In this large study of FSWs in two U.S.-Mexico border cities, HIV prevalence has increased significantly in the last decade. Applying UNAIDS criteria, the HIV epidemics in both cities appear to have transitioned from low-level to concentrated, since overall HIV prevalence among FSWs is now >5% [5
]. FSWs who injected cocaine, smoked, inhaled or snorted methamphetamine, and whose syphilis titers were consistent with active infection were at least three times more likely to be HIV-infected compared to other FSWs. These findings have important implications for cross-border HIV transmission, since both cities adjoin busy ports of entry to the U.S. and are destinations for large numbers of sex tourists.
FSWs who used stimulants had greater odds of HIV infection, even if they did not inject. Injection drug use and sex work have been shown to overlap considerably in various settings [6
]. An elevated risk of HIV among injectors is not surprising; however, the independent association we observed between HIV infection and non-injection methamphetamine use was unexpected. Methamphetamine use has been closely associated with incident HIV infection among MSM [10
], but few reports have examined this association among heterosexuals. In one such study, St. Petersburg, Russia, an independent association was reported between HIV infection and methamphetamine use among heterosexual—and predominantly male—IDUs [11
]. Use of stimulants, including methamphetamine and cocaine, has been associated with high risk sexual behavior in FSWs and other populations [12
]. A study of pregnant women in Tijuana found that HIV was independently associated with methamphetamine use, even after adjusting for injection drug use [4
], which mirrors our findings.
Although we did not observe an independent association between HIV infection and unprotected sex, methamphetamine use may be a proxy measure of high risk sexual behaviors. Since methamphetamine production and use has risen dramatically in Mexico following crackdowns on its production in the U.S. [13
], it is important to identify whether the mechanism(s) responsible are directly or indirectly drug-related so that appropriate interventions can be developed. Volkow and colleagues suggested that stimulants induce sexual desire, which in turn could account for an increased frequency of high risk sexual behaviors [14
]. On the other hand, stimulant use could be a marker of sensation-seeking or other personal attributes which are associated with higher levels of unprotected sex [10
The finding that FSWs who inject cocaine were at elevated risk of HIV infection is also cause for concern. These women may acquire HIV through sharing of needles or other injection paraphernalia, since cocaine injection is associated with frequent, more chaotic injection behavior. Alternatively, cocaine injection could be a marker of high risk sexual behaviors, such as exchanging sex for drugs, which can compromise women's ability to negotiate safer sex. In our sample, HIV prevalence among FSWs who inject drugs was 12% compared to 4% among FSWs who had never injected drugs [9
], which underscores the need for targeted interventions that reduce both sexual and injection risks in this vulnerable subgroup. Although subsidized drug abuse treatment exists in these Mexican cities, there are limited treatment options for persons addicted to stimulants, as in other countries.
While many variables were significantly associated with HIV serostatus in univariate analyses, some such as Chlamydia and Gonorrhea were not independently associated with HIV infection after controlling for other factors (e.g. syphilis). Indeed, high syphilis titers were independently associated with a four-fold odds of HIV infection, which is not surprising since it is an important cofactors of HIV transmission. At enrollment, few women reported being aware of having an infectious STI, which could suggest that the majority of these infections were chronic and untreated or acute, incident infections. Although free treatment for syphilis and other STIs is available in Mexico under the country's universal health system, limited resources compromise the ability to conduct surveillance and treatment for high risk populations. Recently, Mexico's federal and state Ministries of Health provided at least two mobile HIV/STI health clinics to most Mexican states in an effort to reach high risk populations, an important first step towards a more active case-finding approach. Given the clear and immediate need for improved surveillance and aggressive STI treatment in both cities, binational funding strategies to improve financing and delivery of these services should be undertaken.
Since HIV-positive FSWs were more likely to report that their regular partners and clients injected drugs, there is a concomitant need to develop interventions to reduce high risk behaviors among these male partners. We recently reported that more than two thirds of FSWs in Tijuana and Ciudad Juarez reported having U.S. clients and that this subgroup tended to engage in higher risk behaviors [15
], which underscores the necessity for a binational response to preventive interventions among men paying for sex.
The cross-sectional nature of this analysis limited our ability to draw causal inferences. Due to small cell sizes, we may have been under-powered to detect some associations (e.g., unprotected anal sex). Like most studies of sexual behavior, ours is potentially subject to socially desirable responding. Some women may have been aware of having an STI, but were unwilling to report it, fearing that this would impact their ability to practice sex work. However, less than half reported consistent condom use for vaginal sex with clients, and frequency of reported drug use was also high, suggesting that reporting bias may have been minimal. Since subjects were recruited through convenience sampling and eligibility criteria were designed to target high-risk women in need of intervention, our estimates of HIV/STI prevalence should not be generalized beyond the present sample. Indeed, HIV prevalence may be under-estimated, since women who were knowingly HIV-infected were excluded.
Although FSWs in Tijuana and Ciudad Juarez who use stimulants and have high syphilis titers are at increased risk of HIV infection, these risk factors are amenable to interventions. Our data emphasize the need for interventions to identify and treat ulcerative STIs and to reduce the risks associated with injection and non-injection use of stimulants. Multifaceted interventions need to be developed in a binational context to curtail the growing HIV epidemic among Mexican FSWs in these border communities.