New Highly Virulent Strain of Vancomycin-Resistant MRSA Carries High Mortality Rate

Alice Goodman

November 05, 2009

November 5, 2009 (Philadelphia, Pennsylvania) — The USA600 strain of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia is potentially lethal and is associated with vancomycin resistance, according to a study from the Henry Ford Health System in Detroit, Michigan, reported here at the Infectious Diseases Society of America 47th Annual Meeting. This is bad news, experts warned, because vancomycin is the drug most often used to treat MRSA.

"USA600 was associated with a worse outcome compared with other strains of MRSA. Our study did not explore why patients with USA600 do so poorly. It could be patient characteristics, strain characteristics, or resistance. We need to find out what is going on in other big healthcare systems. If USA600 turns out to be present in systems other than Henry Ford Hospital, we will need studies to identify effective drugs, to monitor the incidence, and to make sure this strain is not spreading," said Carol Moore, PharmD, who presented results.

USA600 occurs mainly in older, sicker patients with more comorbidities. In the current study, 38% of those with the strain were from nursing homes. The study was prompted by the observation that patients with USA600 did worse than patients with other strains of MRSA. It has been reported that this strain has been in Germany and the Netherlands for the past 10 years, but outcome studies in this patient group are lacking.

Dr. Moore and colleagues looked at MRSA blood stream infections at Henry Ford Hospital from 2005 to 2008. Of 420 blood stream infections, 16 were USA600-positive. "USA600 accounted for a small percentage of blood stream infections, under 5%," she said. No increase in the proportion of USA600 strains was seen over the 3-year period, suggesting that the strain is stable, at least in blood stream infections.

"We haven't studied this in skin or respiratory infections. We plan to do that," Dr. Moore announced.

Of the 16 patients with USA600, 80% failed (death within 30 days, positive blood cultures after 10 days, or new MRSA blood stream infection within 30 days of treatment) and 20% got better. Mortality in patients with the USA600 strain was 50% (8 deaths), compared with 11% in those with non-USA600 MRSA. The non-USA600 MRSA group was comprised mostly of USA300 and USA100 strains, and was evenly distributed between these 2 strains. Mortality in USA100 patients was 18% and in USA300 patients was 7%. All deaths occurred within the first 15 days of infection, she said.

Analysis of USA600 isolates in the microbiological research lab at Henry Ford Hospital showed that 50% were vancomycin heteroresistant, meaning that in the presence of vancomycin, resistant subcolonies of the strain emerged. Other lab tests of USA600 showed resistance to vancomycin, clindamycin, trimethoprim, and sulfamethoxazole, Dr. Moore said.

"At this point in time, although it would make sense to switch a patient with MRSA USA600 to a different antibiotic [than vancomycin], there are no data to guide clinicians," she said.

Vancomycin Resistance

"This is one of the first reports on this emerging strain of MRSA. USA600 is particularly resistant to vancomycin, which is worrisome because it is the drug we depend on. Vancomycin has had perhaps the slowest rate of development of resistance among antibiotics. This study suggests we are seeing the loss of good traction with this drug in the USA600 strain," said Paul G. Auwaerter, MD, MD, IDSA program chair and clinical director of the Division of Infectious Diseases at Johns Hopkins University School of Medicine in Baltimore, Maryland.

Dr. Moore reports financial ties with Cubist. Dr. Auwaerter has disclosed no relevant financial relationships.

Infectious Diseases Society of America (IDSA) 47th Annual Meeting: Late-Breaking Abstract 40. Presented October 31, 2009.