CA-MRSA emergency room study shows ubiquitous presence

October 1, 2006

National report - Community-acquired methicillin resistant Staphylococcus aureus (CA-MRSA) has become a ubiquitous presence across the United States.

It was the most common identifiable cause of skin and soft tissue infections in a study conducted at 11 university-affiliated emergency rooms during August 2004. The study was conducted by the EMERGEncy ID NET Study Group, and the results appeared in the Aug. 17 issue of New England Journal of Medicine.

"The main message for physicians is to recognize just how common this has become," study leader Gregory J. Moran, M.D., tells Dermatology Times. "In most places in the U.S., this is the most common cause of skin infections."

Nearly half of the 422 patients with skin and soft tissue infections enrolled in the study were non-Hispanic blacks. Dr. Moran, a clinical professor of emergency medicine at University of California, Los Angeles, believes this reflects the populations that participating emergency rooms serve, and sees no evidence that CA-MRSA disproportionately affects any demographic group.

"Doctors used to routinely give Keflex (Lilly) for skin infections, without really thinking about it very much," Dr. Moran says. "But we have to recognize that MRSA has become established in the community. When antibiotics are necessary, it makes sense to use something that has activity against MRSA."

Importance of incision and drainage

A majority of patients in the study received antibiotics. Follow-up showed that it did not matter whether the MRSA was resistant to the drug or not; there was no difference in outcome.

"That just points out the importance of good drainage, which doesn't always get emphasized. I think the patients got better because they were drained, and the antibiotic was unnecessary," Dr. Moran says.

He notes that emergency rooms are set up to do incision and drainage, "but if you aren't set up to do it, it can be a bit of a barrier. It takes five seconds to write a prescription, and sometimes physicians take the easy way out.

"There aren't any well-defined, evidence-based rules on when to use antibiotics," he says. "If it is a simple, limited skin abscess, even if it is greater than 5 cm, if there is not a lot of surrounding cellulitis, if there are no systemic signs such a fever, then I think simple drainage alone should be fine."

When Dr. Moran does use antibiotics, he tends to prescribe short courses of three to five days.

"We still have a number of options that are cheap, generic and pretty safe. We primarily use trimethoprim-sulfamethoxazole, but doxycycline and clindamycin are other options," he says.

Resistance caused by aggressive strain

The USA300 strain was the cause of almost all of these resistant infections.

"Studies have found only small fractions of the population colonized with these strains of MRSA, but because they are more aggressive, they carry these toxin genes like PVL; of those people who are exposed to them, a very high proportion develop a clinical infection," Dr. Moran says. "If you are exposed to it, you are very likely to develop an infection rather than just carrying it asymptomatically.

"We've already gone beyond the tipping point in our hospital, where we just assume that it is MRSA and treat it as such," he says.

He believes it is important for doctors to do enough cultures to understand the prevalence of resistance in their local population, but he does not culture every patient.