The bacterial scourge methicillin-resistant Staphylococcus aureus (MRSA) can cause life-threatening infections that are difficult to beat with antibiotics. Earlier this year, hospitals in France reported a decline in the rates of the infections after 15 years of concerted control efforts, and ongoing analyses from the U.K. have shown decreasing incidence as well. A new study from the U.S. Centers for Disease Control and Prevention (CDC) reports the number of health care-related MRSA bloodstream infections in nine cities across the country dropped substantially between 2005 and 2008.

Although S. aureus lives in relative biotic harmony on about a third of all people, its drug-resistant varieties can evolve rapidly and spread easily in both health care and, more recently, community settings.

The new analysis, published online August 10 in JAMA, Journal of the American Medical Association, assessed 21,503 reported cases of MRSA infection in selected metropolitan areas of California, Colorado, Connecticut, Georgia, Maryland, Minnesota New York, Oregon and Tennessee. Some 17,508 of these infections were linked to contact with health care settings. The researchers, led by Alexander Kallen of the CDC's Division of Healthcare Quality Promotion, found that after adjusting for age and other variables, the number of invasive MRSA infections contracted while in the hospital declined an average of 9.4 percent each year—28 percent in total—during the course of the four-year period. And the number of infections acquired through overall contact with health care settings declined 5.7 percent per year—some 17 percent summed.

The researchers behind the new report suggest one possible explanation for the decrease could be the greater "dissemination of MRSA prevention practices in U.S. hospitals," such as increasing education about how to detect MRSA and prevent its transmission within the hospital. But authors of an editorial published in the same issue of JAMA point out that many of the declines reported both in the U.S. and Europe seem to have started before many of these formal programs were instituted. "The observed declines in MRSA reported may instead be the result of general infection control efforts, such as wider adoption of alcohol-based hand rubs, improved hand hygiene compliances," the authors, Eli Perencevich and Daniel Diekema, both of the University of Iowa Carver College of Medicine, suggested.

Other possible reasons for the decline, the study authors proposed, include a reduction in average hospital stay or a change in MRSA strains.

But focusing on MRSA alone might be a misguided approach if the goal is to vanquish the infection altogether, Perencevich and Diekema noted. "Surveillance programs that are limited to MRSA will always provide an incomplete epidemiological picture," they wrote. "Natural biological trends, including the emergence and disappearance of specific clones, are likely to override the best-laid attempts at infection control. These trends affect the human epidemiology of all S. aureus disease, not solely MRSA."

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