Update on Methicillin-Resistant Staphylococcus Aureus (MRSA)
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Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.

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Update on Methicillin-Resistant Staphylococcus Aureus (MRSA)

I have written two previous posts related to infections caused by methicillin-resistant Staphylococcus aureus (MRSA). Because MRSA infection is such an important topic in medicine, it is time for an update. In the July, 2007 issue of The New England Journal of Medicine, there is an excellent article entitled "Skin and Soft-Tissue Infections Caused by Methicillin-Resistant Staphylococcus aureus" (New Engl J Med 2007;357:380-90), authored by Robert S. Daum, M.D., C.M.

Here is some important information from the article:

MRSA refers to bacteria of the Staphylococcus aureus species that are resistant to all currently available "beta-lactam" antibiotics, including penicillins and cephalosporins (such as cephalexin, or "Keflex"). These resistant bacteria originally appeared in hospitals, but have now appeared in the community, caused by new strains of resistant microbes. The community-associated MRSA strains appear to be susceptible to the antibiotic clindamycin, but this is not absolute.

Reports suggest that the new community-associated strains may be easily transmitted between humans, including within the settings of households, military installations, and among athletes. Current epidemiology suggests that MRSA infections are on the rise.

Skin and soft-tissue infections represent most of the clinical manifestations of community-associated MRSA. These infections often become necrotic, with severe tissue breakdown, and may be incorrectly attributed to spider or insect bites. They often lead to abscess formation.

Treatment regimens are in evolution. One approach is to use topical antibacterial therapy for superficial skin infections. This is done with bacitracin, alone or in combination with polymyxin and neomycin. Other therapies are topical mupirocin (Bactroban) or the new drug retapamulin (Altabax). However, any of these therapies may prove to be ineffective.

For oral (outpatient) drug treatment, beta-lactam antibiotics should not be considered reliable as presumptive therapy for community-acquired skin and soft-tissue infections. In other words, if a person develops a significant skin or soft tissue infection, it is no longer sufficient to administer a drug like dicloxacillin or cephalexin, because of the risk of MRSA infection. A drug to treat MRSA infection must be included in the prescription. Such drugs include clindamycin, trimethoprim-sulfamethoxazole, or a tetracycline; however, it should be noted that the absolute effectiveness of these drugs for community-acquired MRSA infection has not been rigorously evaluated or compared in clinical trials. It should also be noted that these drugs have their own set of side effects, such as diarrhea caused by the bacteria Clostridium difficile as a side effect of clindamycin therapy. Furthermore, the percentage of isolates of MRSA that is becoming resistant to clindamycin is rising.

If trimethoprim-sulfamethoxazole or a tetracycline is prescribed because of suspicion for a MRSA infection, it is prudent to add a beta-lactam antibiotic to cover possible infection with group A streptococci.

Trimethoprim-sulfamethoxazole used alone for MRSA has met with mixed results. Doxycycline or minocycline should not be considered to be automatically effective substitutes for tetracycline. Rifampin is sometimes used in combination with trimethoprim-sulfamethoxazole or doxycycline to treat MRSA infection, but this is not based on scientific data.

Linezolid, which is a fairly new antibiotic, is active against almost all community-associated MRSA strains, as well as against group A streptococci. The reasons that it is not automatically prescribed are high cost, lack of routine availability, possible side effect of lowerering blood cell counts, and desire to avoid overuse that would lead to bacterial resistance.

Fluoroquinolone antibiotics, such as ciprofloxacin (Cipro), should not be used to treat skin and soft-tissue infections caused by community-acquired MRSA, because of bacterial resistance.

Limiting the spread of MRSA is very important. Some recommendations include:

1. Cover all draining wounds with clean bandages.
2. Wash hands frequently, particularly after contact with a (potentially) contaminated wound.
3. Launder clothing after contact with a contaminated area on the skin.
4. Bathe regularly with use of soap.
5. Avoid sharing items that may become contaminated by contact with wounds or skin bacteria.
6. Clean sports equipment with agents that are effective against MRSA: detergent or disinfectant registered by the EPA, such as quarternary ammonium compounds or a solution of dilute bleach).

photo courtesy of the Public Health Image Library of the Centers for Disease Control and Prevention

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Dr. Paul S. Auerbach is the world’s leading authority on wilderness medicine.