermore, it is possible that the observed increase in clinical resolution of MRSA SSTI on empirical antibiotic therapy could be due to factors not directly related to antibiotic choice, such as changes in the virulence of MRSA strains, improved clinical management, earlier presentation or clinical recognition, or other factors not considered here. Although the mean duration of therapy was modestly increased among patients treated with empirical TMP-SMX, this difference was not statistically significant. Future investigations are needed to evaluate the relative importance of active oral antibiotic therapy and incision and drainage for community-onset MRSA SSTI. The relative efficacies of the oral antibiotics commonly used to treat MRSA SSTI should be compared in a prospective trial.
In summary, this study suggests that TMP-SMX represents an appropriate choice for outpatient empirical therapy of suspected S. aureus SSTI, especially when the prevalence of MRSA in the patient population is significant and resistance of local MRSA strains to clindamycin is common.