MRSA on the rise

December 1, 2007
Bob Roehr

Bob Roehr is a medical writer based in Washinton, D.C.

The first active population-based study of invasive methicillin-resistant Staphylococcus aureus (MRSA) has found the infection to be more widespread than previously thought. Its publication in JAMA on October 17 triggered massive ongoing "panic" news coverage of the infection, particularly the community-associated (CA) variant.

Key Points

The study's publication in the Journal of the American Medical Association on Oct. 17 triggered intense news coverage of the infection, particularly the community-associated (CA) variant.

Much of the coverage focused on outbreaks, or even individual cases, within school systems and universities. There was little coverage of infections in healthcare (HA) settings, where the majority and the more deadly of the infections are more likely to occur.

The focus was invasive MRSA, with 8,987 observed cases during the reporting period. Most were HA-MRSA, often with disease onset occurring within the community (5,250, 58.4 percent); hospital-onset infections were next (2,389, 26.6 percent), then community-associated infections (1,234, 13.7 percent), while a portion could not be classified (114, 1.3 percent).

Infection, death rates

The rate of invasive MRSA was an astounding 31.8 per 100,000. Extrapolating from the sample - the sites covered 5.6 percent of the population - the authors estimated that 94,360 cases of invasive MRSA occurred in the United States in 2005, and were associated with 18,650 deaths. That is greater than the number of Americans who die each year of AIDS.

The initial goal of the study was to measure community-associated disease, "but because the epidemiologic characteristics of cases with hospital and community strains were starting to mix, we said, 'Let's look at these across the board,'" lead author R. Monina Klevens, D.D.S., M.P.H., tells Dermatology Times. She is with the surveillance branch of the CDC.

"We did not look at a cohort that started with skin or soft tissue infections; we only focused on invasive disease. So, it might be a little harder for dermatologists to draw comparisons with what they are seeing in their daily practice," Dr. Klevens says.

"But there were cases with concurrent cellulitis. Among these, we know the mortality was low compared with the other syndromes; it was 6.1 percent, which was the lowest of all of the syndromes," she says.

Dr. Klevens says researchers did not perform an analysis by comorbidity, but she readily acknowledges that the hospitalized patients were likely to be older, in poorer health and have weaker immune function.

"We didn't control for all of those factors," she says.

The natural history of MRSA within specific patient populations - who is more likely to progress to invasive disease and who is not - is not well-defined. Dr. Klevens laments that resource limitations preclude following a cohort of patients to get a better sense of those patterns

She says the 39 percent of labs supplying samples was a convenience sample that was not representative.

"We don't want to over-interpret the results," she says. "The goal was to try to capture strain diversity, so each site went to the labs where they felt they were seeing more of the less common, community-associated infections, to make sure we were getting those isolates. By no stretch of the imagination are the isolates representative."

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