Drug Topics - January 2009 - (Page 34) Continuing Education COMMUNITY-ACQUIRED MRSA TABLE 1 HA-MRSA strains tend to possess multi-drug resistance, CA-MRSA resistance is typically limited to beta-lactams and erythromycin. Using polymerase chain reaction (PCR), researchers have shown that CA-MRSA appears to have materialized from a strain of MSSA following the acquisition of a staphylococcal cassette chromosome mec (SCCmec). The five SCCmec types contain genes that encode resistance to various antibiotics. The SCCmec gene carried by CA-MRSA strains is typically SCCmec type IV (and on occasion type V). In contrast, HA-MRSA commonly possesses SCCmec types I, II, and III. The difference in SCCmec gene expression between CA-MRSA and HA-MRSA isolates accounts for the disparity in antimicrobial resistance patterns. SCCmec type IV is a smaller element than types I, II, and III. As a result, isolates possessing the SCCmec type IV contain only the mecA resistance gene, which codes for methicillin resistance. In contrast, SCCmec types I, II, and III are larger genetic elements and can contain multiple resistance genes that yield resistance to additional antibiotic classes. Differences in size of SCCmec genes may also provide insight into the rapid spread that has been noted with CA-MRSA. The smaller size of SCCmec type IV is believed to allow for more efficient transfer among isolates. This may be translated into the explosive spread of resistance. Although CA-MRSA may display a more favorable resistance pattern, it is often more virulent than HA-MRSA. This is due to the expression of virulence genes that encode for enterotoxins and Panton-Valentine leukocidin (PVL). Enterotoxins alter the permeability of the epithelial cells of the intestinal wall by forming pores in cell membranes. This allows interstitial fluid to enter the intestine, causing diarrhea. PVL is a white-blood-cell toxin that was characterized approximately 70 years ago. This toxin can cause tissue necrosis and leukocyte destruction. Expression of PVL has been associated with necrotizing skin and soft-tissue infections and severe necrotizing pneumonia among healthy children and young adults. In comparison to CA-MRSA, HA-MRSA rarely displays PVL genes and is typically associated with less-extensive tissue necrosis. Table 1 summarizes HA-MRSA and CA-MRSA resistance and virulence factors. Comparison of HA-MRSA and CA-MRSA resistance and virulence factors MRSA type HA-MRSA SCCmec type I, II, III Resistance Genotypes Virulence genes factors mecA; possibly Tn554, pT181 mecA USA100, USA500, USA800 USA300, USA400 Rare CA-MRSA IV, V Enterotoxins, PantonValentine Leukocidin (PVL) strains of MRSA. They also lack traditional HA-MRSA risk factors. CA-MRSA risk factors noted by the CDC include the “5 C’s”: crowding, contact (usually skin-to-skin), compromised skin, contaminated surfaces, and cleanliness (or lack of it). Thus, most CA-MRSA infections have been linked to settings involving crowding, contact, and compromised hygienic conditions, such as fitness facilities and team sports. Diagnosis The Centers for Disease Control and Prevention (CDC) has published characteristics commonly noted among patients with CA-MRSA infection. Characteristics of infection and patient include the following criteria: a diagnosis of MRSA made in the outpatient setting or on the basis of a positive culture for MRSA within 48 hours after hospital admission; no medical history of MRSA infection or colonization; no history in the preceding year of hospitalization, dialysis, surgery, or admission to a nursing home, skilled nursing facility, or hospice; and no permanent indwelling catheters or medical devices that pass through the skin into the body. Patients who acquire infections with CA-MRSA tend to be younger than those who become infected with traditional Presentation Similar to methicillin-sensitive S. aureus, MRSA can cause infections at a variety of body sites including, but not limited to, the skin and soft tissue, bone, joints, blood, and heart. However, the clinical presentations and epidemiology of patients infected with HA-MRSA and CA-MRSA may vary. HA-MRSA is typically associated with invasive infections, such as infections of the bloodstream, respiratory tract, and urinary tract. In contrast, CA-MRSA causes invasive infections infrequently; it is more commonly associated with skin and soft-tissue infections such as abscesses, cellulitis, folliculitis, and necrotizing fasciitis. Skin and soft-tissue infections caused by CA-MRSA may initially present as small red bumps resembling pimples and are often mistaken for insect bites. Infection rapidly progresses to deep, painful abscesses with necrotic centers. The severity of CA-MRSA skin and soft-tissue infections varies from mild superficial infections to deep soft-tissue abscesses requiring hospital admission. Early recognition and diagnosis is critical, since CA-MRSA can spread quickly. Prompt treatment may stem the spread of infection and decrease need for extensive debridement and other complications. The potential for the infection to progress to necrotizing fasciitis secondary to PVL production must be considered in patients presenting with suspected CA-MRSA skin infections. Table 2 summarizes the differences in the patient population and presentation between HA-MRSA and CA-MRSA. Treatment In the treatment of CA-MRSA, it is important that clinicians recognize that antimicrobial therapy alone may not always W W W.D R U GTO P I C S .C O M 34 DRUG TOPICS Januar y 2009 http://WWW.DRUGTOPICS.COM
Table of Contents Feed for the Digital Edition of Drug Topics - January 2009 Drug Topics - January 2009 Contents Letters Up Front Up Front in Depth Community Practice Drug Pipeline: What to Watch in 2009 OTC Community-Aquired MRSA Infections New Products Viewpoint Drug Topics - January 2009 Drug Topics - January 2009 - Drug Topics - January 2009 (Page Cover1) Drug Topics - January 2009 - Drug Topics - January 2009 (Page Cover2) Drug Topics - January 2009 - Drug Topics - January 2009 (Page 1) Drug Topics - January 2009 - Drug Topics - January 2009 (Page 2) Drug Topics - January 2009 - Drug Topics - January 2009 (Page 3) Drug Topics - January 2009 - Contents (Page 4) Drug Topics - January 2009 - Contents (Page 5) Drug Topics - January 2009 - Contents (Page 6) Drug Topics - January 2009 - Contents (Page 7) Drug Topics - January 2009 - Contents (Page 8) Drug Topics - January 2009 - Contents (Page 9) Drug Topics - January 2009 - Contents (Page 10) Drug Topics - January 2009 - Contents (Page H1) Drug Topics - January 2009 - Contents (Page H2) Drug Topics - January 2009 - Contents (Page H1) Drug Topics - January 2009 - Contents (Page H2) Drug Topics - January 2009 - Contents (Page H3) Drug Topics - January 2009 - Contents (Page H4) Drug Topics - January 2009 - Contents (Page H5) Drug Topics - January 2009 - Contents (Page H6) Drug Topics - January 2009 - Contents (Page H7) Drug Topics - January 2009 - Contents (Page H8) Drug Topics - January 2009 - Contents (Page 13) Drug Topics - January 2009 - Up Front (Page 14) Drug Topics - January 2009 - Up Front (Page 15) Drug Topics - January 2009 - Up Front (Page 16) Drug Topics - January 2009 - Up Front (Page 17) Drug Topics - January 2009 - Up Front in Depth (Page 18) Drug Topics - January 2009 - Up Front in Depth (Page 19) Drug Topics - January 2009 - Community Practice (Page 20) Drug Topics - January 2009 - Community Practice (Page 20a) Drug Topics - January 2009 - Community Practice (Page 20b) Drug Topics - January 2009 - Community Practice (Page 21) Drug Topics - January 2009 - Drug Pipeline: What to Watch in 2009 (Page 22) Drug Topics - January 2009 - Drug Pipeline: What to Watch in 2009 (Page 23) Drug Topics - January 2009 - Drug Pipeline: What to Watch in 2009 (Page 24) Drug Topics - January 2009 - Drug Pipeline: What to Watch in 2009 (Page 25) Drug Topics - January 2009 - Drug Pipeline: What to Watch in 2009 (Page 26) Drug Topics - January 2009 - Drug Pipeline: What to Watch in 2009 (Page 27) Drug Topics - January 2009 - OTC (Page 28) Drug Topics - January 2009 - OTC (Page 29) Drug Topics - January 2009 - OTC (Page 30) Drug Topics - January 2009 - OTC (Page 31) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 32) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 33) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 34) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 35) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 36) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 37) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 38) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 39) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 40) Drug Topics - January 2009 - Community-Aquired MRSA Infections (Page 41) Drug Topics - January 2009 - New Products (Page 42) Drug Topics - January 2009 - New Products (Page 43) Drug Topics - January 2009 - New Products (Page 44) Drug Topics - January 2009 - New Products (Page 45) Drug Topics - January 2009 - New Products (Page 46) Drug Topics - January 2009 - New Products (Page 47) Drug Topics - January 2009 - Viewpoint (Page 48) Drug Topics - January 2009 - Viewpoint (Page Cover3) Drug Topics - January 2009 - Viewpoint (Page Cover4)
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