August 2019 Issue #32 August 2019 Issue #32 | Page 152

The mucous membranes of the rectum, and the mucus they produce (rectal fluid), are
no exception. Several studies show that HIV can be found in the rectal fluid of a person
living with HIV.2,3,4,5,6 In fact, one study of 64 HIV-positive men (of which about half
were on antiretroviral therapy) found that, overall, the average amount of virus in their
rectal fluid was higher than in their semen and blood.4
Why might rectal fluid contain more HIV than other bodily fluids? It turns out that the
majority of the immune cells in the body – including the cells that are a major target
for HIV – are located in the mucous membranes of the gastrointestinal tract, which
includes the rectum.7 There are a lot of immune cells in the gastrointestinal tract be-
cause it has a very large surface area. Also, a large number of immune cells are need-
ed to help to protect the gut from the “foreign” germs in our food and to control the
growth of the “friendly” germs living in our gut.
The high concentration of immune cells means that the majority of HIV replication in
someone with HIV may be happening in the gastrointestinal tract, including the rec-
tum.8,9,10 This may explain why so much HIV can be found in the rectal fluid.
Implications for HIV transmission and prevention
Anal sex is a common practice among men who have sex with men, heterosexual men
and women, and transgender individuals and is a known risk factor for HIV infection
and transmission.11,12,13,14 In a recent nationally representative survey of almost
6,000 men and women in the United States (of which the majority were heterosexual),
approximately 20% of women between the ages of 18 to 39 reported engaging in anal
sex in the past year, as did approximately 25% of men between the ages of 25 to 49.15
HIV transmission
Rectal fluid has implications for HIV transmission through anal sex when the HIV-neg-
ative person is the insertive partner (that is, inserts their penis into a partner’s anus).
Research show that this type of anal sex can carry a significant risk of HIV transmis-
sion. In fact, the average risk of HIV infection through a single act of condomless in-
sertive anal sex with an HIV-positive partner is slightly higher than through vaginal sex
but much lower than if the HIV-negative person takes the receptive role during anal
sex.16,17
Rectal fluid undoubtedly contributes to the risk of HIV transmission through anal sex
where the insertive partner is HIV negative. We know that for HIV transmission to be
possible, a fluid that contains HIV must come into contact with specific parts of the
body that are vulnerable to HIV infection. If an HIV-negative person has insertive anal
sex with an HIV-positive partner, rectal fluid containing HIV can come into contact with
the urethra and/or the penis foreskin. Both the urethra and foreskin are vulnerable to
HIV infection.
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