Aetiology

Methicillin-resistant Staphylococcus aureus (MRSA) has taken up residence in sport teams, prisons, schools, the military, and even swine. A new article in the Annals of Internal Medicine shows that, at least in Boston and San Francisco, it's also causing a lot of infections in men who have sex with men; more after the jump.

To examine this, the authors looked at MRSA infections from a variety of health care settings: medical centers, community health clinics, HIV clinics, and emergency rooms. These were examined in separate analyses. For example, for the medical centers they looked at in San Francisco, they were able to estimate overall incidence of infection for the city, and for each ZIP code within the city--and they found this incidence to be quite variable. They grouped several contiguous ZIP codes together, and found that the highest-incidence areas averaged an incidence rate of 59 cases of a specific multidrug-resistant (MDR) strain of community-associated MRSA (dubbed USA300) per 100,000, while the other areas averaged only 4 cases/100,000. The areas with highest incidence corresponded to areas that also had the highest proportion of male same-sex couples. For instance, in the Castro district, roughly 26% of the households are male same-sex couples, and their incidence for MDR USA300 was 170 cases per 100,000.

The authors looked at a number of different variables that may play a role in infection, including HIV status, prior MRSA infection, and antibiotic use. They found that, while HIV status was a risk factor for acquisition of the MDR USA300 strain, men having sex with men was an independent risk factor--so men having sex with men who were HIV negative still had a greatly increased risk of acquiring the MDR USA300 strain of MRSA.

They also noted that of the MDR USA300 infections, a high proportion of them involved the buttocks, genitals, or perineum--suggesting transmission during sex. Though Staph aureus is typically carried in the nose or on the skin, it can also be carried rectally (though this is less common)--so there are a number of ways parters could infect each other during sexual activity.

A big limitation of this research is that, while incidence of disease can be calculated, we don't know anything about prevalence of infection with this strain in the community at large. It's likely to be low based on what they found in their emergency department studies in 11 cities around the U.S. (only 2 out of 212 MRSA isolates were MDR USA300), but do the incidence rates they found in the current article accurately reflect carriage rates in MSM or other populations?

[EDIT TO ADD: see also Mike's post on this topic]

Reference

Diep, BA et al. 2008. Emergence of Multidrug-Resistant, Community-Associated, Methicillin-Resistant Staphylococcus aureus Clone USA300 in Men Who Have Sex with Men. Annals of Internal Medicine, in press. Link.