CA-MRSA contention: Prevalence, transmission study sparks controversy

March 1, 2008

A population and genotypic study of the USA300 strain of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) among gay men strongly suggests that the bacteria can be transmitted through skin-to-skin contact during sexual activity.

Key Points

National report - A population and genotypic study of the USA300 strain of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) among gay men strongly suggests that the bacteria can be transmitted through skin-to-skin contact during sexual activity.

Possible transmission of MRSA during sexual activity is relevant to any population group.

A report last year documented the first cases of sexual transmission of CA-MRSA among heterosexuals.

One part of the investigation was based on chart reviews and genetic analysis of clinical isolates from hospitals, representing more than 98 percent of the hospital beds in San Francisco, plus two public outpatient clinics, during the period 2004 to June 2006.

It used a stratified random sample of 532 (21 percent) of the 2,495 San Francisco residents who had culture-proven MRSA infections.

CA-MRSA typically is resistant to beta-lactams and one or two other classes of antibiotics, but remains susceptible to older generations of drugs.

"So, pretty much the first-line antibiotics are all gone," Dr. Diep tells Dermatology Times.

Plotting the cases by ZIP code and overlaying that with demographic information, he found that MDR USA300 was 13 times more likely to occur in areas with the highest concentrations of gay residents.

The epicenter for both was the Castro district (ZIP code 94114), which has the highest percentage (25.7 percent) of male same-sex couples in the United States, and an MDR USA300 incidence rate of 170 cases per 100,000 residents.

However, the total number of cases in that limited geographic area was small, so the confidence interval of the statistic was large (41 – 299).

He 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 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 percent vs. 14 percent).