An uncommon type of MRSA called USA600 appears much more lethal when it infects the bloodstream than more common strains of methicillin-resistant Staphylococcus aureus.
That's the finding from a preliminary study presented Saturday at the Infectious Disease Society of America's annual meeting in Philadelphia.
Infectious disease researcher Carol Moore, PharmD, said her study examined a sample of 80 patients hospitalized with MRSA at Henry Ford Hospital in Detroit over the last three years. Of the 80, 16 had USA600 in their bloodstream. Of those, eight, or 50%, died within 30 days. Of the remaining 64 who had bloodstream infections with other types of MRSA, seven, or 11%, died within 30 days.
"These are obviously very preliminary results in a small number of patients, but it indicates we may have a serious problem with this strain," Moore says. "We need to increase awareness of it, and continue research with larger studies and more analysis to determine what factors are important with this bacteria."
In general, she says, more common types of MRSA infections of the bloodstream have an average 20% to 30% mortality rate within 30 days.
Moore says the 16 patients who were infected with USA600, did tend to be slightly older and sicker than those with other types of MRSA, "but that doesn't fully explain" the increased mortality, Moore says. "We've studied older patients with MRSA in the bloodstream in the past, but we've never seen mortality rates this high."
The average age of patients with USA600 was 64 while the average age of patients with other MRSA strains was 52.
What should also concern healthcare providers is that USA600 appears more likely to be passed on within healthcare settings. Of the 16 USA600 patients, 15 (94%) had a prior tie to a healthcare setting, such as a nursing home, dialysis center or had recently been treated in an acute care hospital, Moore says. In contrast, 42 of the 64 patients with other types of MRSA (65%) had a prior connection with a healthcare setting.
At the Centers for Disease Control and Prevention, health communication specialist Rosa Herrera says USA600 "are still quite rare in the nation." She adds that of all MRSA samples tested by the CDC, "fewer than 1% were USA600."
"It's on our radar, but it's something that still looks to be relatively rare," Herrera says, adding "it is something that needs to be studied further."
At Henry Ford, Moore says the USA600 is only partially resistant to the first line antibiotic for MRSA infections, vancomycin. However, it is more resistant to that antibiotic than most other MRSA bacteria.
According to CDC statistics, the estimated number of people developing a serious MRSA infection in 2005 was about 94,360 and approximately 18,650 persons died during a hospital stay related to that infection.
Moore says her study was launched after hospital officials noticed that among a small number of patients with this strain of MRSA, 60% had died within 30 days. "That was so much higher than what we had seen with other strains," she says.
She says the hospital is conducting research to determine if the bacteria have some identifiable characteristics of virulence.
"We are conducting other studies to see if there is a toxin that may be causing this, but this study doesn't answer that."
Cheryl Clark is a senior editor and California correspondent for HealthLeaders Media Online. She can be reached at cclark@healthleadersmedia.com.