A Centers for Disease Control and Prevention official says there's no national evidence so far to suggest that a rare type of methicillin-resistant Staphylococcus aureus, USA600, is more lethal than other MRSA bacteria.
"USA600 strains are an uncommon cause of human infection in the U.S., and we have no evidence to suggest USA600 is any more or less virulent, nor any more or less resistant to antibiotics, than any of the other strains of the bacteria that we encounter," says Brandi Limbago, of the CDC's division of quality promotion.
Limbago was asked to comment on a report last week from Henry Ford Hospital in Detroit in which researchers discovered MRSA bacteria in 80 patients over the last three years. Of the 80, 16 were infected with USA600, one of 12 groupings of MRSA numbered from 100 to 1200. The MRSA type of bacteria is rarely found in hospitals, and was never believed to be much of a threat for patient safety.
But of the 16 patients found to be infected with USA600, 50% died within 30 days. Of the 64 infected with other strains of MRSA, the fatality rate within 30 days was only 11%.
Carol Moore, the researcher who led the study, presented her finding at the Infectious Disease Society's annual meeting in Philadelphia. It caught MRSA experts off guard because previously the bacteria had not been remarkable.
She acknowledges that the study is premature and based on a very small number of patients. But she adds that she wanted to share the finding so the bacteria can be more carefully studied and so health providers can be forewarned.
Paul G. Auwaerter, clinical director of the Division of Infectious Diseases at Johns Hopkins University School of Medicine in Baltimore, and who was on the panel when Moore presented her report, agrees.
"It's too early to tell" if USA600 represents a new threat to patient safety, he says, because the number—16—is too small. "Who knows?" he says. "This may be a signal.
"Detroit tends to be a place where they have an intensive drug using population—and although it wasn't clear whether these patients had been intravenous drug users," he says. "we remembered that in the 1970s, some of the first signs of bad staph aureus came from Detroit.
"This is interesting to point out and it definitely bears more investigation," he says.