A 65-year-old male with no significant past medical history, recently returned from a trip to the Democratic Republic of the Congo, presented with pain, swelling, and ulceration of his right lower leg. The symptoms had progressed despite oral amoxicillin/clavulanate. Evaluation at the time of admission revealed a large fluid collection in the anterior calf with extensive subcutaneous edema. Blood cultures were positive for methicillin–resistant S. aureus susceptible to clindamycin, erythromycin, tetracycline, trimethoprim-sulfamethoxazole, gentamicin, and tetracycline. His infection was successfully treated with surgical debridement, wound care, and vancomycin.
In 1941, Skinner and colleagues described the seriousness of S. aureus bloodstream infections in their series of 122 consecutive patients. The mortality rate was greater than 80% (1). Despite early success with penicillin the subsequent decades have shown this organism to be capable of elaborating resistance mechanisms that make therapy increasingly difficult (2). Methicillin resistance, which first appeared in the 1960s, has come to characterize many of the S. aureus isolates that are identified in the hospital. Recently, distinct strains of methicillin-resistant S. aureus (MRSA) are more commonly being identified in patients presenting for care from the community. This review will discuss recent developments in the clinical presentation and epidemiology of community-acquired MRSA in adults.
Definitions and Epidemiology
For infection control and epidemiological purposes, infections have been traditionally termed nosocomial if they 1) were not incubating at the time of presentation, 2) developed more than 72 hours after hospital admission, or 3) occurred in patients who were recently discharged from the hospital or who reside in a long-term care or skilled nursing facility. Beyond epidemiology, these definitions have been useful in helping the practicing clinician to employ effective empirical antibiotic therapy. The delivery of health care has evolved, however, and the distinction between outpatients and inpatients has been blurred. A broader term that has been suggested for infectious maladies in experienced patients who have moved in and out of the hospital is “healthcare associated” infections (3).
The evolving understanding of the origin of an infection has influenced efforts to define community MRSA. The term “community onset” or “community associated” MRSA can be used to describe a methicillin-resistant S. aureus infection that began incubating outside the hospital. If a patient has historical ties to a traditional treatment setting, the infection is most likely healthcare associated. Notable risk factors include hospitalization or stay in a nursing facility within the past year, use of broad-spectrum antibiotics, surgery, dialysis, intravenous drug use, or the presence of an indwelling vascular catheter. A MRSA infection in a patient presenting from home without any healthcare risk factors can be deemed “community acquired” MRSA (CaMRSA) (4).
A further understanding of CaMRSA can be gleaned from molecular studies of the organism. Methicillin resistance is mediated by a genetic element called staphylococcal cassette chromosome mecA (SCCmecA). MecA codes for a novel penicillin binding protein, PBP 2a, which is not inhibited by beta-lactam antibiotics (2). There are at least 5 types of SCCmecA. Types I through III are typically present in nosocomial MRSA strains. CaMRSA is distinguishable by the presence of SCCmecA IV (4-6).
Another distinctive feature of CaMRSA is the presence of the Panton-Valentine leukocidin (PVL). Previous work has shown that only 2–3% of strains of S. aureus produce this toxin (7). However this virulence factor, encoded by the genes lukS-PV and lukF-PV, appears to be expressed much more commonly in CaMRSA.
The difficulties with defining CaMRSA have influenced attempts to understand its prevalence. The key question in reviewing the available studies is how rigorous an attempt was made to exclude those patients who had significant healthcare contact. Salgado and colleagues performed a meta-analysis to try to determine the prevalence of true CaMRSA. They found that a significant number of subjects included in prevalence studies had identifiable healthcare risk factors, and that when this was accounted for, the overall prevalence of CaMRSA was less than 0.24% (8). The burden of CaMRSA infection will vary however based on location, and certain areas of the United States have demonstrated a higher prevalence. Researchers from the Emerging Infections Program Network examined CaMRSA in Atlanta, Baltimore, and Minnesota and found the prevalence to range from 8% to 20% (9). Of note, only 41% of suspected cases of CaMRSA were confirmed through interviews.