CA-MRSA infections, on the other hand, occur in individuals who have not had any contact with healthcare facilities. Higher rates of CA-MRSA infection are observed in settings where individuals have close contact with each other, including military trainees, athletes involved in contact sports, patients age 65 and older, men who have sex with other men, and parenteral substance abusers.8,11-13 However, in view of the high prevalence of CA-MRSA in the U.S., most patients, including those without any apparent risk factors, are at risk.8
HA-MRSA has the ability to survive on inanimate objects for extended time periods, increasing the likelihood of transmission to persons who come into contact with those objects. Although evidence has not confirmed that CA-MRSA has a similar capacity, it seems plausible that such spread does contribute to the propagation of CA-MRSA.12
The increasing importance of CA-MRSA also is evident in hospital settings, where it is replacing HA-MRSA as the most common type of Staphylococcus aureus. Because CA-MRSA tends to be susceptible to a larger number of antibiotics than HA-MRSA is, this has led to a reduced incidence of multidrug resistance. Fortunately, unlike HA-MRSA, CA-MRSA is susceptible to non-beta-lactam antibiotics, including tetracyclines, sulfonamides, and clindamycin.9
CA-MRSA most often causes SSTIs, and a tender abscess is a typical presentation.8 Patients commonly misinterpret early skin lesions as an insect or spider bite.12,14 When cutaneous CA-MRSA presents as an abscess, an incision and drainage procedure is essential for adequate treatment of the infection. For some CA-MRSA infections, particularly those characterized by the presence of a relatively small abscess, it might be adequate to do only an incision and drainage procedure, and not administer antibiotics.8,15 However, in most instances, especially when there is an area of cellulitis around the abscess, the initiation of antibiotic therapy improves patients’ clinical outcomes.9,16
When there is no apparent drainable purulent fluid collection, which often occurs with cellulitis, antibiotics should be the mainstay of therapy. The decision about which antibiotic to start can present some challenges, because the organism causing the cellulitis usually is not identified. This is because blood cultures are positive in less than 5% of cases. Also, positive culture results from needle aspiration are only helpful 5% to 40% of the time. Meanwhile, culture of punch biopsy specimens yields a pathogen in only 20% to 30% of cases.3,17-19
Due to increased CA-MRSA incidence, cephalexin should not be prescribed to treat cellulitis in the outpatient setting because it does not provide coverage for the pathogen.13 Instead, oral antibiotics (e.g. clindamycin or trimethoprim/sulfamethoxazole) should be prescribed. Doxycycline, minocycline, rifampin (usually prescribed in combination with fusidic acid to prevent resistance development), and linezolid are additional therapeutic options.
Trimethoprim/sulfamethoxazole and clindamycin have several advantages: good oral bioavailability, familiarity to physicians, and general affordability. A disadvantage to using both trimethoprim/sulfamethoxazole and doxycycline is that they provide inadequate coverage for group A streptococci, which are a common cause of cellulitis. Therefore, the simultaneous use of a beta-lactam antibiotic with either of these medications may improve outcomes for “nonpurulent” cellulitis.13,15 Linezolid has proven effective for SSTIs caused by MRSA, even though it is not bactericidal.
Red and inflamed leg of a 75-year old female patient, showing cellulitis (at right). Cellulitis is a bacterial infection of the skin and its underlying tissues. The leg was already swollen (lymphedema) from previous lymphoma treatment.
Excellent oral bioavailability of this drug is an attractive characteristic, as it facilitates the transition from the use of intravenous to oral antibiotic therapy later in a patient’s hospital course. Although oral linezolid has been studied in clinical trials and provides good coverage for MRSA, its use in the outpatient setting is relatively limited, largely due to its significant cost.20 In 2008, the cost of 10 days of treatment with oral linezolid was $1,286.80. In comparison, the generic trimethoprim/sulfamethoxazole cost $9.40, and generic clindamycin cost $95.10.8 The lack of routine availability in many outpatient pharmacies also hinders the widespread use of linezolid.13