Community-Associated MRSA Strains May Be Supplanting Traditional Nosocomial Strains

Community-associated strains of methicillin-resistant Staphylococcus aureus (MRSA) were responsible for an increasing proportion of all nosocomial MRSA cases over a 7-year period at a large inner-city hospital, according to a retrospective study reported in the March 15 issue of Clinical Infectious Diseases.

The researchers note, however, that their study also found that, so far, the community-associated strains are presenting no more of a danger to patients than the traditional, hospital-associated strains.

Dr. Kyle Popovich, at Stroger Hospital of Cook County, Chicago, and colleagues first updated an established algorithm that uses antibiotic susceptibility (to clindamycin and/or ciprofloxacin) and results from pulsed-field gel electrophoresis to infer the genotypes of MRSA strains. They then applied this rule to all 208 hospital-onset (more than 72 hours after admission) MRSA isolates between January 2000 and December 2006.

Although the total incidence density rates for MRSA bloodstream infections were relatively stable over the study period, the risk for hospital-onset MRSA infections due to community genotype (CG) strains increased (risk ratio 1.9, p = 0.01), while the risk for infections due to hospital genotype (HG) strains decreased (risk ratio 0.7, p = 0.02).

Between the period January 2000 - June 2003 and the period July 2003 - December 2006, the proportion of cases due to CG strains doubled from 24% to 49%.

The evaluated clinical outcomes, including duration of bacteremia, length of hospital stay, hospital readmission within 3 months and all-cause in-hospital mortality, were similar between patients infected with traditional hospital strains and those with community-associated strains.

"While our study showed similarities between CG and HG strains," Dr. Popovich told Reuters Health, "studies have shown virulent community MRSA strains exist leading to necrotizing skin infections, pneumonia and bacteremia. Since community MRSA strains are entering the hospital, additional research into appropriate infection control strategies to reduce transmission and infection is needed."

In an accompanying editorial, Dr. John M. Boyce of Yale University School of Medicine gives a variety of recommendations for improving infection-control measures in hospitals, among them making personnel "aware that patients infected with MRSA are usually colonized at multiple body sites (including normal-appearing areas of skin) and that they often contaminate items in their immediate vicinity."


Reviewed by Dr. Ramaz Mitaishvili


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