WASHINGTON — As many as 2.3 million people in the United States carry methicillin-resistant Staphylococcus aureus, Dr. Daniel B. Jernigan reported at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy.
The estimate comes from a recent study in which nasal swab samples were collected from 9,622 people (at least 1 year of age) as part of the National Health and Nutrition Examination Survey in 2001–2002. Roughly one-third (32%) were nasally colonized with S. aureus and 0.8% were nasally colonized with methicillin-resistant S. aureus (MRSA). Based on those numbers, the population prevalence of all strains of S. aureus is 89.4 million (J. Infect. Dis. 2006;193:172–9).
“MRSA colonization was associated with being over the age of 60 and also with being female,” said Dr. Jernigan, a medical epidemiologist with the National Center for Infectious Diseases.
Concern about community-acquired MRSA (CA-MRSA) is growing. (See box.) Unlike health care-acquired MRSA, CA-MRSA affects healthy individuals, is not associated with traditional risk factors for health care-acquired MRSA, and varies in prevalence by race, age, and geography. In particular, there is an elevated prevalence of MRSA among African Americans and among children.
Population surveillance of bacterial infections in the United States is performed through the Active Bacterial Core surveillance program, which is a collaboration between the Centers for Disease Control and Prevention and several state health departments and universities participating in the Emerging Infections Program Network.
According to this program, CA-MRSA incidence in the United States ranged from 18 to 26 cases per 100,000 people per year in 2001–2002 (N. Engl. J. Med. 2005;352:1436–44). “I believe that is a conservative estimate,” Dr. Jernigan said at the meeting sponsored by the American Society for Microbiology. Overall confirmed or probable CA-MRSA cases accounted for 17% of all MRSA infections in three large cities.
“The majority of infections due to these community-associated cases are skin and soft tissue infections—about 77%,” he said.
In an unpublished analysis of data from the National Ambulatory Medical Care Survey and the National Hospital Ambulatory Medical Care Survey, researchers at the National Center for Health Statistics identified skin conditions likely to be caused by S. aureus infection based on ICD-9 codes.
They estimated that 4,000 outpatient visits per 100,000 people per year are due to such skin and soft tissue infections.
In another study, researchers in California cultured each culturable pus lesion in a Los Angeles area emergency department during 1 month (Emerg. Infect. Dis. 2005;11:928–30).
“What they found was that 59% of culturable pus seen in the emergency department was due to MRSA, and of those the majority was community-associated MRSA strains,” Dr. Jernigan said.
CA-MRSA outbreaks have been reported among athletes, inmates, soldiers, children in schools and day-care centers, Native Americans/Alaskan Natives, men who have sex with other men, and Hurricane Katrina evacuees. Transmission factors that have been associated with outbreaks of CA-MRSA include crowding, frequent skin-to-skin contact, compromised skin, contaminated surfaces, shared items, uncleanliness, and antimicrobial use.
Two recent unpublished studies from the CDC's Epidemic Intelligence Service have documented CA-MRSA transmission among people getting tattoos from unlicensed tattooists and among crystal meth smokers, although not among IV drug users, Dr. Jernigan said.
The CDC's CA-MRSA prevention strategy includes steps to prevent infection, effectively diagnose and treat infections, use antimicrobials wisely, and prevent transmission. “So rather than just a focused area on judicious antimicrobial use, we think—for this particular pathogen, which has colonize status, infective status, susceptible status, and possibly the environment as sources of infection—that you have to have a multipronged approach,” he said.
Experts at a CDC-sponsored meeting on CA-MRSA recommended these clinical steps for combating CA-MRSA:
▸ Increase awareness about CA-MRSA.
▸ Use local data to develop treatment strategies.
▸ Collect diagnostic specimens.
▸ Provide adequate follow-up.
▸ Target treatment with alternative antibiotics.
The experts also recommended steps to better control outbreaks of CA-MRSA:
▸ Enhance surveillance during an outbreak by looking for skin infections.
▸ Target empiric therapy to the outbreak strains.
▸ Provide education about wound care and wound containment.
▸ Promote enhanced personal hygiene and encourage limiting shared use of items.
▸ Consider excluding patients from certain activities that involve skin-to-skin contact, such as athletic events.
▸ Achieve and maintain a clean environment.
Community-Acquired MRSA Emerging in San Francisco
The burden of methicillin-resistant Staphylococcus aureus disease appears to be shifting from health care-acquired strains to community-acquired strains, at least in San Francisco, Dr. Henry F. Chambers III said at the meeting.
The burden of MRSA disease at three hospitals in San Francisco has shifted toward community onset. “About 50%–60% of MRSA disease in San Francisco is now community-acquired,” said Dr. Chambers, chief of the division of infectious diseases at San Francisco General Hospital.