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Methicillin-Resistant Staphylococcus aureus Strain Linked to High Mortality Rates: Presented at IDSA

By Ed Susman

PHILADELPHIA -- November 3, 2009 -- A strain of methicillin-resistant Staphylococcus aureus (MRSA) known as USA600 appears to be 5 times more lethal than other strains, and has shown resistance to the potent antibiotic drug vancomycin, often considered one of the drugs of last resort for treatment of the highly antibiotic-resistant pathogen, researchers stated here at the 47th Annual Meeting of the Infectious Diseases Society of America (IDSA).

"We found that 8 of 16 patients identified with USA600 bloodstream infections -- 50% of the patients infected -- died within 30 days of being diagnosed," said Carol Moore, PharmD, Division of Infectious Diseases, Henry Ford Hospital, Detroit, Michigan. In comparison, of 64 patients infected with non-USA600 strains, about 11% died within 30 days.

"Actually, we found that most of the patients infected with USA600 died within 15 days of diagnosis," Dr. Moore stated at her late-breaker poster presentation here on October 31. The 30-day mortality rate for MRSA bloodstream infections historically ranges from 10% to 30%. The USA600 strain, however, contains unique characteristics that may be linked to the high mortality rate.

Dr. Moore said researchers are not sure yet whether the strain is more lethal or the health conditions of the patients in this study -- 40% of whom were infected in nursing homes -- resulted in the high death rate. It is unclear whether other factors, like the patients' older age, diseases, or the spread of infection, contributed to the poor outcomes collectively or with other factors. The average age of patients with the USA600 strain was 64 years; the average age of patients with other resistant S aureus strains was 52 years.

"While many methicillin-resistant S aureus strains are associated with poor outcomes, the USA600 strain was shown to be more lethal and cause high mortality rates," said Dr. Moore. "In light of the potential for the spread of this virulent and resistant strain and its associated mortality, it is essential that more effort be directed to better understanding this strain to develop measures for managing it."

Paul Auwaerter, MD, Johns Hopkins School of Medicine, Baltimore, Maryland, noted, "This strain of methicillin-resistant S aureus appears to have some resistance to vancomycin. However, we do have alternatives now such as daptomycin or linezolid."

Funding for this study was provided by Henry Ford Health System.

[Presentation title: USA600 Methicillin-Resistant Staphylococcus aureus Bacteremia (MRSA-B) Associated With Reduced Vancomycin Susceptibility and Significant Mortality. Abstract LB-40]

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