Woman to Man Transmission: why are the odds different in Africa?
Jul 30, 2002
In the US, there is a consensus that the risk of heterosexual transmission from woman to man is much lower than from man to woman. Why is this different in Africa?
Response from Mr. Kull
In general, the receptive partner in vaginal or anal sex is considered to be at greater risk for infection with HIV than the insertive partner. This does not mean that women are biologically more prone to infection than men; similar rules apply to the receptive and insertive partners in anal sex. This has more to do with the areas of the body that are exposed to fluids. The vagina and rectum are both warm cavities in the body in which fluids can be deposited (increasing the time of exposure to infected fluids) and may be prone to microscopic tears that can increase the risk for infection. The man's urethra (the tube he pees out of) is the primary location for infection during penetrative sex. There is a much larger surface area for exposure in the vagina and rectum when compared to the small opening at the head of the penis. This transmission dynamic is supported by statisitcs of transmission in the United States. The main modes of sexual transmission in the U.S. are through receptive vaginal and anal sex.
Studies of transmission in Africa seem to contradict this long-held assumption. For many years, reseachers have identified that African men were much more likely to be infected thorough insertive sex than men in the United States. Recent studies confirm that the likelihood of male-female transmission is about the same as female-male. It is believed that certain factors, such as the man being uncircumcised, may put him at higher risk for infection than his circumcised counterparts.
There is increasing, reliable, scientific evidence that heterosexual men who are uncircumcised have a greater risk of being infected during insertive vaginal intercourse when compared to uncircumcised men. This is clear in many regions in Africa, where the rate of infection is much higher among uncircumcised heterosexual men. The interior of the foreskin is a mucous membrane that contains Langerhans cells, cells that are known to be vehicles for HIV's passage to the immune system. Circumcision is a much more common practice in the United States, which might account for the lower rate of transmission to heterosexual men.
The presence of genital STD's among men in Africa may also increase their risk for infection. There is also the obvious fact that heterosexual men in Africa are much more likely to have sexual contact with an HIV infected woman than heterosexual men in the United States, increasing their odds for exposure and infection significantly.
All of these factors have some influence in HIV transmission. I'm sure more will be understood about the biology of transmission in the near future.
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