Multidrug-Resistant Staph Clone Emerges in Gay Men

Bob Roehr

January 16, 2008

January 16, 2008 — Sexually active gay men are up to 13 times more likely than the general population to acquire a highly drug resistant strain of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), according to a study from the University of California, San Francisco. The study was published online January 14 in the Annals of Internal Medicine.

CA-MRSA typically is resistant to beta-lactams and 1 or 2 other classes of antibiotics but remains susceptible to older generations of drugs. However, the multidrug-resistant (MDR) clone of the USA300 strain of CA-MRSA carries a large genetic segment (pUSA03) that also makes it resistant to macrolides, clindamycin, and mupirocin — the topical antibiotic used to decontaminate sites of skin colonization.

The clonal isolate is still susceptible to rifampin and trimethoprim-sulfamethoxazole, but lead author Binh An Diep, PhD, from the San Francisco General Hospital (SFGH) Medical Center, California, expects the bacteria to develop resistance to those drugs as well. "So, pretty much the first-line antibiotics are all gone," he told Medscape Infectious Diseases.

"The second-line drugs are vancomycin, which is intravenous and requires hospitalization, and daptomycin and linezold, both of which are exceedingly expensive."

San Francisco Cases

The study was based on chart reviews and analysis of clinical isolates of MRSA from hospitals representing more than 98% of the hospital beds in San Francisco, plus 2 public outpatient clinics, from 2004 to June 2006. It used a stratified random sample of 532 (21%) of the 2495 San Francisco residents who had culture-proven MRSA infections.

The annual incidence of USA300 infection per 100,000 persons was 275 cases, and the incidence of the MDR isolate with the pUSA03 plasmid was 26 cases. Geographically, 8 contiguous zip codes had an average incidence of 59 cases compared with 4 cases in the remaining zip codes.

Overlaying the incidence with census information, the researchers found that 10.3% of the population in the first cluster of zip codes was same-sex male couples, compared with 2.2% in the rest of the city. The Castro district (zip code 94114) had the highest percentage (25.7%) of male same-sex couples in the United States and a MDR USA300 incidence rate of 170 cases per 100,000 residents. However, the confidence interval in that small sample was large (41 – 299).

An analysis of 183 consecutive patients with MRSA infection treated at the SFGH HIV Clinic found that most cases (179 patients) were skin or soft tissue infections with symptoms of abscess (121), cellulitis (17), folliculitis (18), impetigo (2), ulceration (6), or wound infection (15).

The vast majority of the infections (170) were caused by USA300, with 30 being MDR. MDR infections were more likely than other isolates to involve the buttocks, genitals, and perineum than other anatomical sites (30% vs 14%).

Boston Cases

The Boston cohort was drawn from a baseline study conducted by Fenway Community Health during a roughly similar time period. It involved 130 patients with MRSA; almost all (n = 126) were infected with USA300, nearly half (60) of which had the MDR variant.

The study actively screened patients for colonization with MRSA and so identified a significantly greater number of anatomic sites where the bacterium was present. Nearly half of them showed presence of MRSA at each of the 4 sites sampled.

The broader screening of all participants, not just those with active MRSA infection, found that 4% carried USA300 in the nose and 2% in the perianal area. Dr. Diep said, "This is an extremely high rate of perianal colonization that is practically unheard of."

Comparing the SFGH and Fenway groups, Dr. Diep found that the risk for MDR USA300 on the buttocks, genitals, or perineum was 30% and 47%, respectively. Because the sites are where there is physical contact during anal sex, it strongly suggested that the infections were transmitted during that activity.

The study also identified a Boston patient who regularly traveled to San Francisco; his medical chart specifically mentioned the 94114 zip code. Given the identical genetic sequences of the USA300 clones found in both cities, it seems likely that they shared a common origin and were disbursed by travelers from that site to other — perhaps many other — locations.

Fenway's research director, Kenneth Mayer, MD, said the retrospective nature of the study made it impossible to ascertain the effect of multiple sexual partners on risk of acquisition of MRSA or link that acquisition to any particular venue or sexual activity.

He acknowledged that bathhouses and sex clubs, not to mention gyms, are all possible locations for acquisition of MRSA through contact with surfaces contaminated with the bacteria; sexual activity is not required for transmission. Issues of infection control are likely to become more important in many such venues.

HIV Status

Being HIV-positive carries a 2-fold increased risk of developing MDR USA300 infection. "However, the much stronger predictor, 13-fold, is men having sex with men," said Dr. Diep. "The role of HIV infection is probably minimal in this setting; it is the sexual transmission of this clone that is important."

He said they saw little difference among HIV-positive patients in terms of acquisition, disease progression, or response to therapy. However, most of those patients had a CD4 cell count greater than 200 cells/mm3. Significant risks of opportunistic infections often are not seen until the CD4 count decreases to less than 100 cells/mm3, and the number of patients in that category was too small for meaningful analysis.

Clinical Practice

Derek Jones, MD, a dermatologist in Los Angeles, California, wrote about an outbreak of CA-MRSA in gay men in a 2004 paper in the Journal of the American Academy of Dermatology.

That outbreak began in the summer of 2000 and "is still very much [present] here in Los Angeles," he told Medscape Infectious Diseases. "We'll see upticks and downticks from time to time, but the incidence has been fairly stable." Dr. Jones attributes that to a series of high-profile outbreaks in the region that have alerted clinicians to the risk; primary care physicians have learned to recognize and treat the infection at an early stage.

He sees active infection at various sites on the body; it does not seem to be focused in areas of direct sexual contact, as was the case in Boston and San Francisco. His patients are still sensitive to rifampin and sulfamethoxazole/trimethoprim, but drug–drug interactions generally preclude the use of rifampin in an HIV-positive population receiving therapy.

Dr. Jones maintains aggressive infection control in his office, wiping down all surfaces, including doorknobs, once a patient with MRSA leaves. He also cultures the noses of his staff for the presence of MRSA on an annual basis, although he has not found any yet. He recommends those practices to other clinicians.

Soap and water is highly effective against all forms of MRSA. Patients can help protect themselves from infection by washing well after sexual activity and working out. Good hand hygiene can reduce the risk that nasal or other site colonization can spread to become active infection.

Hype and Homophobia

There has been an element of hysteria in much of the media coverage of recent studies on MRSA. Dr. Diep is concerned about a possible backlash against the gay community because of how his study might be presented. He made the comparison with the early days of the HIV epidemic in this country.

"Even though HIV emerged in the gay community, that was fortuitous." He said the relatively concentrated and closed nature of the gay community often allows it play the role of canary in the coal mine, alerting the broader population to emerging public health pathogens. This early warning can allow for the development of medical and social interventions before the pathogen can spread spreads more broadly.

Ann Intern Med. Published online January 14, 2008.


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