HIV-1 infection among female commercial sex workers in rural Thailand

Objective: To reconstruct past HIV-1 incidence and identify risk factors for HIV-1 seroconversion among female commercial sex workers in a semi-rural setting in northern Thailand. Design: Retrospective cohort of sex workers attending a sexually transmitted disease (STD) clinic at Mae Chan District Hospital, Chiang Rai Province, Thailand. Methods: Information including demographic data, HIV-1 antibody testing and STD diagnosis was collected on 821 sex workers attending the STD clinic from 1989 through 1993. HIV-1 incidence among initially HIV-l-seronegative sex workers who underwent repeat HIV-1 antibody testing during follow-up was determined and possible predictive factors for seroconversion including diagnosis of other STD were examined. Results: HIV-1 seroprevalence among 556 sex workers who underwent antibody testing was 52%, rising from 29% at initial test in 1989 to 53-63% for 1990-1993. Among 96 initially HIV-1-negative sex workers who underwent repeat testing, 64 HIV-1 seroconversions occurred, giving an incidence rate of 12.6/100 person months (PM). Incidence increased from 12.0/100 PM for 1989 and 1990 to 17.0/100 PM for 1991, with a subsequent decline to 9.3/100 PM for 1992-1993. The cumulative risk of seroconversion 6 months following initial negative HIV-1 antibody test was 57%. There were higher rates among younger sex workers (14-19 years old) and those from Hilltribes, but only a diagnosis of chancroid was significantly associated with HIV-1 seroconversion (P = 0.014). Conclusion: An explosive epidemic of HIV-1 occurred among sex workers in a semi-rural area of northern Thailand from 1989. These HIV-1 incidence estimates, which are among the highest rates of sexual transmission ever reported, highlight the extreme vulnerability of female sex workers to HIV infection, even in a non-urban selling.