Invasive MRSA More Pervasive Than Suspected

ATLANTA, Oct. 16 -- Invasive infections of methicillin-resistant Staphylococcus aureus (MRSA) may be more common than suspected, according to the first nationwide surveillance estimates.

The incidence of invasive MRSA infections in 2005 was 31.8 per 100,000 people, reported R. Monina Klevens, D.D.S., M.P.H., of the CDC here, and colleagues, in the Oct. 17 issue of the Journal of the American Medical Association.

Their estimate of 94,360 invasive infections was three times as high as the CDC's previous estimate of 31,440 hospitalizations for MRSA bacteremias in 2000, which was derived from discharge coded data rather than surveillance.

The findings were astounding, commented Elizabeth A. Bancroft, M.D., S.M., of the Los Angeles County Department of Public Health in Los Angeles, in an accompanying editorial.

"To put this number into context, the estimated rate of invasive MRSA is greater than the combined rate in 2005 for invasive pneumococcal disease (14.1 per 100,000), invasive group A streptococcus (3.6 per 100,000), invasive meningococcal disease (0.35 per 100,000), and invasive H influenzae (1.4 per 100,000)," she noted.

And, if the projection by the CDC investigators of 18,650 MRSA-related deaths in 2005 was accurate, "these deaths would exceed the total number of deaths attributable to human immunodeficiency virus/AIDS in the United States," Dr. Bancroft said.

Because the vast majority of MRSA cases are noninvasive, such as skin abscesses, these estimates are likely "only the tip of the drug-resistance iceberg" with respect to disease burden, she added.

The researchers analyzed invasive MRSA infections detected in the CDC's Active Bacterial Core laboratory surveillance system from July 2004 through December 2005.

The active, population-based surveillance was done at nine laboratories covering 16.5 million patients, about 5.6% of the U.S. population. Reports of MRSA were investigated and classified as health care-associated or community-associated.

The analysis included only invasive MRSA, which was defined as infections in normally sterile sites, such as blood, cerebrospinal fluid, and internal organs.

Most of the 8,987 observed invasive MRSA infection cases were among patients whose medical records showed they had risk factors for exposure in a health care setting. The most common of these risk factors were history of hospitalization, surgery, long-term care residence, or prior MRSA infection or colonization.

Overall, 58.4% of cases were health care-associated infections with a community onset and 26.6% were health care-associated cases with onset in the hospital.

Another 13.7% were community-associated infections, which included only patients without any established health care risk factors for MRSA infection. The origin of 1.3% could not be classified.

For 2005, the standardized incidence rate of invasive MRSA was 31.8 per 100,000 after adjusting for age, race and sex. It ranged from an average of 24.4 at the three sites with the lowest incidence to 35.2 at the three with the highest incidence, excluding an outlier where incidence was 116.7 per 100,000 per year.

The standardized annual mortality rate was 6.3 per 100,000 with a similarly calculated interval from 3.3 to 7.5 to 100,000.

Factors associated with increased incidence and mortality rates, respectively, included:

• Age 65 and older (127.7 and 35.3 per 100,000).
• Black race (66.5 and 10.0 per 100,000).
• Male gender (37.5 and 7.4 per 100,000).

A convenience sample of isolates sent to the laboratories showed that the USA300 strain typically associated with community-origin infections showed up in some hospital-onset MRSA cases (15.7%).

However, the majority of invasive infections overall-including 23.0% of community-associated isolates tested--were still associated with the USA 100 MRSA strain typically found in health care settings.

"It appears that what happens in the hospital does not stay in the hospital," Dr. Bancroft commented. "Working vigorously to decrease transmission of MRSA in health care facilities may decrease both nosocomial and community-onset MRSA that occurs in persons with prior health care exposure."

She cautioned, though, that the incidence and mortality estimates might have been subject to misclassification error in determining the origin of MRSA cases because the presence of a health care risk factor does not preclude patients from acquiring the infection in the community and not all risk factors may have been recorded in hospital charts.

Furthermore, the MRSA rates in the surveillance areas may not have been representative of the nationwide distribution since incidence is known to vary geographically.