25. ANAL SEXUALLY TRANSMISSIBLE INFECTIONS AS RISK FACTORS FOR HIV SEROCONVERSION

Objectives: Sexually transmitted infections (STIs) are believed to increase the risk of HIV acquisition, but few studies have focused on homosexual men. We examined sexual behaviour and common STIs as independent risk factors for HIV seroconversion in a community-based cohort of homosexual men in Sydney. Methods: Between 2001 and 2004, 1427 initially HIV-negative men were enrolled. They were tested annually for HIV, for gonorrhoea and chlamydia in the urethra and anus (strand displacement amplification, BDProbeTec), and for herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) using type specific ELISA. Participants also reported diagnoses of STIs since their last interview. Detailed information on sexual risk behaviours was collected every 6 months. Results: There were 49 HIV seroconversions through 2006, an incidence of 0.80 per 100PY. A higher number of episodes of insertive and receptive unprotected anal intercourse (UAI) with HIV positive or HIV status unknown partners was each significantly associated with HIV seroconversion. In multivariate analysis of behavioural risk factors, HIV seroconversion was significantly associated with a higher number of episodes of receptive UAI with a partner of unknown HIV status (p trend <0.001) or with a partner known to be HIV positive (p trend <0.001). After controlling for these sexual behaviours, a study diagnosis of anal gonorrhoea remained strongly related to HIV seroconversion (RR = 7.41, 95% CI 1.75-31.75). Most cases of anal gonorrhoea diagnosed were asymptomatic. In addition, there was an independent association with anal warts (RR = 3.43, 95% CI 1.43-8.19), and prevalent HSV-1 infection was of borderline significance (RR = 2.78, 95% CI 0.99-7.80). Conclusion: Certain anal STIs were associated with HIV seroconversion, even after adjustment for UAI. For some anal conditions, in particular gonorrhoea, infection was frequently asymptomatic. Screening for anal STIs should be investigated as a potential HIV prevention intervention.