A Staff Report from the Straight Dope Science Advisory Board

How do men get sexually transmitted diseases?

January 24, 2006

Dear Straight Dope:

Ever since I took sex ed in high school I've been wondering: How exactly do men get STDs from women? The gym teacher explained that STDs are transmitted in body fluids and, as far as I know, men don't intake bodily fluids from women. Can you help me out?

Pardon the metaphor, but while a gusher may be spectacular, a swamp can be just as wet.

Transmission from females to males is dependent on a lot of factors such as open sores on the penis and vulva, vaginal trauma in women and circumcision in men (non-circumcised males have a higher risk of penile cancer). Female arousal includes a lubrication phase that squeezes water out of vaginal cells through vasodilation. The lubricant that forms is mostly water with a mix of squamous vaginal cells, cervical mucus and small amounts of blood. Luckily for us women, that can help reduce the risk of transmission to partners since more lubricant keeps the vagina safe from the cuts and scrapes of enthusiastic partners, and therefore less blood is introduced into the mix. It pays to keep your girl happy.

As for actual numbers, female-to-male transmission has been studied extensively, mainly in connection with HIV. A 1994 study in Italy (Nicolosi et al--see references below) showed that male-to-female transmission of HIV was 2.3 times higher than female-to-male transmission. In contrast, a study in Uganda in 2000 (Rakai Project Study Group) showed no statistical difference between men and women in HIV transmission--the biggest factor in transmission rates was viral load, not gender or any other variable. Many other studies have confirmed the disparity between transmission rates in Africa and those in the developed world. The factors that probably make the difference are the higher number of penile ulcers in African men, who are also more likely to become infected via a prostitute.

As for other STDs like HPV (human papilloma virus, which causes genital warts), herpes and hepatitis, a few preliminary studies in the U.S. and Europe have been reported at conferences. These suggest the rate of female-to-male transmission isn't much lower than male-to-female transmission and may be roughly equal.

Female-to-female transmission is only beginning to be studied, but HPV, herpes and even syphilis are known to have been transmitted via female-to-female sexual contact. We also know that exclusively lesbian women have higher rates of bacterial vaginosis than straight women (Hughes and Evans, 2003; Fethers et al 2000; Campos-Outcalt and Hurwitz, 2002). As usual, it always pays to play safely, no matter your gender or orientation.

References

Rakai Project Study Group, "Viral load and heterosexual transmissionof human immunodeficiency virus type 1," New England Journal of Medicine, March 30, 2000; 342(13):921-9.

Robinson NJ, Mulder D, Auvert B, Whitworth J, Hayes R, "Type of partnership and heterosexual spread of HIV infection in rural Uganda: results from simulation modelling," International Journal of STD & AIDS, November 1999; 10(11):718-25.

Nicolosi A, Correa Leite ML, Musicco M, Arici C, Gavazzeni G, Lazzarin A, "The efficiency of male-to-female and female-to-male sexual transmission of the human immunodeficiency virus: a study of 730 stable couples. Italian Study Group on HIV Heterosexual Transmission," Epidemiology, November 1994; 5(6):565-7.

Hughes C, Evans E, "Health needs of women who have sex with women," BMJ, October 25, 2003; 327:939-940.

Fethers K, Marks C, Mindel A, Estcourt S, "Sexually transmitted infections and risk behaviours in women who have sex with women," Sexually Transmitted Infections, 76:345-349 (2000).

Campos-Outcalt D, Hurwitz S, "Female-to-female transmission of syphilis: a case report," Sexually Transmitted Diseases, February 2002; 29(2):119-20.

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