MRSA strain hospital stays triple

Collegeville, Pa. - U.S. hospitalizations related to the USA300 strain of methicillin-resistant Staphylococcus aureus (MRSA) more than tripled between 2004 and 2008, Medscape Today reports.

Overall hospitalization rates for S. aureus increased only moderately during that same time period.

Increases in the USA300 strain previously have been documented outside the hospital and among skin and soft tissue infections, but a new study documents the rise among invasive disease at the hospital level, according to lead author Robertino Mera, M.D., Ph.D., director of the Research Statistics Unit of GlaxoSmithKline in Collegeville.

Researchers combined data from the Surveillance Network and the Nationwide Inpatient Sample, identifying three major groups of clonal complexes to correspond with 89 percent of all isolates during the four-year study period. Two of these groups are methicillin-resistant and the other is methicillin-sensitive.

The USA300 strain represented 7.3 percent of all S. aureus hospitalizations in 2004, with a hospitalization rate of 1.03 ± 0.08 per 1,000 discharges. By 2008, the strain represented 23.9 percent of all S. aureus hospitalizations, with a rate of 3.62 ± 0.24 per 1,000 discharges.

Medscape Today quotes Dr. Mera as saying, “The strength of our study is to show (increases in) incidence rates at the national level and among invasive diseases such as pneumonia and bacteremia,” adding that the increase has been greater among children than adults.

“The key factor that explains the expansion of the clonal group USA300 is its capacity to transmit and invade, coupled with innate resistance to most penicillins and macrolide (azithromycin, clarithromycin) antibiotics,” Dr. Mera says.

The good news, he says, is that more recent data from the Centers for Disease Control and Prevention will probably show a plateau in the USA300 strain.

The study, presented at the recent 21st European Congress of Clinical Microbiology and Infectious Diseases in Milan, was supported by GlaxoSmithKline (GSK), which is developing an S. aureus vaccine. Dr. Mera is a GSK employee.