Men who have sex with both men and women (MSM/W) are second in importance to injection drug users as a source of HIV infection in heterosexual women (WSM) in the United States.1 Of the women who contracted HIV through heterosexual contact in 2004, 15% reported sex with MSM/W as their only risk factor2; this is probably an underestimate, as between 33% and 75% of MSM/W do not disclose to female partners that they have sex with men.3,4
MSM/W bridge 2 populations: Men who have sex with men (MSM), a population with high prevalence of sexually-transmitted infections (STI’s), including HIV, and heterosexual females who have lower STI prevalence. MSM/W also report higher numbers of sex partners, more casual sex partners, and less frequent condom use than other groups, increasing their STI risk.4 Women who have sex with women and men (WSW/M) act as a sexual bridge between heterosexual men (MSW) and women who have sex with women (WSW)—a group with relatively low STI prevalence. The impact of MSM/W bridging between MSM and WSM on the AIDS epidemic in the United States has been explored using risk models.1,5 The resulting probabilistic risk assessments suggest that even a small number of MSM/W bridging could profoundly effect HIV transmission, and presumably, other STI, at the population level if MSM/W have a central role in connecting multiple sexual networks that would otherwise remain separate as demonstrated by Hightow et al.5,6 Most data on MSM/W and WSW/M come from select populations, such as persons with AIDS, persons at gay bars or sexual pick-up areas, or patients in STI clinics; there are few general population estimates of MSM/W and WSW/M prevalence. We describe the prevalence of and sexual behaviors among MSM/W and WSW/M identified from participants in a random digit dialing (RDD) survey of individuals aged between 18 and 39.
The RDD survey was conducted in the Seattle area between October 2, 2003, and January 26, 2004 among residents 18 to 39 years of age with fluency in the English language who had ever engaged in vaginal, anal, or oral intercourse. Details of the survey and survey questionnaire are presented elsewhere.7 Of the 2582 eligible individuals contacted, 1194 (46.2%) agreed to participate and completed the interview. We limited our analysis to the 1103 (92.5%) participants who reported ever engaging in vaginal, oral, or anal intercourse and who reported the sex of their partners.
We grouped participants based on responses regarding the number of sex partners of same and opposite sex. Female respondents reporting both female and male partners were classified as WSW/M; those reporting only female partners as WSW, and those reporting only male partners were classified as WSM. Male respondents reporting both male and female partners were classified as MSM/W; those reporting only male partners were designated MSM, and those reporting only female partners were designated MSW.
We used proportions to describe the frequency of sexual behaviors and other STI risk factors. To identify correlates of sexual behaviors and other STI risk factors by gender, and gender of sex partners, we used contingency tables with the chi-squared test to assess statistical significance. We used the Wilcoxon test to determine statistically significant differences in means between the different groups because the responses for each of the questions were not normally distributed. The software package SAS was used for analyses.8
A total of 43 men (3.9% of men) and 94 women (8.5% of women) reported having both same sex and opposite sex partners. The median proportion of lifetime sex partners of the opposite gender was 0.3 among MSM/W and 0.8 among WSW/M. Fewer men (n = 29, 2.6% of males) and substantially fewer women (n = 11, 1.0% of females) reported only same sex partners (Table 1). The median age at first sex was lower for MSM/W (16.0), WSW/M (16.0), and MSM (16.0) than for MSW (17.0) or WSM (17.0). MSM also reported the highest lifetime number of sex partners, followed by MSM/W and WSW/M (Table 2). Concurrency was reported significantly more often for MSM/W and WSW/M compared with individuals of the same sex who reported only opposite sex partners (32% for MSM/W vs. 13% for MSW, P = 0.002, and 23% for WSW/M versus 12% for WSM, P = 0.004), (Table 2). MSM/W were less likely than MSM to consistently use condoms with the most recent partner during anal sex (P = 0.04), but equally likely to report a history of STI; MSM/W: 33%; MSM:35%. Among females, STI was reported more often by WSW/M (32%), than by WSM (19%) (P = 0.002). No WSW reported STI (Table 2).
We defined cross-gender sexual bridging during the past 5 partnerships as a partnership with an opposite-sex partner that either preceded or followed a same-sex partnership regardless of the time interval. At the time of study, not all individuals had been in 5 partnerships and some had only 1 partnership. Of MSM/W who had been in at least 5 partnerships, 22% reported bridging within their past 5 relationships compared to 49% of WSW/M (P = 0.02) (Table 2).
The frequency and type of sex acts performed with the most recent partner differed by sexual orientation (Table 3). Both for MSM/W and WSW/M, the median frequency of oral sex and anal sex engaged in with the most recent partner was similar for opposite sex and same sex partners. Both MSM/W (65%) and WSW/M (37%) were more likely than heterosexuals to have used the internet to recruit sex partners in the past 6 months, and met their sex partners at more diverse venues than heterosexuals (Table 3).
This 2003 RDD Survey is one of few studies of the sexual behavior of bisexual men and women conducted in individuals identified from the general population, and is the first we are aware of to estimate the frequency of cross-gender bridging. Data on sexual risk behaviors, including early age at sexual debut, high numbers of sexual partners, concurrent partnerships, infrequent condom usage, gender of respondent, and gender of respondent’s sex partner(s) generally showed MSM to have highest risks of STI, followed by progressively decreasing risk for MSM/W, then WSW/M, then MSW, then WSM, and then WSW. Thus, bisexual men and women are at higher risk for acquiring and transmitting STIs, and are also epidemiologically important for sexual bridging between populations at relatively higher risk and those at lower risk of STIs, including HIV infection.
When MSM/W partner concurrently with MSM and WSM, they can create a bridge transferring STIs between the networks of the higher risk group of MSM and the lower risk group of WSM.9 Among those reporting at least 5 partnerships, 22% of MSM/W reported at least one instance of sexual bridging across genders; this level of bridging might have a significant impact on STI transmission at a population level. By contrast, 49% of WSW/M reported at least one instance of bridging across genders during the past 5 partnerships; however, the effect of this bridging on STI transmission at the population level may be less, because WSW/M partner with WSM and WSW, groups that have lower STI prevalence. Because we did not interview sex partners, we do not know whether individuals that have partners of both the same and opposite sex are more likely to partner with other individuals who have both same and opposite sex partners rather than individuals with only same or only opposite sex partners, which may increase their risk of STI. We found no similar studies in the literature quantifying the frequency of bridging between opposite- and same-sex partners among MSM/W or WSW/M; however, in a study of MSM that included 85 MSM/W identified at gay bars in New York City, 48% of MSM/W had had sex with both a man and a woman in the past year.10
The use of answering machines and mechanisms to screen and block calls from unidentified numbers has reduced the participation rate for RDD surveys. This occurs before screening can take place, potentially making selection biases difficult to detect. Among the 84% of individuals in our initial sampling frame who were contacted, 46% of those who were eligible agreed to participate, which is consistent with other recent reports.11,12 Another limitation to the representativeness of the study is the exclusive sampling of land-line phones. At the time the survey was conducted, only 5% of households were cell phone only; however, those ages 18 to 29 were almost twice as likely to be in cell phone only households as those ages 30 to 44.13 The smaller likelihood of selecting younger respondents resulted in a high average age of respondents and may have affected estimates of sexual behavior and its disclosure. Another possible limitation is that sexual orientation was based on lifetime sex partners and may not correlate with current risk behaviors, which may result in an underestimation of risky behaviors among bisexual and homosexual individuals.
In summary, while bridging by MSM/W may help accelerate the epidemic in the heterosexual population via the “bisexual bridge” per se, the riskier sexual behaviors and practices among bisexuals make them a particularly high risk group for STI/HIV transmission. Although MSM and MSM/W most frequently reported condom use during anorectal intercourse, they nonetheless reported the highest rates of prior STI, probably attributable to their otherwise riskier behavior and practices, and sexual networks. Further studies are needed that use information gained from this study in dynamic models to better define the conditions where MSM/W could markedly increase STI infection levels among WSM and MSW.
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