ATLANTA – Nearly 19,000 people died in the United States in 2005 after being infected with virulent drug-resistant bacteria that have spread rampantly through hospitals and nursing homes, according to the most thorough study of the disease’s prevalence ever conducted.

The government study, which is being published today in The Journal of the American Medical Association, suggests that such infections may be twice as common as previously thought, according to its lead author, Dr. R. Monina Klevens.

If the mortality estimates are correct, the number of deaths associated with the germ, methicillin-resistant Staphylococcus aureus, or MRSA, would exceed those attributed to HIV-AIDS, Parkinson’s disease, emphysema or homicide each year.

By extrapolating data collected in nine locations, the researchers estimated that 94,360 patients developed an invasive infection from the pathogen in 2005 and that nearly one of every five, or 18,650 of them, died. The study points out that it is not always possible to determine whether a death is caused by MRSA or merely accelerated by it.

The authors, who work for the U.S. Centers for Disease Control and Prevention, cautioned that their methodology differed significantly from previous studies and that direct comparisons were therefore risky. But they said they were surprised by the prevalence of serious infections, which they calculated as 32 cases per 100,000 people.

In an accompanying editorial in the medical journal, Dr. Elizabeth A. Bancroft, an epidemiologist with the Los Angeles County Department of Public Health, characterized that finding as “astounding.” The prevalence of invasive MRSA – when the bacteria has not merely colonized on the skin, but has attacked a normally sterile part of the body, like the organs – is greater, she wrote, than the combined rates for other conditions caused by invasive bacteria, including bloodstream infections, meningitis and flesh-eating disease.

The study also concluded that 85 percent of invasive MRSA infections are associated with healthcare treatment. Previous research had indicated that many hospitals and long-term care centers have become breeding grounds for MRSA because bacteria may be transported from patient to patient by doctors, nurses and unsterilized equipment.

“This confirms in a very rigorous way that this is a huge health problem,” said Dr. John A. Jernigan, the deputy chief of prevention and response in the CDC’s division of healthcare quality promotion. “And it drives home that what we do in health care will have a lot to do with how we control it.”

The findings are likely to further stimulate an already active debate about whether hospitals and other medical centers should test all patients for MRSA upon admission.

Some hospitals have had notable success in reducing their infection rates by isolating infected patients and then taking extra precautions, like requiring workers to wear gloves and gowns for every contact.

But other research has suggested that such techniques may be excessive, and may have the unintended consequence of diminishing medical care for quarantined patients.

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