Nov 20, 2007
Community-associated cases not the majority but are still responsible for invasive infections and, rarely, death.
by Michelle StephensonIDN Correspondent
November 2007
Invasive methicillin-resistant Staphylococcus aureus infections are a major public health problem. Recent study results found that although they are primarily health care-associated, they are no longer confined to a particular type of institution.
Researchers from across the country recently participated in a study to describe the incidence and distribution of invasive MRSA. The Active Bacterial Core surveillance system is an ongoing, population-based, active laboratory surveillance system, and it is a part of the Emerging Infections Program of the CDC. For this study, researchers from several Emerging Infections Program sites conducted surveillance for invasive MRSA infections from July 2004 through December 2005.
The sites included the Atlanta metropolitan area; Baltimore City; the state of Connecticut; Davidson County, Tennessee; the Denver metropolitan area; Monroe County, New York; the Portland, Oregon metropolitan area; Ramsey County (the St. Paul area), Minnesota and the San Francisco Bay Area. In 2005, the total population under surveillance was approximately 16.5 million people, or approximately 5.6% of the U.S. population.
Incidence ratesBetween July 2004 and December 2005, there were 8,987 reported cases of invasive MRSA. Most were health care-associated, with 5,250 (58.4%) community-onset, health care-associated infections; 2,389 (26.6%) hospital-onset, health care-associated infections; 1,234 (13.7%) community-associated infections and 114 (1.3%) that could not be classified.
In 2005, the standardized incidence rate of invasive MRSA was 31.8 per 100,000, and the standardized mortality rate was 6.3 per 100,000. People who were aged 65 years or older had the highest incidence rates (127.7 per 100,000), followed by blacks (66.5 per 100,000) and men (37.5 per 100,000). Children aged 5 to 17 years had the lowest incidence rates (1.4 per 100,000).
Of the total number of patients with MRSA infection, 1,598 died while they were inpatients.
The unadjusted incidence rates of all types of invasive MRSA infections ranged from approximately 20 to 50 per 100,000; however, rates were remarkably higher in Baltimore City (116.7 per 100,000).
The incidence rate of invasive CA-MRSA was five per 100,000 or fewer in all of the sites, and incidence rates were consistently higher among blacks compared with whites in all age groups.
The rate of health care-associated, community-onset MRSA infections was 17.6 per 100,000, which was greater than either health care-associated, hospital-onset infections (8.9 per 100,000) or community-associated infections (4.6 per 100,000). Among patients with MRSA, the mortality rate for health care-associated, community-onset infections was higher (3.2 per 100,000) than for health care-associated, hospital-onset infections (2.5 per 100,000) or for community-associated infections (0.5 per 100,000).
“For the first time, we have a measurement of the burden and distribution of invasive MRSA infections. Given that the majority [of cases] were health care-associated, we need to step up our prevention efforts in health care facilities. The findings also suggest that, in 2005, community-associated cases had invasive infections at a rate of five per 100,000 and death at a rate of 0.5 per 100,000: not the majority but not trivial,” said Monina Klevens, DDS, from the CDC, who was a researcher of the study. The study results were recently published in the Journal of the American Medical Association.
During 2005, 5,287 MRSA infections were reported in the surveillance areas, and after adjusting for age, race, and sex and applying these numbers to the total U.S. population, the researchers estimated that 94,360 patients had an invasive MRSA infection. During 2005, 988 patients died of invasive MRSA infections. After adjusting these numbers to the total U.S. population, the researchers estimated that there were 18,650 in-hospital deaths due to invasive MRSA infections.
Risk factors for MRSA
During the study period, 4,105 (78.2%) of the 5,250 patients with health care-associated, community-onset infections and 1,993 (83.4%) of the 2,389 patients with health care-associated, hospital-onset infections had more than one health care risk factor for MRSA documented in their medical records. The most common health care risk factors among patients with community-onset infections were a history of hospitalization (76.6%), long-term care residence (38.5%), history of surgery (37.0%) and MRSA infection or colonization (30.3%). The most common health care risk factors among patients with hospital-onset infections were a history of hospitalization (57.7%), history of surgery (37.6%), long-term care residence (21.9%) and MRSA infection or colonization (17.4%).
Of the 8,987 observed cases of invasive MRSA, 8,792 cases had complete information in their charts, and of these, the clinical syndrome associated with invasive MRSA disease included bacteremia (75.2%), pneumonia (13.3%), cellulitis (9.7%), osteomyelitis (7.5%), endocarditis (6.3%) and septic shock (4.3%). Of the patients, 8,304 (92.4%) were hospitalized. Additionally, 1,598 (17.8%) of the patients died during hospitalization, and 1,162 (12.9%) of the patients developed recurrent invasive infections.
Ninety-eight percent of patients had a recorded clinical outcome. Interestingly, mortality rates varied according to MRSA-related diagnosis. Patients with septic shock had a high mortality rate of 55.6%; for patients with pneumonia, 32.4%. Patients with endocarditis had a moderate mortality rate of 19.3%; for patients with bacteremia, 10.2%. Patients with cellulitis had a low mortality rate (6.1%).
Pulsed-field typing
The results of pulsed-field gel electrophoresis were available for 864 (71.9%) of the 1,201 isolates received from eight of the nine study sites. Most (81.6%) of the results were from blood cultures, 4.8% from synovial fluid, 4.7% from bone, 1.9% from pleural fluid, 1.5% from peritoneal fluid and 5.5% from other normally sterile sites. Isolates tested were associated with uncomplicated bacteremia (69.8%), pneumonia (19.3%), cellulitis (11.3%), osteomyelitis (10.4%), endocarditis (8.5%) and septic shock (5.0%).
For two-thirds (66.6%) of the isolates from community-associated cases, USA300 was the strain type identified. It was also identified among 22.2% of the isolates from health care-associated, community-onset cases and among 15.7% of health care-associated, hospital-onset cases. USA100 was the strain type found in 35 (23.0%) of the 150 isolates from community-associated cases.
“One of the interesting findings from this study was the report that the USA300 clone, the most common community-associated MRSA strain, caused not only community-associated infection but also health care-associated infection. The epidemiology of MRSA continues to evolve and becomes more and more complex. The frequency of MRSA in the community and hospital and its effect on poor clinical outcomes will hopefully lead to improved hand and personal hygiene,” said Keith S. Kaye, MD, medical director of the hospital infection control committee, Duke University Medical Center, Durham, N.C., and member of the Infectious Disease News editorial advisory board.
Denise M. Cardo, MD, director of the division of health care quality promotion at the CDC’s National Center for Preparedness, Detection and Control of Infectious Diseases, and member of the Infectious Disease News editorial advisory board, also stressed the importance of infection prevention measures. “MRSA is an important problem in health care settings and is a threat to your patients. Most MRSA invasive infections are health care-associated, and these infections can be prevented with adherence to infection prevention recommendations during all patient encounters,” said Cardo.
Because the main mode of transmission of MRSA is the hands, standard precautions such as hand hygiene and gloving are imperative.
For more information:
Klevens RM, Morrison MA, Nadle J, et al. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA. 2007;298:1763-1771.
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