MRSA skin infections associated with sex, drug use and environmental exposure in HIV-positive gay men

A case-control study of HIV-positive gay men has shown that skin infection with methicillin-resistant Staphylococcus aureus (MRSA) is associated with sex, drug use and environmental exposure. However, it is not associated with the use of antiretroviral therapy. The study was published in the 15^th May edition of Clinical Infectious Diseases.

MRSA is a strain of a common type of bacterium that is resistant to many antibiotics. Although infection with MRSA has been associated with hospital treatment, it can also be contracted in people who have not been hospitalised.

To investigate factors associated with the risk of ‘community-acquired’ MRSA, researchers gathered information on 35 gay men with MRSA infection on the skin, using telephone interviews and analysis of medical records. The patients were recruited from three clinics in Los Angeles County between September 2002 and May 2003.

Community-acquired MRSA infection was diagnosed in the outpatient setting or within 72 hours of hospital admission. Genetic analysis of the majority of the MRSA samples confirmed that they were caused by a community-associated strain of the bacterium, and not a strain associated with hospital treatment.

The findings from the men with MRSA were compared to 76 control HIV-positive gay men without skin symptoms. The control patients were matched to the cases based on having the same clinic and doctor and attending the clinic during the same week. Up to three control patients were recruited for every patient with MRSA.

“We found that community-associated MRSA skin infections among HIV-positive men who have sex with men were associated with high-risk sex and drug-using behaviours but not with immune status,” write the investigators. “Our findings indicate that community acquired MRSA spreads by contact transmission – via direct skin-to-skin transmission, such as sexual contact with someone with a skin infection – or by indirect transmission, such as during hot tub or sauna use.”

Most patients with MRSA had an abscess on the skin (55%), cellulitis (redness and inflammation of the skin; 31%) or recurrent boils (31%). Skin lesions were seen most often on the legs (26%), buttocks (26%) or arms (11%).

When they compared the information gathered from the MRSA and control patients, the investigators found that the MRSA patients were more likely to have had close contact with someone with a skin infection (matched odds ratio [OR]: 3.0; p = 0.040) or to have reported routine use of a public sauna or hot tub (matched OR 3.8; p = 0.008). Use of gyms (p = 0.386) or activities such as massage or contact sports were not associated with risk of MRSA.

MRSA patients were also more likely to have reported use of methamphetamines, such as ecstasy or ‘crystal meth’ (matched OR: 6.7; p = 0.004), nitrates (‘poppers’; matched OR: 4.2; p = 0.012) and sildenafil (Viagra; matched OR: 3.3; p = 0.037). However, ketamine, cocaine and gamma hydroxybutyrate (GHB) use were not linked to MRSA infection.

Although the average number of sex partners was not associated with MRSA infection (p = 0.079), the investigators did find significant associations between certain sexual behaviours among the sexually active participants. These included having two or more sexual partners (matched OR: 4.4; p = 0.005), meeting sexual partners in a sex club or bathhouse (matched OR: 4.0; p = 0.018) or via the Internet (matched OR: 5.5; p = 0.011), and having attended a sex party (matched OR: 4.1; p = 0.030).

Consistent use of condoms was found to have a protective effect (matched OR: 0.2; p = 0.015), while history of a sexually transmitted infection other than HIV was associated with an elevated risk of contracting MRSA (matched OR: 6.4; p = 0.007).

There was no significant effect of antiretroviral therapy on the risk of MRSA infection (p = 0.506).

Further risk factors included hospitalisation for any reason other than skin infection (matched OR: 3.7; p = 0.027), past use of ciprofloxacin (Ciproxin; matched OR: 5.9; p = 0.036), and frequent fingernail biting (matched OR: 2.9; p = 0.020). In contrast, use of co-trimoxazole (Septrin) prophylaxis (matched OR: 0.3; p = 0.031) or disposable toilet seat covers (matched OR: 0.4; p = 0.047) were found to be protective.

After controlling for the effects of hospitalisation, ethnicity and the number of sexual partners in a multivariate analysis, the investigators found similar risk factors for MRSA infection. Use of a public hot tub or sauna (adjusted matched OR: 3.9; p = 0.023), methamphetamine use (adjusted matched OR: 8.5; p = 0.012), frequent fingernail biting (adjusted matched OR: 4.2; p = 0.019), hands-on contact with customers at work (adjusted matched OR: 5.7; p = 0.027) and having a sex partner with a skin infection (adjusted matched OR: 9.2; p = 0.022) were associated with MRSA infection. In contrast, consistent condom use (adjusted matched OR: 0.1; p = 0.019) and use of co-trimoxazole prophylaxis (adjusted matched OR: 0.2; p = 0.024) were protective against infection.

The authors note that 15 (52%) of 29 patients with MRSA were initially treated with antibiotics that were not effective against the infection. This indicates “the need to increase awareness of community-associated MRSA diagnosis and treatment among physicians,” they write. “Additional research is needed to determine whether empiric treatment of skin and soft tissue infections should be modified among certain populations.”