Sexual risk-taking may be associated with MRSA in HIV-positive individuals

HIV-positive individuals are 18 times more likely to become infected with community-acquired MRSA than the general population, according to a US study presented to the Sixteenth International AIDS Conference in Toronto on Monday August 14^th. The investigators, from San Diego in California, found that recent infection with syphilis and treatment with b-lactam antibiotics were significantly associated with a diagnosis of community-acquired MRSA.

There has been a marked increase in the incidence of community-acquired methicillin-resistant staph. aureus infection (MRSA) in the general population in recent years, but there are limited data on the incidence of the infection amongst HIV-positive individuals. MRSA causes skin and soft tissue infections that may sometimes be life-threatening, such as necrotising fasciitis, but more commonly it causes painful and stubborn boils, abscesses and bacterial infections of wounds.

Investigators in San Diego undertook a study to identify all cases of community-acquired MRSA amongst patients attending a large HIV clinic. Cases were defined as patients with a positive culture for MRSA without a recent (one year or more) history of hospitalisation. To identify the risk factors for community-acquired MRSA, the investigators gathered data on patient demographics, antibiotic and antiretroviral use, CD4 cell count, viral load, and history of sexually transmitted infections. These factors were then compared between patients diagnosed with community-acquired MRSA and those who remained free of the infection.

Of the 425 patients in the study, a total of 25 (6%) were diagnosed with community acquired-MRSA. All the cases occurred after 2002, with a 18-fold increase in incidence between 2003 and 2005 (p < 0.001).

HIV-positive patients were significantly more likely than the general US population to be diagnosed with community-acquired MRSA. In 2005, the annual incidence amongst HIV-positive patients was 40 cases per 1000 person years, compared to only 2.3 cases per 1000 patient years amongst the HIV-negative population, meaning that the incidence of community-acquired MRSA was 18-fold higher amongst HIV-positive patients.

The investigators then looked at the manifestations of community-acquired MRSA in their patients. They noted that all the patients who developed the infection had soft-tissue involvement, that 16% of individuals with the infection required hospitalisation, and that 67% had positive nasal cultures. None of the patients diagnosed with community- acquired MRSA were taking cotrimoxazole (Septrin) prophylaxis and 56% were taking potent anti-HIV therapy. Sixteen per cent had recurrent MRSA despite receiving appropriate initial treatment.

Risk factors for community-acquired MRSA were then examined. In their initial analysis, the investigators found that lower CD4 cell count, an AIDS diagnosis, a history of the sexually transmitted infection, syphilis, and treatment with beta-lactam antibiotics, a class of antibiotics often used as first-line syphilis treatment, were all associated with an increased risk of community-acquired MRSA. No demographic factors nor the use if HIV therapy had any association with the infection. The investigators then performed a second “multivariate” analysis and found that only recent use of B-lactam antibiotics (p = 0.04) and recent syphilis infection (p = 0.02) predicted community-acquired MRSA infections in HIV-positive individuals.

Presenting the data, Nora Crum-Cianflone of San Diego's TriService AIDS Clinical Consortium said that the fact that the clinic population comprised military personnel had made it impossible to carry out any analysis of sexual behaviour or drug use due to US military policy that forbids continued military service if homosexual activity is disclosed.

The most common manifestation of MRSA skin infections was abscesses on the skin of the buttocks or scrotum, leading an audience member to ask whether shaving of these areas might be implicated in the development of MRSA skin infections. Nora Crum-Cianflone said that while the retrospective nature of the analysis made it impossible to confirm this, it was a plausible explanation.

The San Diego researchers conclude that the incidence of community-acquired MRSA is increasing rapidly amongst HIV-positive patients, and that it occurs with much greater frequency amongst HIV-positive individuals than the general population. They emphasise that sexual risk-taking, evidence by recent infection with syphilis, and use of B-lactam antibiotics appeared to be risk factors for the infection.