News of the WeekInfectious Diseases

Resistant Staph Finds New Niches

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Science  14 Mar 2003:
Vol. 299, Issue 5613, pp. 1639-1641
DOI: 10.1126/science.299.5613.1639

Huge, painful boils and abscesses that must be cut open and drained before they can heal: Those are the scary symptoms of a major outbreak of a drug-resistant microbe among hundreds of jail inmates and gay men in the United States. A recent surge in cases has epidemiologists scrambling to understand what's going on—and wondering whether a well-known pathogen is on the verge of becoming a much bigger problem.

Experts on the microbe, methicillin- resistant Staphylococcus aureus (MRSA), discussed the latest developments last week at a meeting in San Antonio, Texas—only to conclude that they have more questions than answers. But already, says Peter Ruane, a physician at Tower Infectious Diseases Medical Associates in Los Angeles, the outbreak suggests that MRSA “may become much more widespread in the general population.”

For many years, MRSA has been an important cause of infections mainly in hospitals and nursing homes, where it's often resistant to all antibiotics except vancomycin, considered the last resort. Recently, epidemiologists started seeing more cases of MRSA among people who had no specific connection to hospitals—for instance, among Native Americans, athletic teams, intravenous drug users, and schoolchildren.

Initially, some researchers speculated that those “community-acquired” microbes might have “escaped” from hospitals. But a genetic comparison of community- and hospital-acquired MRSA strains by Patrick Schlievert of the University of Minnesota, Twin Cities, and colleagues, published in January, suggests that the community variety arose independently and not long ago, when a wild strain picked up a so-called cassette chromosome—a mobile genetic element—containing a gene for methicillin resistance.

Like regular staph, MRSA can be carried on the skin or in the nasal cavity, where many people never notice it. But sometimes MRSAs can cause severe skin and soft tissue infections and, when they reach the lungs, pneumonia—with fatal consequences if treatment comes too late. Toxic shock syndrome is another potentially serious result. In contrast to hospital strains, community-acquired MRSA is usually susceptible to a range of antibiotics, making treatment relatively easy once it's diagnosed. MRSA infection is not a reportable disease in the United States, so firm numbers are hard to come by. But Schlievert suspects that there are thousands of cases every year in this country and probably hundreds of deaths; five to 10 children die from MRSA in the Minneapolis metropolitan area alone.

On the move.

Methicillin-resistant Staphylococcus infections are increasingly occurring outside hospitals.

CREDIT: K. LOUNATMAA /PHOTO RESEARCHERS

Now, community-acquired strains of the microbe seem to be moving into higher gear. In 2002, more than 900 people in Los Angeles county jails became infected, L.A. health authorities announced in January—by far the biggest single outbreak ever recorded. Late last year, Los Angeles was the first city to record cases among gay men—at least 50 so far, says Ruane. Many, but not all, were HIV infected, but it's unclear whether this made them more susceptible. Boils and abscesses turned up on hands, legs, chests, buttocks, and genitalia, apparently infecting even intact skin. Since then, the infection has cropped up in gay communities in cities such as San Francisco, Boston, Atlanta, and Washington, D.C., suggesting that the microbe has found a new niche in which it can travel fast.

Researchers still don't know whether all these cases are related or how they were spread; nor is it clear to what extent the apparent jump in the number of cases might be due to increased reporting as a result of publicity. “But everybody seems to agree there's something new going on,” says Ruane.

Adding to the concerns are two cases of aggressive MRSA infections that occurred recently in gay men in the Netherlands, says Wim Wannet of the National Institute of Public Health and the Environment in Bilthoven. If the two strains prove to be genetically identical to the U.S. strains, it could mean that the outbreak has crossed the Atlantic.

One reason the current strain is more ferocious than most hospital-acquired strains may be that it produces a particular combination of toxins—although researchers disagree on which ones are important. But whatever the roots of their pathogenicity, epidemiologists fear that if aggressive MRSA strains become firmly established in the community, the bugs might pick up resistance to additional antibiotics, says Scott Fridkin of the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta. Another worry, says Schlievert, is that the strains will move into hospitals, replacing the more benign strains now residing there.

As a step toward finding answers, Matthew Boulton, Michigan's state epidemiologist, has proposed that resistant Staph infections be reported to CDC, as are about 50 other diseases. If the Council of State and Territorial Epidemiologists, the body that advises CDC on this matter, passes Boulton's proposal this summer, he says, “we would at least get a much better idea what's going on here.”

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