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New MRSA Strain Much More Lethal Than Other Types

Cheryl Clark, November 3, 2009

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."

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