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Deadlier Strain of MRSA Emerges

'USA600' Strain Is Partly Resistant to Treatment by Another Antibiotic
By
WebMD Health News
Reviewed by Louise Chang, MD

Nov. 2, 2009 (Philadelphia) -- A newly discovered strain of drug-resistant staph bacteria is five times more deadly than other strains, a new study suggests.

Adding insult to injury, the new superbug appears to have some resistance to the antibiotic commonly used to treat it, researchers report.

Half of patients infected with the new strain of MRSA (methicillin-resistant Staphylococcus aureus) died within 30 days, says Carol Moore, PharmD, a research investigator in infectious diseases at Henry Ford Hospital in Detroit.

That compares to only about 10% of patients infected with other MRSA strains, she tells WebMD.

Moore and colleagues studied 16 people infected with the new strain, called USA600, and 64 people infected with other MRSA strains at their institution.

MRSA strains are typically susceptible to the antibiotic vancomycin, Moore says. But the USA600 strain was at least party immune to vancomycin, she says.

Though the new superbug appears to have unique characteristics that make it deadly, other factors, such as the patients' older age, may have played a role, Moore says. The average age of patients infected with the USA600 strain in the study was 64 vs. 51 for patients with other MRSA strains.

The study was presented at the meeting of the Infectious Diseases Society of America.

While USA600 infections have only been seen in hospitalized patients to date, a growing number of MRSA infections are being seen in otherwise healthy people in the community.

"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," Moore says.

MRSA Spikes in Summer Months

Other new research presented at the meeting shows a spike in MRSA infections in summer months -- probably driven, experts says, by the rise in cases outside the hospital setting in recent years.

Loyola University researchers studied the medical records of more than 400,000 people admitted to a Chicago hospital from 2000 to 2008. They identified about 1,300 people who tested positive for MRSA.

The number of infections did not vary by season during the first four years studied.

But from 2004 to 2008, the risk of being infected was about 30% greater in the summer months than in the winter or spring.

That's likely due to the rise in community-acquired infections, says the CDC's Fernanda Lessa, MD, who tracks MRSA but was not involved with this study.

"We know that the skin is a major entry point for the bacteria, so one would expect more infections in the community in the summer when your wear fewer clothes and are more prone to skin lesions," she tells WebMD.

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