Introduction

Female sex workers (FSWs) are highly vulnerable to HIV and other sexually transmitted infections (STIs), particularly in low- to middle-income countries.¹ In a review of HIV in Mexico, del Rio and Sepulveda² concluded that from 1980 to 2000, HIV seroprevalence among FSWs was less than 1%. However, more recent estimates in Tijuana and Ciudad (Cd.) Juarez yielded HIV prevalences ranging from 6% among non-injecting FSWs to 12% in FSW-IDUs.³ In contrast to HIV, STI prevalence among FSWs in Mexico has been high for decades. In the late 1990s, the prevalences of active syphilis, chlamydia, and gonorrhea among FSWs in Mexico City were 23.7, 12.8 and 11.6%, respectively;⁴ the prevalence of HSV-2 was 60%.⁵ Among FSW-IDUs residing in Tijuana or Cd. Juarez, 50% had at least one active STI, compared to 25% among non-IDU FSWs.³

The present study used a social ecological framework to explore the prevalence of HIV and other STIs and their correlates among 1 092 FSWs in 13 sites across Mexico. The Social Ecological Model describes five levels of influence on behavior, including individual (e.g., education), interpersonal (e.g., substance use with clients), institutional (e.g., work venue), community (e.g., poverty, access to health care), and policy (e.g., policing).⁶Given recent emphasis in prevention research on the “risk environment”,⁷ which includes all levels of influence described in our conceptual model, we hypothesized that site-specific, community-level, municipal characteristics might explain variations in HIV/STI prevalence independent of individual-, interpersonal-, institutional, and policy-level factors. We also hypothesized that settings with fewer resources and less-educated populations would have FSWs who were riskier. The availability in Mexico of two government-monitored indices, the Human Development Score (which comprises life expectancy, education, income, and other factors) and the Marginalization Index (an indicator of decreased access to basic public services and greater poverty), provided us with ready-made potential predictor variables related to the risk environments of the sites studied. If correlations could be found, the findings could help rationalize the allocation of limited resources for HIV/STI prevention and treatment.

Materials and methods

Data for this analysis were obtained from baseline interviews and HIV/STI screenings of 1092 FSWs enrolled in the Mujer Segura implementation study.⁸ This hybrid type-2 study simultaneously tested the efficacy of the Mujer Segura safer-sex intervention and a train-the-trainer implementation strategy at multiple sites.⁹ The protocol was reviewed and approved by the Institutional Review Boards of the University of California, San Diego; Universidad Autónoma de Ciudad Juárez, which provided training to staff; and the Mexican Foundation for Family Planning (Mexfam), which operates the clinics at which the study was carried out. All participants provided written informed consent.

Results

Discussion

The prevalence of HIV among the participants was low at all of the study sites, and prevalence of other STIs varied widely, ranging from 9.9 to 46%. Similarly, self-reported sex risk varied widely, with rates of unprotected sex ranging from 20 to 60% between sites. While illicit drug use was low overall, alcohol abuse was common with 73% overall meeting criteria for hazardous drinking. Neither drug nor alcohol use correlated with STI prevalence in univariate analyses. In univariate analyses, a high Human Development Score decreased the odds of combined HIV/STI prevalence, but this variable was not significant in multivariate analyses. The Human Development Scores are broadly representative of each state and may not reflect the micro-environments inhabited by individual sex workers. Therefore, future research should focus on individual markers of economic status.

The prevalence of HIV among the sample of 1 092 FSWs was 0.4%. When combined with previous estimates,^3,13,14 data from the present study suggest that FSWs from the 13 study sites had about 5.7 times (0.40/.07%) the risk of HIV compared to women from the general population, but they have only 0.06 times the risk of HIV compared to FSWs in Mexico’s northern border region, providing further evidence that sub-epidemics of HIV are occurring in specific regions and subpopulations in Mexico, particularly in border cities where migration patterns, drug trafficking routes, and sexual tourism contribute to higher risk behaviors among FSWs.¹⁵

Overall 23% of FSWs had either HIV or another STI. While this is comparable to STI rates observed historically among FSWs in Mexico,⁴ it is high relative to the general population (e.g., 2.3% prevalence of syphilis among women in the general population in 2007),¹⁶ suggesting high sex risk and thus the potential for an escalation of the HIV epidemic among FSWs in larger areas of Mexico. However, compared to FSWs in Tijuana and Cd. Juárez,¹⁷ FSWs in the present study were at about half the risk (.48 times) of gonorrhea, similar in chlamydia risk (1.18 times), and at lower risk for active syphilis titers (.18 times).

The final multivariate model suggested that, after controlling for variability in municipal characteristics, a greater risk of STI/HIV infection was related to four factors: lower education, having a spouse who has been diagnosed or treated for an STI in the past six months, not being able to afford a condom, and having clients who were from outside of Mexico. These results suggest a number of targets for interventions. Poverty and low education among FSWs have been identified previously as risk factors for negative health outcomes, Hence, HIV/STI prevention interventions could include structural components such as microloans and small business training.¹⁸

Providing more free condoms is an obvious and relatively inexpensive structural intervention. However, even FSWs who receive free condoms might be induced not to use them by offers from clients of higher fees for unprotected sex. This disincentive to condom use could be counteracted by cash transfers that are dependent on FSWs’ remaining free of sexually transmitted infections.¹⁹

The need for couples-based interventions is highlighted by our finding that having a spouse who has been diagnosed or treated for an STI in the past six months is related to increased risk for HIV/STIs. A number of investigators have tested couples-based interventions that, suitably adapted, could be effective for FSWs in Mexico.²⁰

Finally, the correlation between having foreign-born male clients and being at elevated risk for HIV/STIs is of unclear significance. It is unclear whether the foreign-born clients reported in this study were primarily tourists, resident or transient migrants, or some combination. Sex tourism²¹ is not a likely explanation for the presence of foreign-born male clients, since the sites were predominantly located in cities or towns in the interior of Mexico that are not well known for their tourist trades. If the foreign-born clients were predominantly migrants, then the literature that indicates higher HIV risk among migrant men²² might shed some light on our finding. Future studies should examine this question in greater detail.

A limitation of this study stems from our sampling design, which involved recruiting women in high-risk areas and venues through time location sampling. This type of sampling can introduce bias by omitting unidentified areas and venues, and by excluding FSWs who do not visit these areas and venues, or refuse to be screened.¹⁰ The sample also consisted of volunteers in a sexual risk reduction intervention and thus may not be representative of the broader population of FSWs in each study site. Also, the $30 US compensation for participation in the interview and intervention could have affected the representativeness of the sample; most likely it resulted in greater motivation to participate. Although the low overall HIV prevalence could reflect the success of ongoing HIV/STI surveillance and prevention efforts, our cross-sectional data prevent us from assessing whether overall HIV/STI prevalence is decreasing among FSWs in these cities. Site-specific prevalence estimates may be unstable due to relatively small samples at each site and the fact that entry criteria included reporting high-risk behavior.

This study contributes to our understanding of the HIV epidemic in Mexico by adopting a multi-level approach to identifying correlates of HIV/STI prevalence that includes community-level municipal characteristics that have been largely ignored in previous research. Also, unlike previous work,²³ this multi-site study involved a national sample of FSWs that included both urban and rural regions of Mexico. Our data suggest that interventions are advisable to mitigate the risks associated with FSW lifestyles. Evidence-based behavioral interventions for HIV prevention have been shown to be highly cost-effective and to potentially save public health resources in LMIC such as Mexico.²⁴