Although it’s not something commonly discussed, people have claimed that HIV transmission is primarily homosexual rather than heterosexual, because heterosexual men do not contract HIV from their sexual partners. The idea is a controversial one, so I thought I would look into it for today. If it is true, the reason would probably be mechanical, a simple consequence of the inefficiency of the female-to-male transmission route. Although female secretions do come into contact with the male’s external urethral orifice in heterosexual sex, male ejaculate introduces seminal fluid directly into the female reproductive tract; the male’s very act of orgasm may even wash female fluids out of the male urethra.
This is, of course, a rational argument, but not an empirical one. Do observations support expectations? One 1991 study, “Female-to-Male Transmission of Human Immunodeficiency Virus,” reported on data from 379 couples:
The majority of couples were monogamous since 1978, white, and in their 30s. Most partners did not know their serostatus at entry into the study.
We observed one probable instance (1%) of female-to-male transmission compared with 20% transmission rates in the female partners of infected men. All couples were sampled in the same way. Male index cases were more likely to be symptomatic than female index cases.
The odds of male-to-female transmission were significantly greater than female-to-male transmission. The one case of female-to-male transmission was unique in that the couple reported numerous unprotected sexual contacts and noted several instances of vaginal and penile bleeding during intercourse. (Padian, Shiboski, & Jewell, 1991)
A follow-up study replicated these findings:
Participants were recruited from health care providers, research studies, and health departments throughout Northern California, and they were interviewed and examined at various study clinic sites. A total of 82 infected women and their male partners and 360 infected men and their female partners were enrolled. Over 90% of the couples were monogamous for the year prior to entry into the study; <3% had a current sexually transmitted disease (STD). The median age of participants was 34 years, and the majority were white.
…Overall, 68 (19%) of the 360 female partners of HIV-infected men (95% confidence interval (CI) 15.0–23.3%) and two (2.4%) of the 82 male partners of HIV-infected women (95% CI 0.3–8.57%) were infected. History of sexually transmitted diseases was most strongly associated with transmission. Male-to-female transmission was approximately eight times more efficient than female-to-male transmission and male-to-female per contact infectivity was estimated to be 0.0009 (95% CI 0.0005–0.001). Over time, the authors observed increased condom use (p < 0.001) and no new infections. Infectivity for HIV through heterosexual transmission is low, and STDs may be the most important cofactor for transmission. (Padian et al., 1997)
Studies carried out by other research teams returned similar results:
To compare the efficiency of male-to-female and female-to-male sexual transmission of human immunodeficiency virus (HIV), we studied 524 female partners of HIV-infected men and 206 male partners of HIV-infected women in 16 Italian clinical centers. All of the partners had had a sexual relationship with the index case lasting for at least 6 months and presented no other risk factor than sexual exposure to the HIV-infected partner… [T]he efficiency of male-to-female transmission was 2.3 (95% confidence interval = 1.1-4.8) times greater than that of female-to-male transmission. Between-gender differences in the contact surfaces and the intensity of exposure to HIV during sexual intercourse are possible reasons for the difference in efficiency of transmission. (Nicolosi et al., 1994)
The chance for contracting HIV from any single sexual act is actually quite low, but the highest probability for transmission appears to be for anal sex with a homosexual man. Considering the per-act risk of contracting HIV from unprotected sex with a partner of unknown HIV status, it appears that the chance for contracting HIV as a receptive female during penile-vaginal sex is approximately 0.0001%, while for a receptive man during penile-anal sex, the chance is roughly 0.003%, and thirty-fold difference. (Varghese, 2002)Does this mean that AIDS is a gay disease? No, particularly since people are not always consistent in their sexual patterns. It does, however, help to explain why HIV disproportionately affects the homosexual community, and it sheds some light on the epidemiology of HIV, and possibly other sexually transmitted diseases as well.If the primary mechanism for infection is reception of infected seminal fluid, it seems inevitable that sexually transmitted diseases would have trouble spreading. They would either need a significant proportion of homosexuals who regularly play both insertive and receptive roles during sex, or, they would need to create lesions through which fluids or blood could pass from females to males, or lastly, they would need to be able to survive externally on the male genitals for a significant period of time. Although I’ve previously discussed the downsides to circumcision, the practice may have arisen as a means of combating this route of infection; a circumcised penis is likely to be inhospitable to pathogens.
Nicolosi, A., Leite, M. L. C., Musicco, M., Arid, C., Gavazzeni, G., & Lazzarin, A. (1994). The Efficiency of Male-to Female and Female-to-Male Sexual Transmission of the Human Immunodeficiency Virus: A Study of 730 Stable Couples. Epidemiology, 5(6), 570-575.
Padian, N. S., Shiboski, S. C., & Jewell, N. P. (1991). Female-to-male transmission of human immunodeficiency virus. Jama, 266(12), 1664-1667.
Padian, N. S., Shiboski, S. C., Glass, S. O., & Vittinghoff, E. (1997). Heterosexual transmission of human immunodeficiency virus (HIV) in northern California: results from a ten-year study. American journal of epidemiology, 146(4), 350-357.
Varghese, B., Maher, J. E., Peterman, T. A., Branson, B. M., & Steketee, R. W. (2002). Reducing the risk of sexual HIV transmission: quantifying the per-act risk for HIV on the basis of choice of partner, sex act, and condom use. Sexually transmitted diseases, 29(1), 38-43.