Dear Carol,

Is the likelihood of transmitting HIV from a man to a woman greater than from a woman to a man? In addition to small tears in penile skin, I’m aware that other STDs like herpes and genital warts can increase risk of men contracting the virus. I’ve read about the greater HIV transmission rates of uncircumcised men in Africa — is this an issue here? —DB

Dear DB,

All other things being equal (even though they often aren’t), the likelihood is greater that a man will transmit HIV to a woman than the other way around. In part, this is simply statistical probability: more men than women are infected with HIV, so the likelihood is greater in any heterosexual coupling that the man will carry the virus. Also, a small area of mucosal tissue on a man will be exposed to a female partner’s vaginal secretions during intercourse — unless the man still has his foreskin (i.e., is uncircumcised), his urethral opening may be bathed in potentially HIV-infected juices, but that’s a small area. Yes, his risk will go up a lot if he has other sexually transmitted diseases, especially ones that result in broken or irritated skin on the penile shaft or urethral meatus. Such ports of entry for the virus wouldn’t exist on intact skin, which is a barrier to HIV. And speaking of intact, I’ll get to the circumcision-related part of your question presently.

Let’s consider the reverse situation: If an HIV-positive man and a seronegative woman have unprotected penile-vaginal intercourse that results in internal ejaculation, HIV-laden semen will contact a much greater mucosal area, namely the vagina. Vaginal mucosa isn’t as delicate and prone to irritation and micro-tears as rectal mucosa, but it’s also not the barrier that non-mucosal skin is. As with males, if the woman has another STD or other condition that results in mucosal or genital irritation, her risk goes up. (Such conditions may include, as recent prostitute studies in Africa indicate, irritation caused by using nonoxynol-9 as a germicide — an ugly irony given that for the first 20 or so years of the epidemic we’ve been encouraged to consider n-9 an important layer of protection against HIV.)

Men and women do not only have penile-vaginal intercourse. There’s also anal and oral sex. The penetrated person's risk is greater than the penetrating partner’s for essentially the same reasons as stated above: lots of exposed, delicate mucosa. Orally, the difference has to do with the relative differences in HIV infectivity between semen and vaginal juice. Semen can carry a higher concentration of HIV; therefore, it’s posited that a blow job with ejaculation is more risky than cunnilingus. There are a couple of wild cards here: a menstruating woman will have blood in her vaginal juices, and if the oral sex-giver’s mouth is in bad shape (with bleeding gums or other viral ports of entry) that person’s risk will go up regardless of whether s/he’s going down on a woman or a man. All that said, oral sex is considered by most to be less risky than intercourse, anal or vaginal — but if those gums are really bleeding, the risk may not be all that minimal. There are cases of oral transmission, though they are less common than transmission through intercourse.

Whew! Now let’s go back to the question of circumcision. As I’ve written here before, HIV transmission in Africa may not be the best guide to understanding transmission in the US. (Tell that to the African prostitutes who are awash in nonoxynol-9). But first: Why would circumcision make a difference? In a word, mucosa. The inside of the foreskin is mucosal and thus more delicate than the keratinized skin of a circumcised penis. All other things being equal (as, I repeat, they frequently are not), the virus likes mucosal tissue better. But that doesn’t mean transmission is automatic. Much research indicates that HIV is not always transmitted during p/v or even penile-anal intercourse, for example. Discordant couples (one positive, one negative) have been a bit of a research mystery, especially those that do not always use condoms. It may be that some are resistant to the virus, but just as likely is the explanation that mucosal micro-tears or irritation are responsible for all HIV transmission during intercourse. Don’t like the chances in that particular game of roulette? Condoms, my friends!

A brief recap of my take on the Africa circumcision research: All kinds of factors may affect how readily we can generalize this information to US populations. These include the following: Access to medical care and sanitary facilities (sometimes even including clean water) can be very difficult in parts of Africa where the epidemic has hit hardest. Other sexually transmitted conditions, when present, will certainly act as HIV cofactors in uncircumcised men. Sexual mores may be quite different in circumcised populations (in Africa these men are largely Moslem; we do not know for sure that they engage in comparably risky behaviors as their uncircumcised neighbors). We don’t know what difference these sorts of factors make on the research, but they might affect it greatly. What we can infer is that uncircumcised men should be aware and concerned about avoiding other STDs and keeping the skin of the prepuce intact.

Having said all this, though, I should add that the easiest way for women and men to transmit HIV to each other is non-sexual: in the US, sharing infected needles results in heterosexual HIV transmission more frequently than anything else. And I should probably say, because there still seem to be a few folks who are unclear on the concept: You can’t get HIV/AIDS from someone who is not infected. All this talk about risk is moot unless one partner has the virus.