Researchers report that a multiple-drug-resistant strain of methicillin-resistant staphylococcus aureus (MRSA) is common among gay men in San Francisco and Boston and provide evidence that the bacterium can be transmitted sexually. This strain of MRSA, dubbed “USA300,” and its impact in both HIV-positive and HIV-negative gay men has been reported widely in the press today and is described in the February 19 issue of the Annals of Internal Medicine.

Though not necessarily more virulent or infectious than other strains of MRSA, USA300 is notable in that it is resistant to at least two of the drugs typically used to treat other strains of MRSA. Also unique are the higher rates found in gay men and other men who have sex with men, and the body parts sometimes affected, such as the genitals and buttocks.

In the study, Binh An Diep, PhD, a postdoctoral fellow from the University of California in San Francisco, and his colleagues first examined hospital records of patients diagnosed with MRSA in San Francisco. They determined that the overall prevalence of infection was 26 cases per 100,000 people. However, the prevalence of infections among people living in the Castro area of San Francisco—an area of residence for many gay men—was 170 cases per 100,000 people, or 6.5 times higher.

Diep’s team next examined the medical records of 183 HIV-positive people at an HIV clinic at a San Francisco hospital and 130 people, some of whom were HIV positive, attending a health clinic in Boston, all of whom had been diagnosed with MRSA.

Among the San Francisco study subjects, the authors found that 30, or 16 percent, were infected with the USA300 strain. Upon further analysis, men who reported having sex with other men, were far more likely to have been infected with USA300.

Among the Boston study subjects, 60, or 46 percent, were infected with USA300 and all of them were men who had sex with men. HIV-positive men were somewhat more likely to have been infected with USA300 than HIV-negative men, but both were infected at high rates.

Diep’s team notes that because of case reports of gay men having MRSA infections in the buttocks and genitals, it is likely that at least some of the spread of USA300 is through sexual contact. However, the researchers were unable to determine the types of sexual activity that were most closely associated with the transmission of the bacterium.

Previous reports of outbreaks of MRSA among gay men have also focused on steam rooms at gyms and saunas. It has also been well document in the general population, including athletes and school children.  

Though MRSA is a growing health concern for the general population, people with HIV whose immune systems are weakened may be especially vulnerable. The researchers note, however, that the risk of transmission can be reduced through vigorous washing with soap and water after sexual contact.