Invasive MRSA rises in the community

Infection prevention a huge challenge

While the news that methicillin-resistant Staphylococcus aureus (MRSA) had eclipsed the annual death toll of HIV drew most of the attention, there was another disturbing finding in a recently published study that was largely overlooked: Nearly 14% of the invasive MRSA cases found were acquired in the community.

Researchers at the Centers for Disease Control and Prevention analyzed population-based surveillance data for invasive MRSA infections in nine sites participating in the Active Bacterial Core system. Of the 8,987 observed cases of invasive MRSA 1,234 (13.7%) were considered community-associated.1

"The community-associated cases were defined as those with no documented health care risk factor. The majority of those were USA300, so we know that strain evolved de novo in the community," says R. Monina Klevens, DDS, MPH, lead author of the study and an epidemiologist in the CDC's division of health care quality promotion. "There were concerns that community-associated infections were occurring frequently and we wanted to measure that. No one should trivialize 14%. When you just look at the community-associated cases, the rate is 4.6 per 100,000 [people]. That's a lot."

The study found that (58%) of MRSA cases were among patients who had health care risk factors but community onset of disease. The majority of those patients had the USA100 genotype, a typical hospital strain; however, some of those cases were caused by USA300 and could have also been community-acquired, adds Elizabeth A. Bancroft, MD, a medical epidemiologist at the Los Angeles County health department, who wrote an accompanying editorial to the study.2

"When you look at strain typing, it still appears the majority are hospital strains, but there was a significant minority that had the community strain," she tells Hospital Infection Control. "So it wouldn't surprise me if some of those that were classified as health care associated really were purely community acquired. The fact that the person had surgery three months before had absolutely nothing to do with their invasive disease."

Strategies to prevent sporadic community-associated MRSA are not as well described as hospital measures, though hand washing, not sharing personal items, and keeping wounds clean, dry, and covered are commonly mentioned prevention methods, she noted in the editorial. Indeed, the findings not only raise concerns about how fast CA-MRSA is emerging — the CDC reported some of the first cases in 1999 — but beg the difficult question of how to stop staph transmission in the community.

"We need to recognize that the control measures that can even be envisioned for CA-MRSA are problematic at best," says William Jarvis, MD, a former leading CDC hospital outbreak investigator now in private consulting. "Look at prisons, intravenous drug users, and the homeless. How are you going to improve hygiene, reduce close contacts, prevent sharing of towels, soap, and needles that lead to transmission of MRSA? Infection control in the community is very problematic. I've not seen any data by anybody that has shown effective control measures [among such community populations who] are incubating and transmitting MRSA. [They will] increase the burden of disease and allow it to get into other populations."

Complicating the issue, the typically proposed solution of controlling antibiotic use is unlikely to have much impact on the rise of MRSA in the community, particularly among the aforementioned high-risk groups, Jarvis says. "There has been a lot of talk about antibiotic controls, but in the majority of studies of CA-MRSA antibiotics have not been a risk factor," he says. "So to say antibiotic controls are going to have impact on the community-acquired cases is ridiculous. [Improved] hygiene is really the only way."


  1. Klevens RM, Morrian MA, Nadle J. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA 2007; 298:1,763-1,771.
  2. Bancroft EA. Editorial: Antimicrobial resistance — It's not just for hospitals. JAMA 2007; 298:1,803-1,804.