MRSA: No white flag

May 1, 2007

Community-acquired MRSA, which often presents cutaneously, is a growing public problem that challenges dermatologists. In healthy adolescents and adults with superficial infections, many MRSA strains will respond to antibiotics to which the strains are not sensitive, such as cephalosporin or clindamycin.

Key Points

Since many community-acquired MRSA infections present on the skin, dermatologists are bound to feel the brunt.

Some areas of the country are particularly hard hit by the infection, says Moise L. Levy, M.D., pediatric dermatologist, professor of dermatology and pediatrics at Baylor College of Medicine and chief of the dermatology service at Texas Children's Hospital, Houston.

The community-acquired strain can be traced to the outpatient healthcare setting, including doctors' practices, and to emergency department visits that are 72 hours and less in duration.

MRSA differences

As distinguished from other types of staph infections, MRSA has a fairly narrow pattern of antibiotic resistance and a typical, or predictable, pattern of antibiotic activity.

Most cases are going to be sensitive to clindamycin, trimethoprim sulfamethoxazole, tetracycline (in particular doxycycline or minocycline), as well as systemic vancomycin and the quinolones, according to Dr. Levy.

"Clinically, MRSA presents as deep-seated hair follicle infections. It can also present as infected eczema or atopic dermatitis, as well as any other primary skin disease (but secondarily infected)," he says.

Dermatologists should culture suspected cases. But, in reality, many may not perform cultures when patients walk through their clinic doors with superficial skin infections. These patients are often arbitrarily started on a cephalosporin, Dr. Levy says.

Relapse risk

Even though the antibiotic sensitivity pattern might suggest otherwise, it is clear that many MRSA strains found in healthy adolescents and adults will respond to antibiotics to which the strains are not sensitive, such as cephalosporin or clindamycin.

"Those are the cases that are more likely to relapse," Dr. Levy says. "Even in the setting of resistance, according to standard antibiotic sensitivity testing, it has been seen clinically that superficial infection will, in fact, spawn in healthy hosts but are more apt to relapse."

The issue of resistance is becoming more of a concern with clindamycin. In Houston, for example, resistance of community-acquired MRSA in the last three years or so has gone from 2 percent to 3 percent resistance to near 9 percent, according to Dr. Levy.

Forms of resistance

There are two forms of resistance to clindamycin.

The most important form for dermatologists to consider is inducible resistance, which is genetically driven. It is a genetic trait that is passed from erythromycin to clindamycin. The only way to detect inducible clindamycin resistance is with a D test - not a standard practice at all labs, Dr. Levy says.

"What you do not know from this standard testing is whether that particular strain is manifesting inducible resistance to clindamycin from erythromycin. If you have inducible resistance based upon a positive D test, you will get a reading that says it is clindamycin-resistant, which alerts the physician not to use it," he says.

Another caveat regarding MRSA is that while most of these infections involve skin and soft tissue and are uncomplicated, some can present as deep-seated pneumonias, osteomyelitis and the like. Especially the more invasive disease is often associated with strains manifesting as Panton-Valentine leukocidin (PVL). PVL is a protein which is increasingly occurring in MRSA according to Dr. Levy.

"The strain of MRSA that is predominant in our country is called the USA 300. That strain happens to elaborate PVL quite frequently. What that means is, clinically, we might end up having more invasive disease, although most cases of community-acquired MRSA are skin and soft tissue," he says.