Condomless anal sex carries some of the highest risks for the transmission of HIV. Despite what many people apparently think (70 percent think hetero anal sex carries low risks), female receptive partners are also at risk (in fact they are more than 17 times more at risk through anal than vaginal sex). But what is true is that the top or insertive partner does have lower risks than the bottom or receptive partner—86 percent less likely, in fact.
We know that reasons condomless anal sex is so risky include the increased chance of tears occurring in the rectal lining. But what about rectal fluid? Previous studies have shown that viral loads are much higher in rectal fluid than in blood or semen (500 percent higher than in blood, 2500 percent higher than in semen), according to AIDS Map.
Let’s actually pause a moment to talk about rectal fluid. What is it?
Rectal fluid is the mucus that lines the rectum to help in the elimination of waste. It’s a slippery secretion similar to that produced by other parts in our body called mucous membranes, which are located at each entrance (what some like to call the holes), i.e. the mouth, vagina, urethra, nose and rectum, to name a few.
Mucous membranes have a heavy viral load in people living with HIV because they are loaded with the very cells HIV likes to infect: Immune cells. In fact, most of the immune cells in our body are located in the mucous membranes of the gastrointestinal tract (like a tube that leads from the mouth through the intestines and out the rectum). They’re meant to protect us from germs, while also herding in the good kinds of microbes. Because HIV is so attracted to these kinds of cells (which it uses to replicate itself), the mucus can contain high levels of the virus.
Still, for an HIV-negative insertive partner to contract HIV via the rectal fluid of a person with HIV is highly unlikely — even if the sex is condomless and the HIV-positive person's viral load hasn't been controlled — but not impossible. Rectal fluid (and blood) can also secrete into the urethra (where pee comes out) and transmit HIV that way, although this is also unlikely. The bottom line is that the risk for the insertive partner in sex is only 10 out of 10,000 exposures.
So what about coming into contact with HIV in rectal secretions via rimming? It may seem like oral contact with the mucus would have high risks, but actually once swallowed, HIV cannot survive stomach acids. On the other hand; you can certainly pick up other nasty STIs from rimming.
In a nutshell, the different viral load levels in different bodily fluids isn't the best predictor for whether they are most likely to transmit HIV. And the good news is that if you are on treatment and have lowered your blood viral loads to undetectable levels, you cannot transmit HIV others — no matter if you're a top or a bottom.