Community Associated Methicillin-Resistant Staphylococcus Aureus: A Review

Michael J. Rybak, PharmD; Kerry L. LaPlante, PharmD


Pharmacotherapy. 2005;25(1):74-85. 

In This Article


In 2000, the CDC began working closely with four states with a combined population of about 12 million persons to study the epidemiology of CA-MRSA infections. Information from these studies is helping the CDC understand the nature of the disease, the reasons why people get infected, and the types of research needed to help prevent these infections. These data are being collected in Connecticut, Minnesota, Georgia, and Maryland as part of CDC's Emerging Infections Program. This program was expanded to six states in 2004.

In one of the more recent studies, a group of investigators characterized the epidemiologic and microbiologic characteristics of both CA- and HA-MRSA in 1100 MRSA infections.[31] Based on pulsed field gel electrophoresis (PFGE) and staphylococcal exotoxin gene testing, they determined that 12% of the infecting strains were CA-MRSA and 85% were HA-MRSA. Seventy-five percent of skin and soft tissue infections were caused by CA-MRSA, whereas 37% were caused by HA-MRSA (odds ratio [OR] 4.25; 95% confidence interval [CI] 2.97-5.90). The CA-MRSA isolates typically possessed different exotoxin gene profiles than the HA-MRSA isolates.

Earlier studies investigated the epidemiology and clonality of CA-MRSA in Minnesota health care facilities.[23] Ten health care facilities contributed data on 354 patients with CA-MRSA from 1996-1998. Patient records were examined for demographic data, and all infection types were recorded. All available isolates were analyzed with PFGE to determine if they were of hospital or community origin. The median age of patients was 16 years (range 1-78 yrs), and the most common infection type was skin and soft tissue (84%). Examination of the isolates by PFGE indicated that most (86%) were distinctly different from HA-MRSA organisms. In addition, as has been found in other evaluations of CA-MRSA strains, these pathogens tended to be more susceptible to antimicrobial agents than HA-MRSA.

To estimate the prevalence of CA-MRSA infections in Finland, a study evaluated MRSA culture-positive patients from a national hospital discharge register.[24] The definition for CA-MRSA was the lack of hospitalization for a minimum of 2 years. Of the 526 patients identified with MRSA-positive status, 21% were determined to have MRSA of community origin. Three MRSA strains were identified as community strains on the basis of phage typing, PFGE, and ribotyping. Of interest, none of the community-acquired strains were multidrug resistant, and all strains demonstrated a mec hypervariable region. The authors concluded that CA-MRSA may arise de novo, through acquisition of the mecA gene.

A hospital-based observational study compared nosocomial and community-acquired S. aureus bacteremias.[22] The researchers classified 32% of all bacteremias as hospital acquired, whereas 18.5% were deemed community acquired. However, on further examination, all patients with CA-MRSA were found to have regular contact with health care settings, making the term community-acquired misleading.

A meta-analysis applied to various types of CA-MRSA publications yielded several sets of key statistics.[32] In nine studies where researchers obtained culture samples before making risk assessments, the pooled MRSA colonization rate was 2.1% (among 4825 patients). Another revealing finding was that nearly one half of patients with CA-MRSA had one or more risk factors for HA-MRSA, and among the remaining 3525 patients the colonization rate of CA-MRSA strains dropped to only 0.2%. Patients from whom samples were obtained in a health care facility were 2.4 times more likely to carry MRSA than community members cultured outside of a health care setting (95% CI 1.56-3.53). Finally, 17.8% of household contacts of patients colonized with HA-MRSA were found to also carry the index strain. To date, the true incidence of CA-MRSA is unknown, since most studies have characterized this organism in a relatively small group of patients over a short, fixed time interval.


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