Possible Sexual Transmission of Resistant CA-MRSA Clone Among Gay Men

The latest in the continuing story of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) is a study published online in the Annals of Internal Medicine documenting the emergence of multidrug-resistant (MDR) CA-MRSA infections, possibly sexually transmitted, among gay men in San Francisco and Boston. Skin and soft-tissue infections caused by a specific antibiotic-resistant clone of the CA-MRSA strain USA300, the most common cause of community-level skin or soft-tissue infections in the United States, were found with high frequency among traditionally gay districts of San Francisco and in gay patients receiving treatment at a health clinic in Boston. A high rate of these infections involved the buttocks, genitals, or perineum, suggesting the possibility of sexually related transmission.

According to data from a San Francisco HIV clinic, the most prominent risk factor for infection with the MDR USA300 clone was engaging in male-male sex, followed by a prior MRSA infection within the last 12 months and past use of the lincosamide antibiotic clindamycin. However, past use of clindamycin or the topical antibiotic mupirocin was not observed among clone-infected Boston patients. Notably, investigators observed this specific MDR clone in less than 1% of sample MRSA isolates from emergency departments across the United States.

The MDR USA300 clone, which is defined by the presence of the large plasmid pUSA03, expresses multiple genes conferring resistance to β-lactams, fluoroquinolones, tetracycline, macrolide, clindamycin, and mupirocin; 2 of these resistance genes are carried on the pUSA03 plasmid. Generally, clindamycin, tetracycline, and trimethoprim–sulfamethoxazole, are prescribed for less serious CA-MRSA infections. Investigators noted that the MDR clone in this study was not resistant to trimethoprim-sulfamethoxazole (or rifampin).

One of the most intriguing aspects of the study was the identification of an actively gay Boston patient who acknowledged frequent travel to San Francisco's highly gay-populated Castro District, where investigators found the highest annual incidence of MDR USA300—170 cases per 100,000 persons.* The MDR USA300 isolate from this Boston patient was determined to be the USA300-0114 subtype, a subtype accounting for the vast majority of MDR isolates in San Francisco. Moreover, the plasmid in the Boston subtype was identical to the plasmid in MDR USA300 isolates from San Francisco, suggesting cross-country dissemination of this MDR clone.

Although sexual transmission of the MDR USA300 isolate, at least in some cases, is suggested, it is unclear what particular sexual behavior (eg, anal sex) may increase the risk of transmission—as these data were not available to the study investigators. The authors cite case reports in 2007 of heterosexually transmitted CA-MRSA, including a disturbing case of Fournier gangrene.

*The 95% confidence interval for this value was wide: 41, 299.